On Wed, Aug 03, 2022, Michal Luczaj wrote:
> TRY_ASM() mishandles #UD thrown by the forced-emulation-triggered emulator.
> While the faulting address stored in the exception table points at forced
> emulation prefix, when #UD comes, RIP is 5 bytes (size of KVM_FEP) ahead
> and the exception ends up unhandled.
Ah, but that's only the behavior if FEP is allowed. If FEP is disabled, then the
#UD will be on the prefix.
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Heh, I didn't really suggest this because I didn't even realize it was a problem :-)
> Signed-off-by: Michal Luczaj <mhal@xxxxxxx>
> ---
> While here, I've also took the opportunity to merge both 32 and 64-bit
> versions of ASM_TRY() (.dc.a for .long and .quad), but perhaps there
> were some reasons for not using .dc.a?
This should be a separate patch, and probably as the very last patch in case dc.a
isn't viable for whatever reason. I've never seen/used dc.a so I really have no
idea whether or not it's ok to use.
As a "safe" alternative that can be done before adding ASM_TRY_FEP(), we can steal
the kernel's __ASM_SEL() approach so that you don't have to implement two versions
of ASM_TRY_FEP(). That's worth doing because __ASM_SEL() can probably be used in
other places too. Patch at the bottom.
> lib/x86/desc.h | 11 +++++------
> x86/emulator.c | 4 ++--
> 2 files changed, 7 insertions(+), 8 deletions(-)
>
> diff --git a/lib/x86/desc.h b/lib/x86/desc.h
> index 2a285eb..99cc224 100644
> --- a/lib/x86/desc.h
> +++ b/lib/x86/desc.h
> @@ -80,21 +80,20 @@ typedef struct __attribute__((packed)) {
> u16 iomap_base;
> } tss64_t;
>
> -#ifdef __x86_64
> #define ASM_TRY(catch) \
> "movl $0, %%gs:4 \n\t" \
> ".pushsection .data.ex \n\t" \
> - ".quad 1111f, " catch "\n\t" \
> + ".dc.a 1111f, " catch "\n\t" \
> ".popsection \n\t" \
> "1111:"
> -#else
> -#define ASM_TRY(catch) \
> +
> +#define ASM_TRY_PREFIXED(prefix, catch) \
Rather than a generic ASM_TRY_PREFIXED, I think it makes sense to add an explicit
ASM_TRY_FEP() that takes in only the label and hardcodes the FEP prefix. The "#UD
skips the prefix" behavior is unique to "successful" forced emulation, so this is
literally the only scenario where skipping a prefix is expected/correct.
*sigh*
That'll require moving the KVM_FEP definition, but that's a good change on its
own. Probably throw it in lib/x86/processor.h?
> "movl $0, %%gs:4 \n\t" \
> ".pushsection .data.ex \n\t" \
> - ".long 1111f, " catch "\n\t" \
> + ".dc.a 1111f, " catch "\n\t" \
> ".popsection \n\t" \
> + prefix "\n\t" \
> "1111:"
> -#endif
>
> /*
> * selector 32-bit 64-bit
> diff --git a/x86/emulator.c b/x86/emulator.c
> index df0bc49..d2a5302 100644
> --- a/x86/emulator.c
> +++ b/x86/emulator.c
> @@ -900,8 +900,8 @@ static void test_illegal_lea(void)
> {
> unsigned int vector;
>
> - asm volatile (ASM_TRY("1f")
> - KVM_FEP ".byte 0x8d; .byte 0xc0\n\t"
> + asm volatile (ASM_TRY_PREFIXED(KVM_FEP, "1f")
> + ".byte 0x8d; .byte 0xc0\n\t"
Put this patch earlier in the series, before test_illegal_lea() is introduced.
Both so that there's no unnecessary churn, and also because running the illegal
LEA testcase without this patch will fail.
> "1:"
> : : : "memory", "eax");
---
From: Sean Christopherson <seanjc@xxxxxxxxxx>
Date: Wed, 3 Aug 2022 11:09:41 -0700
Subject: [PATCH] x86: Dedup 32-bit vs. 64-bit ASM_TRY() by stealing kernel's
__ASM_SEL()
Steal the kernel's __ASM_SEL() implementation and use it to consolidate
ASM_TRY(). The only difference between the 32-bit and 64-bit versions is
the size of the address stored in the table.
No functional change intended.
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
lib/x86/desc.h | 19 +++++--------------
lib/x86/processor.h | 12 ++++++++++++
2 files changed, 17 insertions(+), 14 deletions(-)
diff --git a/lib/x86/desc.h b/lib/x86/desc.h
index 2a285eb6..e670ebf2 100644
--- a/lib/x86/desc.h
+++ b/lib/x86/desc.h
@@ -80,21 +80,12 @@ typedef struct __attribute__((packed)) {
u16 iomap_base;
} tss64_t;
-#ifdef __x86_64
-#define ASM_TRY(catch) \
- "movl $0, %%gs:4 \n\t" \
- ".pushsection .data.ex \n\t" \
- ".quad 1111f, " catch "\n\t" \
- ".popsection \n\t" \
+#define ASM_TRY(catch) \
+ "movl $0, %%gs:4 \n\t" \
+ ".pushsection .data.ex \n\t" \
+ __ASM_SEL(.long, .quad) " 1111f, " catch "\n\t" \
+ ".popsection \n\t" \
"1111:"
-#else
-#define ASM_TRY(catch) \
- "movl $0, %%gs:4 \n\t" \
- ".pushsection .data.ex \n\t" \
- ".long 1111f, " catch "\n\t" \
- ".popsection \n\t" \
- "1111:"
-#endif
/*
* selector 32-bit 64-bit
diff --git a/lib/x86/processor.h b/lib/x86/processor.h
index 03242206..30e2de87 100644
--- a/lib/x86/processor.h
+++ b/lib/x86/processor.h
@@ -19,6 +19,18 @@
# define S "4"
#endif
+#ifdef __ASSEMBLY__
+#define __ASM_FORM(x, ...) x,## __VA_ARGS__
+#else
+#define __ASM_FORM(x, ...) " " xstr(x,##__VA_ARGS__) " "
+#endif
+
+#ifndef __x86_64__
+#define __ASM_SEL(a,b) __ASM_FORM(a)
+#else
+#define __ASM_SEL(a,b) __ASM_FORM(b)
+#endif
+
#define DB_VECTOR 1
#define BP_VECTOR 3
#define UD_VECTOR 6
base-commit: a106b30d39425b7afbaa3bbd4aab16fd26d333e7
--
