Re: [PATCH v6 0/4] x86, kasan: add KASAN checks to atomic operations

Ingo Molnar
2018-01-31T07:29:03Z

* Will Deacon <will.deacon@arm.com> wrote:

> Hi Dmitry,
>
> On Mon, Jan 29, 2018 at 06:26:03PM +0100, Dmitry Vyukov 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.
> >
> > Atomic operations are defined in arch files, but KASAN instrumentation
> > is required for several archs that support KASAN. Later we will need
> > similar hooks for KMSAN (uninit use detector) and KTSAN (data race
> > detector).
> >
> > This change introduces wrappers around atomic operations that can be
> > used to add KASAN/KMSAN/KTSAN instrumentation across several archs,
> > and adds KASAN checks to them.
> >
> > This patch uses the wrappers only for x86 arch. Arm64 will be switched
> > later. And we also plan to instrument bitops in a similar way.
>
> One way you could reduce the intrusivness for each architecture would be
> to leave the existing macro names as-is, and redefine them in the
> asm-generic header. It's certainly ugly, but it makes the porting work
> a lot smaller. Apologies if you've considered this approach before, but
> I figured it was worth mentioning just in case.
>
> e.g. for atomic[64]_read, your asm-generic header looks like:
>
> #ifndef _LINUX_ATOMIC_INSTRUMENTED_H
> #define _LINUX_ATOMIC_INSTRUMENTED_H
>
> #include <linux/build_bug.h>
> #include <linux/kasan-checks.h>
>
> static __always_inline int __atomic_read_instrumented(const atomic_t *v)
> {
> kasan_check_read(v, sizeof(*v));
> return atomic_read(v);
> }
>
> static __always_inline s64 __atomic64_read_instrumented(const atomic64_t *v)
> {
> kasan_check_read(v, sizeof(*v));
> return atomic64_read(v);
> }
>
> #undef atomic_read
> #undef atomic64_read
>
> #define atomic_read __atomic_read_instrumented
> #define atomic64_read __atomic64_read_instrumented
>
> #endif /* _LINUX_ATOMIC_INSTRUMENTED_H */
>
> and the arch code just includes that in asm/atomic.h once it's done with
> its definitions.
>
> What do you think? Too stinky?

Hm, so while this could work - I actually *like* the low level changes: they are
straightforward, trivial, easy to read and they add the arch_ prefix that makes it
abundantly clear that this isn't the highest level interface.

The KASAN callbacks in the generic methods are also abundantly clear and very easy
to read. I could literally verify the sanity of the series while still being only
half awake. ;-)

Also note that the arch renaming should be 'trivial', in the sense that any
missing rename results in a clear build breakage. Plus any architecture making use
of this new KASAN feature should probably be tested before it's enabled - and the
renaming of the low level atomic APIs kind of forces that too.

So while this approach creates some churn, this series is IMHO a marked
improvement over the previous iterations.

Thanks,

Ingo