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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 23 Apr 2015 15:29:27 -0700
Message-ID: <CALCETrVX_O+vu+GJXJKJa7w83DnKn0nrjp2_9xtNdhEJW79XQA@mail.gmail.com>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Borislav Petkov <bp@alien8.de>, Denys Vlasenko <vda.linux@googlemail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Apr 23, 2015 at 2:37 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> On 04/23/2015 02:10 PM, Borislav Petkov wrote:
>> On Thu, Apr 23, 2015 at 10:01:16PM +0200, Denys Vlasenko wrote:
>>> Naturally, CS can't be NULL, and up until today
>>> I thought SS also can't. But the bit is probably implemented
>>> for all eight cached descriptors.
>>
>> There's this section about NULL selector in APM v2. It says that NULL
>> selectors are used to invalidate segment registers and software can load
>> a NULL selector in SS in CPL0.
>>
>> So, if an interrupt happens and as you quoted earlier that SS gets set
>> to NULL as a result of an interrupt, there's that SS leak causing the SS
>> exception.
>>
>
> Yes, the NULL SS is a special thing in 64-bit mode.  I agree that
> context-switching it is probably the way to go; it should be cheap
> enough.  We might even be able to conditionalize it on an X86_BUG_ flag.

I still don't see why context switches are a better place than just
before sysret, but I could be convinced.

I updated my test at
https://git.kernel.org/cgit/linux/kernel/git/luto/misc-tests.git/.  I
want to figure out whether this is a problem for sysretq, too.

--Andy
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