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Date: Tue, 6 Mar 2018 22:21:19 +0100
From: Arnd Bergmann <arnd@...db.de>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>, Daniel Micay <danielmicay@...il.com>,
Ingo Molnar <mingo@...nel.org>, Kees Cook <keescook@...omium.org>,
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Kernel Hardening <kernel-hardening@...ts.openwall.com>, PaX Team <pageexec@...email.hu>,
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Will Deacon <will.deacon@....com>, X86 ML <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()
On Tue, Mar 6, 2018 at 10:01 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Mar 6, 2018 at 12:42 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>
>> Forcing gcc to allocate a stack slot and zero-initialize it should
>> find many bugs by adding valid warnings, but also add lots of
>> false positives as well as prevent important optimizations in other
>> places that are actually well-defined.
>
> Oh, no, the "force gcc to allocate a stack slot" would be absolutely insane.
>
> You should never do that. Anybody who does that should be shot.
>
> But you don't have to force any stack allocation. You should just
> initialize it to zero (*without* the stack allocation).
>
> Then the optimization passes will just remove the initialization in
> 99.9% of all cases. Only very occasionally - when gcc cannot see it
> being overwritten - would it remain. And those are exactly the cases
> where you *want* it to remain.
Right, there are two separate problems: the missing warnings
and the actual uninitialized use. Allocating the stack slots would
address both but only at an enormous cost. Assigning a value
would still have a cost, as it would prevent certain other optimizations,
and it wouldn't help find the missing initializations,
but the cost would obviously be much lower.
Arnd
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