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riscv: Support RANDOMIZE_KSTACK_OFFSET

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Inspired from arm64's implement -- commit 70918779ae
("arm64: entry: Enable random_kstack_offset support")

Add support of kernel stack offset randomization while handling syscall,
the offset is defaultly limited by KSTACK_OFFSET_MAX() (i.e. 10 bits).

In order to avoid trigger stack canaries (due to __builtin_alloca) and
slowing down the entry path, use __no_stack_protector attribute to
disable stack protector for do_trap_ecall_u() at the function level.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Link: https://lore.kernel.org/r/20231109133751.212079-1-songshuaishuai@tinylab.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
This commit is contained in:
Song Shuai 2023-11-09 21:37:51 +08:00 committed by Palmer Dabbelt
parent dded618c07
commit 05d450aabd
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GPG Key ID: 2E1319F35FBB1889
2 changed files with 17 additions and 1 deletions
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1
arch/riscv/Kconfig
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@ -109,6 +109,7 @@ config RISCV
select HAVE_ARCH_KGDB_QXFER_PKT
select HAVE_ARCH_MMAP_RND_BITS if MMU
select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_THREAD_STRUCT_WHITELIST
select HAVE_ARCH_TRACEHOOK

17
arch/riscv/kernel/traps.c
View File

@ -6,6 +6,7 @@
#include <linux/cpu.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/randomize_kstack.h>
#include <linux/sched.h>
#include <linux/sched/debug.h>
#include <linux/sched/signal.h>
@ -310,7 +311,8 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
}
}
asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
asmlinkage __visible __trap_section __no_stack_protector
void do_trap_ecall_u(struct pt_regs *regs)
{
if (user_mode(regs)) {
long syscall = regs->a7;
@ -322,10 +324,23 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
syscall = syscall_enter_from_user_mode(regs, syscall);
add_random_kstack_offset();
if (syscall >= 0 && syscall < NR_syscalls)
syscall_handler(regs, syscall);
else if (syscall != -1)
regs->a0 = -ENOSYS;
/*
* Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
* so the maximum stack offset is 1k bytes (10 bits).
*
* The actual entropy will be further reduced by the compiler when
* applying stack alignment constraints: 16-byte (i.e. 4-bit) aligned
* for RV32I or RV64I.
*
* The resulting 6 bits of entropy is seen in SP[9:4].
*/
choose_random_kstack_offset(get_random_u16());
syscall_exit_to_user_mode(regs);
} else {