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

Andy Lutomirski
2015-04-27T15:56:46Z

On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@alien8.de> 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@alien8.de> 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