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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 27 Apr 2015 08:56:22 -0700
Message-ID: <CALCETrVKSfontY9U9Ci_Q1Wizyp5GSXZv10Vaa9N-pcPonweoA@mail.gmail.com>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue
To: Borislav Petkov <bp@alien8.de>
Cc: 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>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.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>
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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
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