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

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Message-ID: <CALCETrVX_O+vu+GJXJKJa7w83DnKn0nrjp2_9xtNdhEJW79XQA@mail.gmail.com>
Date:	Thu, 23 Apr 2015 15:29:27 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	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>,
	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 2:37 PM, H. Peter Anvin <hpa@...or.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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