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

Brian Gerst
2015-04-27T16:04:46Z

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.

--
Brian Gerst