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x86/kvm: Fix SETcc emulation for return thunks

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commit af2e140f34208a5dfb6b7a8ad2d56bda88f0524d upstream.

Prepare the SETcc fastop stuff for when RET can be larger still.

The tricky bit here is that the expressions should not only be
constant C expressions, but also absolute GAS expressions. This means
no ?: and 'true' is ~0.

Also ensure em_setcc() has the same alignment as the actual FOP_SETCC()
ops, this ensures there cannot be an alignment hole between em_setcc()
and the first op.

Additionally, add a .skip directive to the FOP_SETCC() macro to fill
any remaining space with INT3 traps; however the primary purpose of
this directive is to generate AS warnings when the remaining space
goes negative. Which is a very good indication the alignment magic
went side-ways.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
[cascardo: ignore ENDBR when computing SETCC_LENGTH]
[cascardo: conflict fixup]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Zijlstra 2022-06-14 23:15:42 +02:00 committed by Greg Kroah-Hartman
parent 0bfe8be824
commit 5b43965d58
1 changed files with 14 additions and 12 deletions
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26
arch/x86/kvm/emulate.c
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@ -321,13 +321,15 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
#define FOP_RET(name) \
__FOP_RET(#name)
#define FOP_START(op) \
#define __FOP_START(op, align) \
extern void em_##op(struct fastop *fake); \
asm(".pushsection .text, \"ax\" \n\t" \
".global em_" #op " \n\t" \
".align " __stringify(FASTOP_SIZE) " \n\t" \
".align " __stringify(align) " \n\t" \
"em_" #op ":\n\t"
#define FOP_START(op) __FOP_START(op, FASTOP_SIZE)
#define FOP_END \
".popsection")
@ -431,15 +433,14 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
/*
* Depending on .config the SETcc functions look like:
*
* SETcc %al [3 bytes]
* RET [1 byte]
* INT3 [1 byte; CONFIG_SLS]
*
* Which gives possible sizes 4 or 5. When rounded up to the
* next power-of-two alignment they become 4 or 8.
* SETcc %al [3 bytes]
* RET | JMP __x86_return_thunk [1,5 bytes; CONFIG_RETPOLINE]
* INT3 [1 byte; CONFIG_SLS]
*/
#define SETCC_LENGTH (4 + IS_ENABLED(CONFIG_SLS))
#define SETCC_ALIGN (4 << IS_ENABLED(CONFIG_SLS))
#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETPOLINE)) + \
IS_ENABLED(CONFIG_SLS))
#define SETCC_LENGTH (3 + RET_LENGTH)
#define SETCC_ALIGN (4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))
static_assert(SETCC_LENGTH <= SETCC_ALIGN);
#define FOP_SETCC(op) \
@ -447,13 +448,14 @@ static_assert(SETCC_LENGTH <= SETCC_ALIGN);
".type " #op ", @function \n\t" \
#op ": \n\t" \
#op " %al \n\t" \
__FOP_RET(#op)
__FOP_RET(#op) \
".skip " __stringify(SETCC_ALIGN) " - (.-" #op "), 0xcc \n\t"
asm(".pushsection .fixup, \"ax\"\n"
"kvm_fastop_exception: xor %esi, %esi; " ASM_RET
".popsection");
FOP_START(setcc)
__FOP_START(setcc, SETCC_ALIGN)
FOP_SETCC(seto)
FOP_SETCC(setno)
FOP_SETCC(setc)