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- Use LEA ...%rsp instead of ADD %rsp in the Zen1/2 SRSO return sequence

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as latter clobbers flags which interferes with fastop emulation in
   KVM, leading to guests freezing during boot
 
 - A fix for the DIV(0) quotient data leak on Zen1 to clear the divider
   buffers at the right time
 
 - Disable the SRSO mitigation on unaffected configurations as it got
   enabled there unnecessarily
 
 - Change .text section name to fix CONFIG_LTO_CLANG builds
 
 - Improve the optprobe indirect jmp check so that certain configurations
   can still be able to use optprobes at all
 
 - A serious and good scrubbing of the untraining routines by PeterZ:
  - Add proper speculation stopping traps so that objtool is happy
  - Adjust objtool to handle the new thunks
  - Make the thunk pointer assignable to the different untraining
    sequences at runtime, thus avoiding the alternative at the return
    thunk. It simplifies the code a bit too.
  - Add a entry_untrain_ret() main entry point which selects the
    respective untraining sequence
  - Rename things so that they're more clear
  - Fix stack validation with FRAME_POINTER=y builds
 
 - Fix static call patching to handle when a JMP to the return thunk is
   the last insn on the very last module memory page
 
 - Add more documentation about what each untraining routine does and
   why
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Merge tag 'x86_urgent_for_v6.5_rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Borislav Petkov:
 "Extraordinary embargoed times call for extraordinary measures. That's
  why this week's x86/urgent branch is larger than usual, containing all
  the known fallout fixes after the SRSO mitigation got merged.

  I know, it is a bit late in the game but everyone who has reported a
  bug stemming from the SRSO pile, has tested that branch and has
  confirmed that it fixes their bug.

  Also, I've run it on every possible hardware I have and it is looking
  good. It is running on this very machine while I'm typing, for 2 days
  now without an issue. Famous last words...

   - Use LEA ...%rsp instead of ADD %rsp in the Zen1/2 SRSO return
     sequence as latter clobbers flags which interferes with fastop
     emulation in KVM, leading to guests freezing during boot

   - A fix for the DIV(0) quotient data leak on Zen1 to clear the
     divider buffers at the right time

   - Disable the SRSO mitigation on unaffected configurations as it got
     enabled there unnecessarily

   - Change .text section name to fix CONFIG_LTO_CLANG builds

   - Improve the optprobe indirect jmp check so that certain
     configurations can still be able to use optprobes at all

   - A serious and good scrubbing of the untraining routines by PeterZ:
      - Add proper speculation stopping traps so that objtool is happy
      - Adjust objtool to handle the new thunks
      - Make the thunk pointer assignable to the different untraining
        sequences at runtime, thus avoiding the alternative at the
        return thunk. It simplifies the code a bit too.
      - Add a entry_untrain_ret() main entry point which selects the
        respective untraining sequence
      - Rename things so that they're more clear
      - Fix stack validation with FRAME_POINTER=y builds

   - Fix static call patching to handle when a JMP to the return thunk
     is the last insn on the very last module memory page

   - Add more documentation about what each untraining routine does and
     why"

* tag 'x86_urgent_for_v6.5_rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/srso: Correct the mitigation status when SMT is disabled
  x86/static_call: Fix __static_call_fixup()
  objtool/x86: Fixup frame-pointer vs rethunk
  x86/srso: Explain the untraining sequences a bit more
  x86/cpu/kvm: Provide UNTRAIN_RET_VM
  x86/cpu: Cleanup the untrain mess
  x86/cpu: Rename srso_(.*)_alias to srso_alias_\1
  x86/cpu: Rename original retbleed methods
  x86/cpu: Clean up SRSO return thunk mess
  x86/alternative: Make custom return thunk unconditional
  objtool/x86: Fix SRSO mess
  x86/cpu: Fix up srso_safe_ret() and __x86_return_thunk()
  x86/cpu: Fix __x86_return_thunk symbol type
  x86/retpoline,kprobes: Skip optprobe check for indirect jumps with retpolines and IBT
  x86/retpoline,kprobes: Fix position of thunk sections with CONFIG_LTO_CLANG
  x86/srso: Disable the mitigation on unaffected configurations
  x86/CPU/AMD: Fix the DIV(0) initial fix attempt
  x86/retpoline: Don't clobber RFLAGS during srso_safe_ret()
This commit is contained in:
Linus Torvalds 2023-08-19 10:46:02 +02:00
commit bf98bae3d8
18 changed files with 238 additions and 135 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
Documentation/admin-guide/hw-vuln/srso.rst
View File

