Re: [PATCH 1/5] include/linux: Add instrumented.h infrastructure

[Marco Elver <elver@google.com>]

On Mon, 20 Jan 2020 at 15:34, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, Jan 20, 2020 at 3:19 PM Marco Elver <elver@google.com> wrote:
> >
> > This adds instrumented.h, which provides generic wrappers for memory
> > access instrumentation that the compiler cannot emit for various
> > sanitizers. Currently this unifies KASAN and KCSAN instrumentation. In
> > future this will also include KMSAN instrumentation.
> >
> > Note that, copy_{to,from}_user require special instrumentation,
> > providing hooks before and after the access, since we may need to know
> > the actual bytes accessed (currently this is relevant for KCSAN, and is
> > also relevant in future for KMSAN).
>
> How will KMSAN instrumentation look like?
>
> > Suggested-by: Arnd Bergmann <arnd@arndb.de>
> > Signed-off-by: Marco Elver <elver@google.com>
> > ---
> >  include/linux/instrumented.h | 153 +++++++++++++++++++++++++++++++++++
> >  1 file changed, 153 insertions(+)
> >  create mode 100644 include/linux/instrumented.h
> >
> > diff --git a/include/linux/instrumented.h b/include/linux/instrumented.h
> > new file mode 100644
> > index 000000000000..9f83c8520223
> > --- /dev/null
> > +++ b/include/linux/instrumented.h
> > @@ -0,0 +1,153 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +/*
> > + * This header provides generic wrappers for memory access instrumentation that
> > + * the compiler cannot emit for: KASAN, KCSAN.
> > + */
> > +#ifndef _LINUX_INSTRUMENTED_H
> > +#define _LINUX_INSTRUMENTED_H
> > +
> > +#include <linux/compiler.h>
> > +#include <linux/kasan-checks.h>
> > +#include <linux/kcsan-checks.h>
> > +#include <linux/types.h>
> > +
> > +/**
> > + * instrument_read - instrument regular read access
> > + *
> > + * Instrument a regular read access. The instrumentation should be inserted
> > + * before the actual read happens.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +static __always_inline void instrument_read(const volatile void *v, size_t size)
> > +{
> > +       kasan_check_read(v, size);
> > +       kcsan_check_read(v, size);
> > +}
> > +
> > +/**
> > + * instrument_write - instrument regular write access
> > + *
> > + * Instrument a regular write access. The instrumentation should be inserted
> > + * before the actual write happens.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +static __always_inline void instrument_write(const volatile void *v, size_t size)
> > +{
> > +       kasan_check_write(v, size);
> > +       kcsan_check_write(v, size);
> > +}
> > +
> > +/**
> > + * instrument_atomic_read - instrument atomic read access
> > + *
> > + * Instrument an atomic read access. The instrumentation should be inserted
> > + * before the actual read happens.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +static __always_inline void instrument_atomic_read(const volatile void *v, size_t size)
> > +{
> > +       kasan_check_read(v, size);
> > +       kcsan_check_atomic_read(v, size);
> > +}
> > +
> > +/**
> > + * instrument_atomic_write - instrument atomic write access
> > + *
> > + * Instrument an atomic write access. The instrumentation should be inserted
> > + * before the actual write happens.
> > + *
> > + * @ptr address of access
> > + * @size size of access
> > + */
> > +static __always_inline void instrument_atomic_write(const volatile void *v, size_t size)
> > +{
> > +       kasan_check_write(v, size);
> > +       kcsan_check_atomic_write(v, size);
> > +}
> > +
> > +/**
> > + * instrument_copy_to_user_pre - instrument reads of copy_to_user
> > + *
> > + * Instrument reads from kernel memory, that are due to copy_to_user (and
> > + * variants).
> > + *
> > + * The instrumentation must be inserted before the accesses. At this point the
> > + * actual number of bytes accessed is not yet known.
> > + *
> > + * @dst destination address
> > + * @size maximum access size
> > + */
> > +static __always_inline void
> > +instrument_copy_to_user_pre(const volatile void *src, size_t size)
> > +{
> > +       /* Check before, to warn before potential memory corruption. */
> > +       kasan_check_read(src, size);
> > +}
> > +
> > +/**
> > + * instrument_copy_to_user_post - instrument reads of copy_to_user
> > + *
> > + * Instrument reads from kernel memory, that are due to copy_to_user (and
> > + * variants).
> > + *
> > + * The instrumentation must be inserted after the accesses. At this point the
> > + * actual number of bytes accessed should be known.
> > + *
> > + * @dst destination address
> > + * @size maximum access size
> > + * @left number of bytes left that were not copied
> > + */
> > +static __always_inline void
> > +instrument_copy_to_user_post(const volatile void *src, size_t size, size_t left)
> > +{
> > +       /* Check after, to avoid false positive if memory was not accessed. */
> > +       kcsan_check_read(src, size - left);
>
> Why don't we check the full range?
> Kernel intending to copy something racy to user already looks like a
> bug to me, even if user-space has that page unmapped. User-space can
> always make the full range succeed. What am I missing?

Fair enough. I can move this into the pre-hooks in v2.

However, note that, that leaves us with a bunch of empty post-hooks in
the patch. While this will probably change when we get KMSAN, is it
reasonable to keep them empty for now?

Thanks,
-- Marco