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

[Brian Gerst]

On Mon, Apr 27, 2015 at 11:56 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 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

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
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