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

Denys Vlasenko
2015-04-27T16:10:59Z

On 04/27/2015 06:04 PM, Brian Gerst wrote:
> On Mon, Apr 27, 2015 at 11:56 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>> 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
>
> Another alternative is to do the canonical check in the paths that can
> set user RIP with an untrusted value, ie, sigreturn and exec.

It is already done only on that path. Fast path doesn't check
RCX for canonicalness.