@ -141,8 +141,8 @@ sequence.
To ensure the safety of this mitigation, the kernel must ensure that the
safe return sequence is itself free from attacker interference. In Zen3
and Zen4, this is accomplished by creating a BTB alias between the
untraining function srso_untrain_ret_alias() and the safe return
function srso_safe_ret_alias() which results in evicting a potentially
untraining function srso_alias_untrain_ret() and the safe return
function srso_alias_safe_ret() which results in evicting a potentially
poisoned BTB entry and using that safe one for all function returns.
In older Zen1 and Zen2, this is accomplished using a reinterpretation

1
arch/x86/include/asm/entry-common.h
View File

@ -92,6 +92,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
static __always_inline void arch_exit_to_user_mode(void)
{
mds_user_clear_cpu_buffers();
amd_clear_divider();
}
#define arch_exit_to_user_mode arch_exit_to_user_mode

49
arch/x86/include/asm/nospec-branch.h
View File

@ -272,9 +272,9 @@
.endm
#ifdef CONFIG_CPU_UNRET_ENTRY
#define CALL_ZEN_UNTRAIN_RET "call zen_untrain_ret"
#define CALL_UNTRAIN_RET "call entry_untrain_ret"
#else
#define CALL_ZEN_UNTRAIN_RET ""
#define CALL_UNTRAIN_RET ""
#endif
/*
@ -282,7 +282,7 @@
* return thunk isn't mapped into the userspace tables (then again, AMD
* typically has NO_MELTDOWN).
*
* While zen_untrain_ret() doesn't clobber anything but requires stack,
* While retbleed_untrain_ret() doesn't clobber anything but requires stack,
* entry_ibpb() will clobber AX, CX, DX.
*
* As such, this must be placed after every *SWITCH_TO_KERNEL_CR3 at a point
@ -293,14 +293,20 @@
defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
VALIDATE_UNRET_END
ALTERNATIVE_3 "", \
CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET, \
CALL_UNTRAIN_RET, X86_FEATURE_UNRET, \
"call entry_ibpb", X86_FEATURE_ENTRY_IBPB, \
__stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
#endif
.endm
#ifdef CONFIG_CPU_SRSO
ALTERNATIVE_2 "", "call srso_untrain_ret", X86_FEATURE_SRSO, \
"call srso_untrain_ret_alias", X86_FEATURE_SRSO_ALIAS
.macro UNTRAIN_RET_VM
#if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
VALIDATE_UNRET_END
ALTERNATIVE_3 "", \
CALL_UNTRAIN_RET, X86_FEATURE_UNRET, \
"call entry_ibpb", X86_FEATURE_IBPB_ON_VMEXIT, \
__stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
#endif
.endm
@ -309,15 +315,10 @@
defined(CONFIG_CALL_DEPTH_TRACKING)
VALIDATE_UNRET_END
ALTERNATIVE_3 "", \
CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET, \
CALL_UNTRAIN_RET, X86_FEATURE_UNRET, \
"call entry_ibpb", X86_FEATURE_ENTRY_IBPB, \
__stringify(RESET_CALL_DEPTH_FROM_CALL), X86_FEATURE_CALL_DEPTH
#endif
#ifdef CONFIG_CPU_SRSO
ALTERNATIVE_2 "", "call srso_untrain_ret", X86_FEATURE_SRSO, \
"call srso_untrain_ret_alias", X86_FEATURE_SRSO_ALIAS
#endif
.endm
@ -341,17 +342,24 @@ extern retpoline_thunk_t __x86_indirect_thunk_array[];
extern retpoline_thunk_t __x86_indirect_call_thunk_array[];
extern retpoline_thunk_t __x86_indirect_jump_thunk_array[];
#ifdef CONFIG_RETHUNK
extern void __x86_return_thunk(void);
extern void zen_untrain_ret(void);
#else
static inline void __x86_return_thunk(void) {}
#endif
extern void retbleed_return_thunk(void);
extern void srso_return_thunk(void);
extern void srso_alias_return_thunk(void);
extern void retbleed_untrain_ret(void);
extern void srso_untrain_ret(void);
extern void srso_untrain_ret_alias(void);
extern void srso_alias_untrain_ret(void);
extern void entry_untrain_ret(void);
extern void entry_ibpb(void);
#ifdef CONFIG_CALL_THUNKS
extern void (*x86_return_thunk)(void);
#else
#define x86_return_thunk (&__x86_return_thunk)
#endif
#ifdef CONFIG_CALL_DEPTH_TRACKING
extern void __x86_return_skl(void);
@ -478,9 +486,6 @@ enum ssb_mitigation {
SPEC_STORE_BYPASS_SECCOMP,
};
extern char __indirect_thunk_start[];
extern char __indirect_thunk_end[];
static __always_inline
void alternative_msr_write(unsigned int msr, u64 val, unsigned int feature)
{

