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Message-ID: <CALCETrVVo4aQosdjfUPBf=JWyVWE_cHavbBKX5Swv_HbV__RNw@mail.gmail.com>
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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