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Date: Tue, 14 Mar 2017 08:39:09 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>, 
	Martin Schwidefsky <schwidefsky@...ibm.com>, Heiko Carstens <heiko.carstens@...ibm.com>, 
	David Howells <dhowells@...hat.com>, Arnd Bergmann <arnd@...db.de>, Al Viro <viro@...iv.linux.org.uk>, 
	Dave Hansen <dave.hansen@...el.com>, René Nyffenegger <mail@...enyffenegger.ch>, 
	Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, 
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Andy Lutomirski <luto@...nel.org>, 
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Nicolas Pitre <nicolas.pitre@...aro.org>, 
	Petr Mladek <pmladek@...e.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Helge Deller <deller@....de>, 
	Rik van Riel <riel@...hat.com>, John Stultz <john.stultz@...aro.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Oleg Nesterov <oleg@...hat.com>, 
	Stephen Smalley <sds@...ho.nsa.gov>, Pavel Tikhomirov <ptikhomirov@...tuozzo.com>, 
	Frederic Weisbecker <fweisbec@...il.com>, Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>, 
	Ingo Molnar <mingo@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Dmitry Safonov <dsafonov@...tuozzo.com>, Borislav Petkov <bp@...en8.de>, 
	Josh Poimboeuf <jpoimboe@...hat.com>, Brian Gerst <brgerst@...il.com>, Jan Beulich <JBeulich@...e.com>, 
	Christian Borntraeger <borntraeger@...ibm.com>, Fenghua Yu <fenghua.yu@...el.com>, 
	He Chen <he.chen@...ux.intel.com>, Russell King <linux@...linux.org.uk>, 
	Vladimir Murzin <vladimir.murzin@....com>, Will Deacon <will.deacon@....com>, 
	Catalin Marinas <catalin.marinas@....com>, Mark Rutland <mark.rutland@....com>, 
	James Morse <james.morse@....com>, "David A . Long" <dave.long@...aro.org>, 
	Pratyush Anand <panand@...hat.com>, Laura Abbott <labbott@...hat.com>, 
	Andre Przywara <andre.przywara@....com>, Chris Metcalf <cmetcalf@...lanox.com>, 
	linux-s390 <linux-s390@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Linux API <linux-api@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 2/4] x86/syscalls: Specific usage of verify_pre_usermode_state

On Tue, Mar 14, 2017 at 8:17 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
> On Tue, Mar 14, 2017 at 2:40 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 03/13/17 17:04, H. Peter Anvin wrote:
>>> On 03/11/17 01:42, Ingo Molnar wrote:
>>>>>
>>>>> +   /*
>>>>> +    * Check user-mode state on fast path return, the same check is done
>>>>> +    * under the slow path through syscall_return_slowpath.
>>>>> +    */
>>>>> +#ifdef CONFIG_BUG_ON_DATA_CORRUPTION
>>>>> +   call    verify_pre_usermode_state
>>>>> +#else
>>>>> +   /*
>>>>> +    * Similar to set_fs(USER_DS) in verify_pre_usermode_state without a
>>>>> +    * warning.
>>>>> +    */
>>>>> +   movq    PER_CPU_VAR(current_task), %rax
>>>>> +   movq    $TASK_SIZE_MAX, %rcx
>>>>> +   cmp     %rcx, TASK_addr_limit(%rax)
>>>>> +   jz      1f
>>>>> +   movq    %rcx, TASK_addr_limit(%rax)
>>>>> +1:
>>>>> +#endif
>>>>> +
>>>
>>> How about simply doing...
>>>
>>>       movq    PER_CPU_VAR(current_task), %rax
>>>       movq    $TASK_SIZE_MAX, %rcx
>>> #ifdef CONFIG_BUG_ON_DATA_CORRUPTION
>>>       cmpq    %rcx, TASK_addr_limit(%rax)
>>>       jne     syscall_return_slowpath
>>> #else
>>>       movq    %rcx, TASK_addr_limit(%rax)
>>> #endif
>>>
>>> ... and let the slow path take care of BUG.  This should be much faster,
>>> even with the BUG, and is simpler to boot.
>>>
>>
>> In fact, we could even to the cmpq/jne unconditionally.  I'm guessing
>> the occasional branch mispredict will be offset by occasionally touching
>> a clean cacheline in the case of an unconditional store.
>>
>> Since this is something that should never happen, performance doesn't
>> matter.
>
> Ingo: Which approach do you favor? I want to keep the fast path as
> fast as possible obviously.

Even though my name isn't Ingo, Linus keeps trying to get me to be the
actual maintainer of this file.  :)  How about (sorry about whitespace
damage):

#ifdef CONFIG_BUG_ON_DATA_CORRUPTION
       movq    PER_CPU_VAR(current_task), %rax
       bt $63, TASK_addr_limit(%rax)
       jc     syscall_return_slowpath
#endif

Now the kernel is totally unchanged if the config option is off and
it's fast and simple if the option is on.

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