Message-ID: <55396617.4020303@zytor.com>
Date: Thu, 23 Apr 2015 14:37:27 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Borislav Petkov <bp@alien8.de>, Denys Vlasenko <vda.linux@googlemail.com>
CC: Andy Lutomirski <luto@amacapital.net>,
        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>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary
References: <1429792491-5978-1-git-send-email-dvlasenk@redhat.com> <CA+55aFwDP1QxvHs9wKE9ZpjAzSHhzc+YQS1Mhwt=oYLrB+Rp1A@mail.gmail.com> <CAMzpN2j3K9B4UDPB8suz15PjoDY2aZBr8bvEZ5r4ag5FLgCVrw@mail.gmail.com> <CA+55aFxre3Jk-CG+mBs6Bp9a1J=riCQaA3dwUTFRph1OsCMSuA@mail.gmail.com> <CALCETrX_RCGH0C4GJ0Zxm1emcasMbZ5s29HCfFy=GrEC9w28+A@mail.gmail.com> <CAK1hOcPp3Xav4iR2mZ4oq4zB5XY+ZNuEds32Rf0Z3fB8A7h1PQ@mail.gmail.com> <20150423211015.GS28327@pd.tnic>
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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.

	-hpa

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