Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

Denys Vlasenko
2015-04-27T12:08:48Z

On 04/27/2015 01:35 PM, Borislav Petkov wrote:
> On Mon, Apr 27, 2015 at 10:53:05AM +0200, Borislav Petkov wrote:
>> ALTERNATIVE "",
>> "shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \
>> sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx \
>> cmpq %rcx, %r11 \
>> jne opportunistic_sysret_failed"
>> X86_BUG_SYSRET_CANONICAL_RCX
>
> Right, so I can do this:
>
> /*
> * Change top 16 bits to be the sign-extension of 47th bit, if this
> * changed %rcx, it was not canonical.
> */
> ALTERNATIVE "", \
> "shl $(64 - (47+1)), %rcx; \
> sar $(64 - (47+1)), %rcx; \
> cmpq %rcx, %r11; \
> jne opportunistic_sysret_failed", X86_BUG_SYSRET_CANON_RCX
>
> If I use the __VIRTUAL_MASK_SHIFT macro *in* the ALTERNATIVE macro, I get some
> really cryptic gas error:
>
> arch/x86/kernel/entry_64.S: Assembler messages:
> arch/x86/kernel/entry_64.S:441: Error: can't resolve `L0' {*ABS* section} - `L0' {*UND* section}
> scripts/Makefile.build:294: recipe for target 'arch/x86/kernel/entry_64.o' failed
> make[1]: *** [arch/x86/kernel/entry_64.o] Error 1
> Makefile:1536: recipe for target 'arch/x86/kernel/entry_64.o' failed
> make: *** [arch/x86/kernel/entry_64.o] Error 2
>
> but I guess we can simply use the naked "47" because a couple of lines
> above, we already have the sanity-check:
>
> .ifne __VIRTUAL_MASK_SHIFT - 47
> .error "virtual address width changed -- SYSRET checks need update"
> .endif
>
> so we should be guarded just fine.
>
> Anyway, if we do it this way, we get 17 NOPs added at build time which is the
> length of the 4 instructions:
>
> ffffffff819ef40c: 48 c1 e1 10 shl $0x10,%rcx
> ffffffff819ef410: 48 c1 f9 10 sar $0x10,%rcx
> ffffffff819ef414: 49 39 cb cmp %rcx,%r11
> ffffffff819ef417: 0f 85 ff 9c bc ff jne ffffffff815b911c <opportunistic_sysret_failed>

This looks strange. opportunistic_sysret_failed label is just a few
instructions below. Why are you getting "ff 9c bc ff" offset in JNE
instead of short jump of 0x5f bytes I see without ALTERNATIVE?