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Date: Tue, 14 Mar 2017 09:29:28 -0700
From: Thomas Garnier <thgarnie@...gle.com>
To: Andy Lutomirski <luto@...capital.net>
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>,
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"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 8:39 AM, Andy Lutomirski <luto@...capital.net> wrote:
> 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):
:D
>
> #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.
I like using bt for fast comparison.
We want to enforce the address limit by default, not only when
CONFIG_BUG_ON_DATA_CORRUPTION is enabled. I tested this one:
/* Check user-mode state on fast path return. */
movq PER_CPU_VAR(current_task), %rax
btq $63, TASK_addr_limit(%rax)
jnc 1f
#ifdef CONFIG_BUG_ON_DATA_CORRUPTION
call syscall_return_slowpath
jmp return_from_SYSCALL_64
#else
movq $TASK_SIZE_MAX, %rcx
movq %rcx, TASK_addr_limit(%rax)
#endif
1:
I saw that syscall_return_slowpath is supposed to be called not jumped
to. I could just call verify_pre_usermode_state that would be about
the same.
If we want to avoid if/def then I guess this one is the best I can think of:
/* Check user-mode state on fast path return. */
movq PER_CPU_VAR(current_task), %rax
btq $63, TASK_addr_limit(%rax)
jnc 1f
call verify_pre_usermode_state
1:
The check is fast and the call will happen only on corruption.
What do you think?
--
Thomas
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