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

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Date:	Mon, 27 Apr 2015 12:04:44 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Mon, Apr 27, 2015 at 11:56 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Apr 27, 2015 at 8:46 AM, Borislav Petkov <bp@...en8.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@...en8.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
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