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Date: Mon, 6 Mar 2017 14:01:07 +0100
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] x86, kasan: add KASAN checks to atomic operations
Message-ID: <20170306130107.GK6536@twins.programming.kicks-ass.net>
References: <20170306124254.77615-1-dvyukov@google.com>
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To: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Ingo Molnar <mingo@redhat.com>, kasan-dev <kasan-dev@googlegroups.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>

On Mon, Mar 06, 2017 at 01:58:51PM +0100, Peter Zijlstra wrote:
> On Mon, Mar 06, 2017 at 01:50:47PM +0100, Dmitry Vyukov wrote:
> > On Mon, Mar 6, 2017 at 1:42 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
> > > KASAN uses compiler instrumentation to intercept all memory accesses.
> > > But it does not see memory accesses done in assembly code.
> > > One notable user of assembly code is atomic operations. Frequently,
> > > for example, an atomic reference decrement is the last access to an
> > > object and a good candidate for a racy use-after-free.
> > >
> > > Add manual KASAN checks to atomic operations.
> > > Note: we need checks only before asm blocks and don't need them
> > > in atomic functions composed of other atomic functions
> > > (e.g. load-cmpxchg loops).
> > 
> > Peter, also pointed me at arch/x86/include/asm/bitops.h. Will add them in v2.
> > 
> 
> > >  static __always_inline void atomic_add(int i, atomic_t *v)
> > >  {
> > > +       kasan_check_write(v, sizeof(*v));
> > >         asm volatile(LOCK_PREFIX "addl %1,%0"
> > >                      : "+m" (v->counter)
> > >                      : "ir" (i));
> 
> 
> So the problem is doing load/stores from asm bits, and GCC
> (traditionally) doesn't try and interpret APP asm bits.
> 
> However, could we not write a GCC plugin that does exactly that?
> Something that interprets the APP asm bits and generates these KASAN
> bits that go with it?

Another suspect is the per-cpu stuff, that's all asm foo as well.

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