4
arch/x86/kernel/alternative.c
View File

@ -687,10 +687,6 @@ void __init_or_module noinline apply_retpolines(s32 *start, s32 *end)
#ifdef CONFIG_RETHUNK
#ifdef CONFIG_CALL_THUNKS
void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
#endif
/*
* Rewrite the compiler generated return thunk tail-calls.
*

1
arch/x86/kernel/cpu/amd.c
View File

@ -1329,3 +1329,4 @@ void noinstr amd_clear_divider(void)
asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
:: "a" (0), "d" (0), "r" (1));
}
EXPORT_SYMBOL_GPL(amd_clear_divider);

27
arch/x86/kernel/cpu/bugs.c
View File

@ -63,6 +63,8 @@ EXPORT_SYMBOL_GPL(x86_pred_cmd);
static DEFINE_MUTEX(spec_ctrl_mutex);
void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
/* Update SPEC_CTRL MSR and its cached copy unconditionally */
static void update_spec_ctrl(u64 val)
{
@ -165,6 +167,11 @@ void __init cpu_select_mitigations(void)
md_clear_select_mitigation();
srbds_select_mitigation();
l1d_flush_select_mitigation();
/*
* srso_select_mitigation() depends and must run after
* retbleed_select_mitigation().
*/
srso_select_mitigation();
gds_select_mitigation();
}
@ -1035,6 +1042,9 @@ do_cmd_auto:
setup_force_cpu_cap(X86_FEATURE_RETHUNK);
setup_force_cpu_cap(X86_FEATURE_UNRET);
if (IS_ENABLED(CONFIG_RETHUNK))
x86_return_thunk = retbleed_return_thunk;
if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD &&
boot_cpu_data.x86_vendor != X86_VENDOR_HYGON)
pr_err(RETBLEED_UNTRAIN_MSG);
@ -1044,6 +1054,7 @@ do_cmd_auto:
case RETBLEED_MITIGATION_IBPB:
setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
mitigate_smt = true;
break;
@ -2417,9 +2428,10 @@ static void __init srso_select_mitigation(void)
* Zen1/2 with SMT off aren't vulnerable after the right
* IBPB microcode has been applied.
*/
if ((boot_cpu_data.x86 < 0x19) &&
(!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED)))
if (boot_cpu_data.x86 < 0x19 && !cpu_smt_possible()) {
setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
return;
}
}
if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
@ -2448,11 +2460,15 @@ static void __init srso_select_mitigation(void)
* like ftrace, static_call, etc.
*/
setup_force_cpu_cap(X86_FEATURE_RETHUNK);
setup_force_cpu_cap(X86_FEATURE_UNRET);
if (boot_cpu_data.x86 == 0x19)
if (boot_cpu_data.x86 == 0x19) {
setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);
else
x86_return_thunk = srso_alias_return_thunk;
} else {
setup_force_cpu_cap(X86_FEATURE_SRSO);
x86_return_thunk = srso_return_thunk;
}
srso_mitigation = SRSO_MITIGATION_SAFE_RET;
} else {
pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");
@ -2696,6 +2712,9 @@ static ssize_t retbleed_show_state(char *buf)
static ssize_t srso_show_state(char *buf)
{
if (boot_cpu_has(X86_FEATURE_SRSO_NO))
return sysfs_emit(buf, "Mitigation: SMT disabled\n");
return sysfs_emit(buf, "%s%s\n",
srso_strings[srso_mitigation],
(cpu_has_ibpb_brtype_microcode() ? "" : ", no microcode"));

