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SubjectRe: [PATCH v2 1/2] x86/retpoline: Simplify vmexit_fill_RSB()
On Fri, Jan 26, 2018 at 09:07:25PM +0100, Borislav Petkov wrote:
> +.macro FILL_RETURN_BUFFER nr:req ftr:req
> #ifdef CONFIG_RETPOLINE
> + ALTERNATIVE "", "call __clear_rsb", \ftr
> #endif
> .endm
>
> @@ -206,15 +174,10 @@ extern char __indirect_thunk_end[];
> static inline void vmexit_fill_RSB(void)
> {
> #ifdef CONFIG_RETPOLINE
> + alternative_input("",
> + "call __fill_rsb",
> + X86_FEATURE_RETPOLINE,
> + ASM_NO_INPUT_CLOBBER("memory"));
> #endif
> }
>


> @@ -19,6 +20,37 @@ ENDPROC(__x86_indirect_thunk_\reg)
> .endm
>
> /*
> + * Google experimented with loop-unrolling and this turned out to be
> + * the optimal version — two calls, each with their own speculation
> + * trap should their return address end up getting used, in a loop.
> + */
> +.macro BOINK_RSB nr:req sp:req
> + push %_ASM_AX
> + mov $(\nr / 2), %_ASM_AX
> + .align 16
> +771:
> + call 772f
> +773: /* speculation trap */
> + pause
> + lfence
> + jmp 773b
> + .align 16
> +772:
> + call 774f
> +775: /* speculation trap */
> + pause
> + lfence
> + jmp 775b
> + .align 16
> +774:
> + dec %_ASM_AX
> + jnz 771b
> + add $((BITS_PER_LONG/8) * \nr), \sp
> + pop %_ASM_AX
> +.endm
> +
> +
> +/*
> * Despite being an assembler file we can't just use .irp here
> * because __KSYM_DEPS__ only uses the C preprocessor and would
> * only see one instance of "__x86_indirect_thunk_\reg" rather
> @@ -46,3 +78,15 @@ GENERATE_THUNK(r13)
> GENERATE_THUNK(r14)
> GENERATE_THUNK(r15)
> #endif
> +
> +ENTRY(__fill_rsb)
> + BOINK_RSB RSB_FILL_LOOPS, %_ASM_SP
> + ret
> +END(__fill_rsb)
> +EXPORT_SYMBOL_GPL(__fill_rsb)
> +
> +ENTRY(__clear_rsb)
> + BOINK_RSB RSB_CLEAR_LOOPS, %_ASM_SP
> + ret
> +END(__clear_rsb)
> +EXPORT_SYMBOL_GPL(__clear_rsb)


One thing I feel this ought to mention (in the Changelog probably) is
that it looses one RET for SKL+. That is, where we used to have 16
'safe' RETs before this, we now have 15.

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