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Message-ID: <7389c6e7-87dc-ea0d-5b2a-7925b8c8d33e@zytor.com>
Date: Tue, 14 Mar 2017 09:44:56 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Thomas Garnier <thgarnie@...gle.com>,
Andy Lutomirski <luto@...capital.net>
Cc: Ingo Molnar <mingo@...nel.org>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
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René Nyffenegger <mail@...enyffenegger.ch>,
Andrew Morton <akpm@...ux-foundation.org>,
Kees Cook
<keescook@...omium.org>,
"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
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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 03/14/17 09:29, Thomas Garnier wrote:
>
> 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.
I wanted to comment on that thing: why on earth isn't
verify_pre_usermode_state() an inline? Making it an out-of-line
function adds pointless extra overhead to the C code when we are talking
about a few instructions.
Second, you never do a branch-around to handle an exceptional condition
on the fast path: you jump *out of line* to handle the special
condition; a forward branch is preferred since it is slightly more
likely to be predicted not taken.
Now, I finally had a chance to actually look at the full file (I was
preoccupied yesterday), and am a bit disappointed, to say the least.
First of all, the jump target you need is only a handful of instructions
further down the code path; you need to do *exactly* that is done when
the test of _TIF_ALLWORK_MASK right above is tested! Not only that, but
you already have PER_CPU_VAR(current_task) in %r11 just ready to be
used! This was all in the three instructions immediately prior to the
code you modified...
So, all you'd need would be:
movq $TASK_SIZE_MAX, %rcx
cmpq %rcx, TASK_addr_limit(%r11)
jne 1f
We even get a short jump instruction!
(Using bt saves one more instruction, but see previous caveats about it.)
-hpa
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