40
arch/x86/kernel/kprobes/opt.c
View File

@ -226,7 +226,7 @@ static int copy_optimized_instructions(u8 *dest, u8 *src, u8 *real)
}
/* Check whether insn is indirect jump */
static int __insn_is_indirect_jump(struct insn *insn)
static int insn_is_indirect_jump(struct insn *insn)
{
return ((insn->opcode.bytes[0] == 0xff &&
(X86_MODRM_REG(insn->modrm.value) & 6) == 4) || /* Jump */
@ -260,26 +260,6 @@ static int insn_jump_into_range(struct insn *insn, unsigned long start, int len)
return (start <= target && target <= start + len);
}
static int insn_is_indirect_jump(struct insn *insn)
{
int ret = __insn_is_indirect_jump(insn);
#ifdef CONFIG_RETPOLINE
/*
* Jump to x86_indirect_thunk_* is treated as an indirect jump.
* Note that even with CONFIG_RETPOLINE=y, the kernel compiled with
* older gcc may use indirect jump. So we add this check instead of
* replace indirect-jump check.
*/
if (!ret)
ret = insn_jump_into_range(insn,
(unsigned long)__indirect_thunk_start,
(unsigned long)__indirect_thunk_end -
(unsigned long)__indirect_thunk_start);
#endif
return ret;
}
/* Decode whole function to ensure any instructions don't jump into target */
static int can_optimize(unsigned long paddr)
{
@ -334,9 +314,21 @@ static int can_optimize(unsigned long paddr)
/* Recover address */
insn.kaddr = (void *)addr;
insn.next_byte = (void *)(addr + insn.length);
/* Check any instructions don't jump into target */
if (insn_is_indirect_jump(&insn) ||
insn_jump_into_range(&insn, paddr + INT3_INSN_SIZE,
/*
* Check any instructions don't jump into target, indirectly or
* directly.
*
* The indirect case is present to handle a code with jump
* tables. When the kernel uses retpolines, the check should in
* theory additionally look for jumps to indirect thunks.
* However, the kernel built with retpolines or IBT has jump
* tables disabled so the check can be skipped altogether.
*/
if (!IS_ENABLED(CONFIG_RETPOLINE) &&
!IS_ENABLED(CONFIG_X86_KERNEL_IBT) &&
insn_is_indirect_jump(&insn))
return 0;
if (insn_jump_into_range(&insn, paddr + INT3_INSN_SIZE,
DISP32_SIZE))
return 0;
addr += insn.length;

13
arch/x86/kernel/static_call.c
View File

@ -186,6 +186,19 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
*/
bool __static_call_fixup(void *tramp, u8 op, void *dest)
{
unsigned long addr = (unsigned long)tramp;
/*
* Not all .return_sites are a static_call trampoline (most are not).
* Check if the 3 bytes after the return are still kernel text, if not,
* then this definitely is not a trampoline and we need not worry
* further.
*
* This avoids the memcmp() below tripping over pagefaults etc..
*/
if (((addr >> PAGE_SHIFT) != ((addr + 7) >> PAGE_SHIFT)) &&
!kernel_text_address(addr + 7))
return false;
if (memcmp(tramp+5, tramp_ud, 3)) {
/* Not a trampoline site, not our problem. */
return false;

2
arch/x86/kernel/traps.c
View File

@ -206,8 +206,6 @@ DEFINE_IDTENTRY(exc_divide_error)
{
do_error_trap(regs, 0, "divide error", X86_TRAP_DE, SIGFPE,
FPE_INTDIV, error_get_trap_addr(regs));
amd_clear_divider();
}
DEFINE_IDTENTRY(exc_overflow)

