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Date: Mon, 27 Apr 2015 18:10:17 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Brian Gerst <brgerst@gmail.com>, Andy Lutomirski <luto@amacapital.net>
CC: Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Denys Vlasenko <vda.linux@googlemail.com>,
        Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute
 issue
References: <5d120f358612d73fc909f5bfa47e7bd082db0af0.1429841474.git.luto@kernel.org>	<20150425211206.GE32099@pd.tnic>	<CALCETrWvqyCYOzCYXz7ZnzaM0obbidqo5CxVp2Hn1ELEiC3m3g@mail.gmail.com>	<20150427085305.GB6774@pd.tnic>	<20150427113506.GG6774@pd.tnic>	<CA+55aFykNBrdHFSVeLftG7Ujx4Tk=MuUJXD2rNYn9kwL+kLN5w@mail.gmail.com>	<20150427154631.GB28871@pd.tnic>	<CALCETrVKSfontY9U9Ci_Q1Wizyp5GSXZv10Vaa9N-pcPonweoA@mail.gmail.com> <CAMzpN2j0oOJ7W1c3QBdMXUv5zSQHiZeb9Ynko1=JxUr=WRKZFQ@mail.gmail.com>
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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.