linux-kernel - Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

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Message-ID: <20150423211015.GS28327@pd.tnic>
Date:	Thu, 23 Apr 2015 23:10:15 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/32: Restore %ss before SYSRETL if necessary

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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