20
arch/x86/kernel/vmlinux.lds.S
View File

@ -133,27 +133,25 @@ SECTIONS
KPROBES_TEXT
SOFTIRQENTRY_TEXT
#ifdef CONFIG_RETPOLINE
__indirect_thunk_start = .;
*(.text.__x86.indirect_thunk)
*(.text.__x86.return_thunk)
__indirect_thunk_end = .;
*(.text..__x86.indirect_thunk)
*(.text..__x86.return_thunk)
#endif
STATIC_CALL_TEXT
ALIGN_ENTRY_TEXT_BEGIN
#ifdef CONFIG_CPU_SRSO
*(.text.__x86.rethunk_untrain)
*(.text..__x86.rethunk_untrain)
#endif
ENTRY_TEXT
#ifdef CONFIG_CPU_SRSO
/*
* See the comment above srso_untrain_ret_alias()'s
* See the comment above srso_alias_untrain_ret()'s
* definition.
*/
. = srso_untrain_ret_alias | (1 << 2) | (1 << 8) | (1 << 14) | (1 << 20);
*(.text.__x86.rethunk_safe)
. = srso_alias_untrain_ret | (1 << 2) | (1 << 8) | (1 << 14) | (1 << 20);
*(.text..__x86.rethunk_safe)
#endif
ALIGN_ENTRY_TEXT_END
*(.gnu.warning)
@ -523,7 +521,7 @@ INIT_PER_CPU(irq_stack_backing_store);
#endif
#ifdef CONFIG_RETHUNK
. = ASSERT((__ret & 0x3f) == 0, "__ret not cacheline-aligned");
. = ASSERT((retbleed_return_thunk & 0x3f) == 0, "retbleed_return_thunk not cacheline-aligned");
. = ASSERT((srso_safe_ret & 0x3f) == 0, "srso_safe_ret not cacheline-aligned");
#endif
@ -538,8 +536,8 @@ INIT_PER_CPU(irq_stack_backing_store);
* Instead do: (A | B) - (A & B) in order to compute the XOR
* of the two function addresses:
*/
. = ASSERT(((ABSOLUTE(srso_untrain_ret_alias) | srso_safe_ret_alias) -
(ABSOLUTE(srso_untrain_ret_alias) & srso_safe_ret_alias)) == ((1 << 2) | (1 << 8) | (1 << 14) | (1 << 20)),
. = ASSERT(((ABSOLUTE(srso_alias_untrain_ret) | srso_alias_safe_ret) -
(ABSOLUTE(srso_alias_untrain_ret) & srso_alias_safe_ret)) == ((1 << 2) | (1 << 8) | (1 << 14) | (1 << 20)),
"SRSO function pair won't alias");
#endif

2
arch/x86/kvm/svm/svm.c
View File

@ -4006,6 +4006,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_in
guest_state_enter_irqoff();
amd_clear_divider();
if (sev_es_guest(vcpu->kvm))
__svm_sev_es_vcpu_run(svm, spec_ctrl_intercepted);
else

7
arch/x86/kvm/svm/vmenter.S
View File

@ -222,10 +222,7 @@ SYM_FUNC_START(__svm_vcpu_run)
* because interrupt handlers won't sanitize 'ret' if the return is
* from the kernel.
*/
UNTRAIN_RET
/* SRSO */
ALTERNATIVE "", "call entry_ibpb", X86_FEATURE_IBPB_ON_VMEXIT
UNTRAIN_RET_VM
/*
* Clear all general purpose registers except RSP and RAX to prevent
@ -362,7 +359,7 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
* because interrupt handlers won't sanitize RET if the return is
* from the kernel.
*/
UNTRAIN_RET
UNTRAIN_RET_VM
/* "Pop" @spec_ctrl_intercepted. */
pop %_ASM_BX

