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Date: Fri, 24 Apr 2015 13:33:01 -0400
Message-ID: <CAMzpN2gDdENpa0vXrCmgvkznFiNzm=z7oPybtCZt76O6j6ZqVg@mail.gmail.com>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue
From: Brian Gerst <brgerst@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>, Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Borislav Petkov <bp@alien8.de>,
        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>
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On Fri, Apr 24, 2015 at 12:25 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Apr 24, 2015 at 5:00 AM, Brian Gerst <brgerst@gmail.com> wrote:
>>
>> So actually this isn't a preemption issue, as the NULL SS is coming
>> from an interrupt from userspace (timer tick, etc.).
>
> It *is* a preemption issue, in the sense that the interrupt that
> clears SS also then returns to user space using an "iret" that will
> properly restore it.
>
> So the only case we need to worry about is the preemption case, where
> the interrupt has caused a task switch (typically because it woke
> something up or it was the timer interrupt and the timeslice of the
> previous task is up), and we switch to another context that returns to
> user space using "sysret" instead.

To clarify, I was thinking of the CONFIG_PREEMPT case.  A nested
interrupt wouldn't change SS, and IST interrupts can't schedule.

--
Brian Gerst
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