Re: [PATCH] x86/retpoline: Avoid return buffer underflows on context switch

["Woodhouse, David" <dwmw@amazon.co.uk>]

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On Mon, 2018-01-08 at 16:15 -0800, Paul Turner wrote:
> On Mon, Jan 8, 2018 at 2:11 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, Jan 08, 2018 at 12:15:31PM -0800, Andi Kleen wrote:
> >> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> >> index b8c8eeacb4be..e84e231248c2 100644
> >> --- a/arch/x86/include/asm/nospec-branch.h
> >> +++ b/arch/x86/include/asm/nospec-branch.h
> >> @@ -53,6 +53,35 @@
> >>  #endif
> >>  .endm
> >>
> >> +/*
> >> + * We use 32-N: 32 is the max return buffer size,
> >> + * but there should have been at a minimum two
> >> + * controlled calls already: one into the kernel
> >> + * from entry*.S and another into the function
> >> + * containing this macro. So N=2, thus 30.
> >> + */
> >> +#define NUM_BRANCHES_TO_FILL 30
> >> +
> >> +/*
> >> + * Fill the CPU return branch buffer to prevent
> >> + * indirect branch prediction on underflow.
> >> + * Caller should check for X86_FEATURE_SMEP and X86_FEATURE_RETPOLINE
> >> + */
> >> +.macro FILL_RETURN_BUFFER
> >> +#ifdef CONFIG_RETPOLINE
> >> +     .rept   NUM_BRANCHES_TO_FILL
> >> +     call    1221f
> >> +     pause   /* stop speculation */
> >> +1221:
> >> +     .endr
> >> +#ifdef CONFIG_64BIT
> >> +     addq    $8*NUM_BRANCHES_TO_FILL, %rsp
> >> +#else
> >> +     addl    $4*NUM_BRANCHES_TO_FILL, %esp
> >> +#endif
> >> +#endif
> >> +.endm
> >
> > So pjt did alignment, a single unroll and per discussion earlier today
> > (CET) or late last night (PST), he only does 16.
> >
> > Why is none of that done here? Also, can we pretty please stop using
> > those retarded number labels, they make this stuff unreadable.
> >
> > Also, pause is unlikely to stop speculation, that comment doesn't make
> > sense. Looking at PJT's version there used to be a speculation trap in
> > there, but I can't see that here.
> >
> 
> You definitely want the speculation traps.. these entries are
> potentially consumed.
> Worse: The first entry that will be consumed is the last call in your
> linear chain, meaning that it immediately gets to escape into
> alternative execution.
> (When I was experimenting with icache-minimizing constructions here I
> actually used intentional backwards jumps in linear chains to avoid
> this.)
> 
> The sequence I reported is what ended up seeming optimal.

On IRC, Arjan assures me that 'pause' here really is sufficient as a
speculation trap. If we do end up returning back here as a
misprediction, that 'pause' will stop the speculative execution on
affected CPUs even though it isn't *architecturally* documented to do
so.

Arjan, can you confirm that in email please?

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