141
arch/x86/lib/retpoline.S
View File

@ -13,7 +13,7 @@
#include <asm/frame.h>
#include <asm/nops.h>
.section .text.__x86.indirect_thunk
.section .text..__x86.indirect_thunk
.macro POLINE reg
@ -133,75 +133,106 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array)
#ifdef CONFIG_RETHUNK
/*
* srso_untrain_ret_alias() and srso_safe_ret_alias() are placed at
* srso_alias_untrain_ret() and srso_alias_safe_ret() are placed at
* special addresses:
*
* - srso_untrain_ret_alias() is 2M aligned
* - srso_safe_ret_alias() is also in the same 2M page but bits 2, 8, 14
* - srso_alias_untrain_ret() is 2M aligned
* - srso_alias_safe_ret() is also in the same 2M page but bits 2, 8, 14
* and 20 in its virtual address are set (while those bits in the
* srso_untrain_ret_alias() function are cleared).
* srso_alias_untrain_ret() function are cleared).
*
* This guarantees that those two addresses will alias in the branch
* target buffer of Zen3/4 generations, leading to any potential
* poisoned entries at that BTB slot to get evicted.
*
* As a result, srso_safe_ret_alias() becomes a safe return.
* As a result, srso_alias_safe_ret() becomes a safe return.
*/
#ifdef CONFIG_CPU_SRSO
.section .text.__x86.rethunk_untrain
.section .text..__x86.rethunk_untrain
SYM_START(srso_untrain_ret_alias, SYM_L_GLOBAL, SYM_A_NONE)
SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
UNWIND_HINT_FUNC
ANNOTATE_NOENDBR
ASM_NOP2
lfence
jmp __x86_return_thunk
SYM_FUNC_END(srso_untrain_ret_alias)
__EXPORT_THUNK(srso_untrain_ret_alias)
jmp srso_alias_return_thunk
SYM_FUNC_END(srso_alias_untrain_ret)
__EXPORT_THUNK(srso_alias_untrain_ret)
.section .text.__x86.rethunk_safe
#endif
/* Needs a definition for the __x86_return_thunk alternative below. */
SYM_START(srso_safe_ret_alias, SYM_L_GLOBAL, SYM_A_NONE)
#ifdef CONFIG_CPU_SRSO
add $8, %_ASM_SP
UNWIND_HINT_FUNC
#endif
.section .text..__x86.rethunk_safe
#else
/* dummy definition for alternatives */
SYM_START(srso_alias_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
ANNOTATE_UNRET_SAFE
ret
int3
SYM_FUNC_END(srso_safe_ret_alias)
SYM_FUNC_END(srso_alias_untrain_ret)
#endif
.section .text.__x86.return_thunk
SYM_START(srso_alias_safe_ret, SYM_L_GLOBAL, SYM_A_NONE)
lea 8(%_ASM_SP), %_ASM_SP
UNWIND_HINT_FUNC
ANNOTATE_UNRET_SAFE
ret
int3
SYM_FUNC_END(srso_alias_safe_ret)
.section .text..__x86.return_thunk
SYM_CODE_START(srso_alias_return_thunk)
UNWIND_HINT_FUNC
ANNOTATE_NOENDBR
call srso_alias_safe_ret
ud2
SYM_CODE_END(srso_alias_return_thunk)
/*
* Some generic notes on the untraining sequences:
*
* They are interchangeable when it comes to flushing potentially wrong
* RET predictions from the BTB.
*
* The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
* Retbleed sequence because the return sequence done there
* (srso_safe_ret()) is longer and the return sequence must fully nest
* (end before) the untraining sequence. Therefore, the untraining
* sequence must fully overlap the return sequence.
*
* Regarding alignment - the instructions which need to be untrained,
* must all start at a cacheline boundary for Zen1/2 generations. That
* is, instruction sequences starting at srso_safe_ret() and
* the respective instruction sequences at retbleed_return_thunk()
* must start at a cacheline boundary.
*/
/*
* Safety details here pertain to the AMD Zen{1,2} microarchitecture:
* 1) The RET at __x86_return_thunk must be on a 64 byte boundary, for
* 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for
* alignment within the BTB.
* 2) The instruction at zen_untrain_ret must contain, and not
* 2) The instruction at retbleed_untrain_ret must contain, and not
* end with, the 0xc3 byte of the RET.
* 3) STIBP must be enabled, or SMT disabled, to prevent the sibling thread
