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

[Andy Lutomirski]

On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Mon, Apr 27, 2015 at 07:57:36AM -0700, Linus Torvalds wrote:
>> On Mon, Apr 27, 2015 at 4:35 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>> >
>> >         /*
>> >          * 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
>>
>> Guys, if we're looking at cycles for this, then don't do the "exact
>> canonical test". and go back to just doing
>>
>>         shr $__VIRTUAL_MASK_SHIFT, %rcx
>>         jnz opportunistic_sysret_failed
>>
>> which is much smaller.
>
> Right, what about the false positives:
>
> 17be0aec74fb ("x86/asm/entry/64: Implement better check for canonical addresses")
>
> ? We don't care?

The false positives only matter for very strange workloads, e.g.
vsyscall=native with old libc.  If it's a measurable regression, we
could revert it.

--Andy
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