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SubjectRe: [PATCH -v1.2] kvm/emulate: Fix SETcc emulation function offsets with SLS
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On 3/19/22 14:50, Borislav Petkov wrote:
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 5719d8cfdbd9..f321abb9a4a8 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -429,8 +429,11 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
> FOP_END
>
> /* Special case for SETcc - 1 instruction per cc */
> +
> +#define SETCC_ALIGN (4 * (1 + IS_ENABLED(CONFIG_SLS)))
> +
> #define FOP_SETCC(op) \
> - ".align 4 \n\t" \
> + ".align " __stringify(SETCC_ALIGN) " \n\t" \
> ".type " #op ", @function \n\t" \
> #op ": \n\t" \
> #op " %al \n\t" \
> @@ -1047,7 +1050,7 @@ static int em_bsr_c(struct x86_emulate_ctxt *ctxt)
> static __always_inline u8 test_cc(unsigned int condition, unsigned long flags)
> {
> u8 rc;
> - void (*fop)(void) = (void *)em_setcc + 4 * (condition & 0xf);
> + void (*fop)(void) = (void *)em_setcc + SETCC_ALIGN * (condition & 0xf);
>
> flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
> asm("push %[flags]; popf; " CALL_NOSPEC

So this is what I squashed in:

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f321abb9a4a8..e86d610dc6b7 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -430,7 +430,19 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);

/* Special case for SETcc - 1 instruction per cc */

-#define SETCC_ALIGN (4 * (1 + IS_ENABLED(CONFIG_SLS)))
+/*
+ * 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.
+ */
+#define SETCC_LENGTH (4 + IS_ENABLED(CONFIG_SLS))
+#define SETCC_ALIGN (4 << IS_ENABLED(CONFIG_SLS))
+static_assert(SETCC_LENGTH <= SETCC_ALIGN);

#define FOP_SETCC(op) \
".align " __stringify(SETCC_ALIGN) " \n\t" \
Paolo

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