* from re-poisioning the BTB prediction.
*/
.align 64
.skip 64 - (__ret - zen_untrain_ret), 0xcc
SYM_START(zen_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
.skip 64 - (retbleed_return_thunk - retbleed_untrain_ret), 0xcc
SYM_START(retbleed_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
ANNOTATE_NOENDBR
/*
* As executed from zen_untrain_ret, this is:
* As executed from retbleed_untrain_ret, this is:
*
* TEST $0xcc, %bl
* LFENCE
* JMP __x86_return_thunk
* JMP retbleed_return_thunk
*
* Executing the TEST instruction has a side effect of evicting any BTB
* prediction (potentially attacker controlled) attached to the RET, as
* __x86_return_thunk + 1 isn't an instruction boundary at the moment.
* retbleed_return_thunk + 1 isn't an instruction boundary at the moment.
*/
.byte 0xf6
/*
* As executed from __x86_return_thunk, this is a plain RET.
* As executed from retbleed_return_thunk, this is a plain RET.
*
* As part of the TEST above, RET is the ModRM byte, and INT3 the imm8.
*
@ -213,13 +244,13 @@ SYM_START(zen_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
* With SMT enabled and STIBP active, a sibling thread cannot poison
* RET's prediction to a type of its choice, but can evict the
* prediction due to competitive sharing. If the prediction is
* evicted, __x86_return_thunk will suffer Straight Line Speculation
* evicted, retbleed_return_thunk will suffer Straight Line Speculation
* which will be contained safely by the INT3.
*/
SYM_INNER_LABEL(__ret, SYM_L_GLOBAL)
SYM_INNER_LABEL(retbleed_return_thunk, SYM_L_GLOBAL)
ret
int3
SYM_CODE_END(__ret)
SYM_CODE_END(retbleed_return_thunk)
/*
* Ensure the TEST decoding / BTB invalidation is complete.
@ -230,16 +261,16 @@ SYM_CODE_END(__ret)
* Jump back and execute the RET in the middle of the TEST instruction.
* INT3 is for SLS protection.
*/
jmp __ret
jmp retbleed_return_thunk
int3
SYM_FUNC_END(zen_untrain_ret)
__EXPORT_THUNK(zen_untrain_ret)
SYM_FUNC_END(retbleed_untrain_ret)
__EXPORT_THUNK(retbleed_untrain_ret)
/*
* SRSO untraining sequence for Zen1/2, similar to zen_untrain_ret()
* SRSO untraining sequence for Zen1/2, similar to retbleed_untrain_ret()
* above. On kernel entry, srso_untrain_ret() is executed which is a
*
* movabs $0xccccccc308c48348,%rax
* movabs $0xccccc30824648d48,%rax
*
* and when the return thunk executes the inner label srso_safe_ret()
* later, it is a stack manipulation and a RET which is mispredicted and
@ -251,22 +282,44 @@ SYM_START(srso_untrain_ret, SYM_L_GLOBAL, SYM_A_NONE)
ANNOTATE_NOENDBR
.byte 0x48, 0xb8
/*
* This forces the function return instruction to speculate into a trap
* (UD2 in srso_return_thunk() below). This RET will then mispredict
* and execution will continue at the return site read from the top of
* the stack.
*/
SYM_INNER_LABEL(srso_safe_ret, SYM_L_GLOBAL)
add $8, %_ASM_SP
lea 8(%_ASM_SP), %_ASM_SP
ret
int3
int3
int3
/* end of movabs */
lfence
call srso_safe_ret
int3
ud2
SYM_CODE_END(srso_safe_ret)
SYM_FUNC_END(srso_untrain_ret)
__EXPORT_THUNK(srso_untrain_ret)
SYM_FUNC_START(__x86_return_thunk)
ALTERNATIVE_2 "jmp __ret", "call srso_safe_ret", X86_FEATURE_SRSO, \
"call srso_safe_ret_alias", X86_FEATURE_SRSO_ALIAS
SYM_CODE_START(srso_return_thunk)
UNWIND_HINT_FUNC
ANNOTATE_NOENDBR
call srso_safe_ret
ud2
SYM_CODE_END(srso_return_thunk)
SYM_FUNC_START(entry_untrain_ret)
ALTERNATIVE_2 "jmp retbleed_untrain_ret", \
"jmp srso_untrain_ret", X86_FEATURE_SRSO, \
"jmp srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
SYM_FUNC_END(entry_untrain_ret)
__EXPORT_THUNK(entry_untrain_ret)
SYM_CODE_START(__x86_return_thunk)
UNWIND_HINT_FUNC
ANNOTATE_NOENDBR
ANNOTATE_UNRET_SAFE
ret
int3
SYM_CODE_END(__x86_return_thunk)
EXPORT_SYMBOL(__x86_return_thunk)

