Date: Thu, 23 Apr 2015 23:10:15 +0200
From: Borislav Petkov <bp@alien8.de>
To: 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>,
        "H. Peter Anvin" <hpa@zytor.com>, 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
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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.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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