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Message-ID: <CAJcbSZG6F3DsiBMjizTbzaccvU5_RKdspU06wQ8yi+JT+EW2Jw@mail.gmail.com>
Date: Tue, 14 Mar 2017 08:17:08 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: 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,
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 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.
>
> -hpa
>
>
--
Thomas
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