11
tools/objtool/arch/x86/decode.c
View File

@ -824,8 +824,11 @@ bool arch_is_retpoline(struct symbol *sym)
bool arch_is_rethunk(struct symbol *sym)
{
return !strcmp(sym->name, "__x86_return_thunk") ||
!strcmp(sym->name, "srso_untrain_ret") ||
!strcmp(sym->name, "srso_safe_ret") ||
!strcmp(sym->name, "__ret");
return !strcmp(sym->name, "__x86_return_thunk");
}
bool arch_is_embedded_insn(struct symbol *sym)
{
return !strcmp(sym->name, "retbleed_return_thunk") ||
!strcmp(sym->name, "srso_safe_ret");
}

45
tools/objtool/check.c
View File

@ -389,7 +389,7 @@ static int decode_instructions(struct objtool_file *file)
if (!strcmp(sec->name, ".noinstr.text") ||
!strcmp(sec->name, ".entry.text") ||
!strcmp(sec->name, ".cpuidle.text") ||
!strncmp(sec->name, ".text.__x86.", 12))
!strncmp(sec->name, ".text..__x86.", 13))
sec->noinstr = true;
/*
@ -455,7 +455,7 @@ static int decode_instructions(struct objtool_file *file)
return -1;
}
if (func->return_thunk || func->alias != func)
if (func->embedded_insn || func->alias != func)
continue;
if (!find_insn(file, sec, func->offset)) {
@ -1288,16 +1288,33 @@ static int add_ignore_alternatives(struct objtool_file *file)
return 0;
}
/*
* Symbols that replace INSN_CALL_DYNAMIC, every (tail) call to such a symbol
* will be added to the .retpoline_sites section.
*/
__weak bool arch_is_retpoline(struct symbol *sym)
{
return false;
}
/*
* Symbols that replace INSN_RETURN, every (tail) call to such a symbol
* will be added to the .return_sites section.
*/
__weak bool arch_is_rethunk(struct symbol *sym)
{
return false;
}
/*
* Symbols that are embedded inside other instructions, because sometimes crazy
* code exists. These are mostly ignored for validation purposes.
*/
__weak bool arch_is_embedded_insn(struct symbol *sym)
{
return false;
}
static struct reloc *insn_reloc(struct objtool_file *file, struct instruction *insn)
{
struct reloc *reloc;
@ -1576,14 +1593,14 @@ static int add_jump_destinations(struct objtool_file *file)
struct symbol *sym = find_symbol_by_offset(dest_sec, dest_off);
/*
* This is a special case for zen_untrain_ret().
* This is a special case for retbleed_untrain_ret().
* It jumps to __x86_return_thunk(), but objtool
* can't find the thunk's starting RET
* instruction, because the RET is also in the
* middle of another instruction. Objtool only
* knows about the outer instruction.
*/
if (sym && sym->return_thunk) {
if (sym && sym->embedded_insn) {
add_return_call(file, insn, false);
continue;
}
@ -2502,6 +2519,9 @@ static int classify_symbols(struct objtool_file *file)
if (arch_is_rethunk(func))
func->return_thunk = true;
if (arch_is_embedded_insn(func))
func->embedded_insn = true;
if (arch_ftrace_match(func->name))
func->fentry = true;
@ -2630,12 +2650,17 @@ static int decode_sections(struct objtool_file *file)
return 0;
}
static bool is_fentry_call(struct instruction *insn)
static bool is_special_call(struct instruction *insn)
{
if (insn->type == INSN_CALL &&
insn_call_dest(insn) &&
insn_call_dest(insn)->fentry)
return true;
if (insn->type == INSN_CALL) {
struct symbol *dest = insn_call_dest(insn);
if (!dest)
return false;
if (dest->fentry || dest->embedded_insn)
return true;
}
return false;
}
@ -3636,7 +3661,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
if (ret)
return ret;
if (opts.stackval && func && !is_fentry_call(insn) &&
if (opts.stackval && func && !is_special_call(insn) &&
!has_valid_stack_frame(&state)) {
WARN_INSN(insn, "call without frame pointer save/setup");
return 1;

1
tools/objtool/include/objtool/arch.h
View File

@ -90,6 +90,7 @@ int arch_decode_hint_reg(u8 sp_reg, int *base);
bool arch_is_retpoline(struct symbol *sym);
bool arch_is_rethunk(struct symbol *sym);
bool arch_is_embedded_insn(struct symbol *sym);
int arch_rewrite_retpolines(struct objtool_file *file);

1
tools/objtool/include/objtool/elf.h
View File

@ -66,6 +66,7 @@ struct symbol {
u8 fentry : 1;
u8 profiling_func : 1;
u8 warned : 1;
u8 embedded_insn : 1;
struct list_head pv_target;
struct reloc *relocs;
};

4
tools/perf/util/thread-stack.c
View File

@ -1038,9 +1038,7 @@ static int thread_stack__trace_end(struct thread_stack *ts,
static bool is_x86_retpoline(const char *name)
{
const char *p = strstr(name, "__x86_indirect_thunk_");
return p == name || !strcmp(name, "__indirect_thunk_start");
return strstr(name, "__x86_indirect_thunk_") == name;
}
/*