linux-kernel - Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue

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Message-ID: <CALCETrVjcw9TAKm+GGzJtzUUPpovJYZMn4vXc=cYG+s5B57h5Q@mail.gmail.com>
Date:	Fri, 24 Apr 2015 13:50:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	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>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64, asm: Work around AMD SYSRET SS descriptor
 attribute issue

On Fri, Apr 24, 2015 at 1:46 PM, Denys Vlasenko
<vda.linux@...glemail.com> wrote:
> On Fri, Apr 24, 2015 at 10:21 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Thu, Apr 23, 2015 at 7:15 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>> AMD CPUs don't reinitialize the SS descriptor on SYSRET, so SYSRET
>>> with SS == 0 results in an invalid usermode state in which SS is
>>> apparently equal to __USER_DS but causes #SS if used.
>>>
>>> Work around the issue by replacing NULL SS values with __KERNEL_DS
>>> in __switch_to, thus ensuring that SYSRET never happens with SS set
>>> to NULL.
>>>
>>> This was exposed by a recent vDSO cleanup.
>>>
>>> Fixes: e7d6eefaaa44 x86/vdso32/syscall.S: Do not load __USER32_DS to %ss
>>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>>> ---
>>>
>>> Tested only on Intel, which isn't very interesting.  I'll tidy up
>>> and send a test case, too, once Borislav confirms that it works.
>>>
>>> Please don't actually apply this until we're sure we understand the
>>> scope of the issue.  If this doesn't affect SYSRETQ, then we might
>>> to fix it on before SYSRETL to avoid impacting 64-bit processes
>>> at all.
>>>
>>
>> After sleeping on it, I think I want to offer a different, more
>> complicated approach.  AFAIK there are really only two ways that this
>> issue can be visible:
>>
>> 1. SYSRETL.  We can fix that up in the AMD SYSRETL path.  I think
>> there's a decent argument that that path is less performance-critical
>> than context switches.
>>
>> 2. SYSRETQ.  The only way that I know of to see the problem is SYSRETQ
>> followed by a far jump or return.  This is presumably *extremely*
>> rare.
>>
>> What if we fixed #2 up in do_stack_segment.  We should double-check
>> the docs, but I think that this will only ever manifest as #SS(0) with
>> regs->ss == __USER_DS and !user_mode_64bit(regs).  We need to avoid
>> infinite retry looks, but this might be okay.  I think that #SS(0)
>> from userspace under those conditions can *only* happen as a result of
>> this issue.  Even if not, we could come up with a way to only retry
>> once per syscall (e.g. set some ti->status flag in the 64-bit syscall
>> path on AMD and clear it in do_stack_segment).
>>
>> This might be way more trouble than it's worth.
>
> Exactly my feeling. What are you trying to save? About four CPU
> cycles of checking %ss != __KERNEL_DS on each switch_to?
> That's not worth bothering about. Your last patch seems to be perfect.

We'll have to do the write to ss almost every time an AMD CPU sleeps
in a syscall.  Maybe that's still not a big deal.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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