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From:   Alexander Potapenko <glider@google.com>
Date:   Wed, 3 Aug 2022 13:17:55 +0200
Message-ID: <CAG_fn=W0bfUZBc-tDMFgeEzgZgezse+mOjQnoO3vBALm9+1Q3w@mail.gmail.com>
Subject: Re: [PATCH v4 18/45] instrumented.h: add KMSAN support
To:     Marco Elver <elver@google.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Christoph Hellwig <hch@lst.de>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kees Cook <keescook@chromium.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Vegard Nossum <vegard.nossum@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux-Arch <linux-arch@vger.kernel.org>,
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On Tue, Jul 12, 2022 at 3:52 PM Marco Elver <elver@google.com> wrote:
>
> On Fri, 1 Jul 2022 at 16:24, Alexander Potapenko <glider@google.com> wrot=
e:
> >
> > To avoid false positives, KMSAN needs to unpoison the data copied from
> > the userspace. To detect infoleaks - check the memory buffer passed to
> > copy_to_user().
> >
> > Signed-off-by: Alexander Potapenko <glider@google.com>
>
> Reviewed-by: Marco Elver <elver@google.com>
>
> With the code simplification below.
>
> [...]
> > --- a/mm/kmsan/hooks.c
> > +++ b/mm/kmsan/hooks.c
> > @@ -212,6 +212,44 @@ void kmsan_iounmap_page_range(unsigned long start,=
 unsigned long end)
> >  }
> >  EXPORT_SYMBOL(kmsan_iounmap_page_range);
> >
> > +void kmsan_copy_to_user(void __user *to, const void *from, size_t to_c=
opy,
> > +                       size_t left)
> > +{
> > +       unsigned long ua_flags;
> > +
> > +       if (!kmsan_enabled || kmsan_in_runtime())
> > +               return;
> > +       /*
> > +        * At this point we've copied the memory already. It's hard to =
check it
> > +        * before copying, as the size of actually copied buffer is unk=
nown.
> > +        */
> > +
> > +       /* copy_to_user() may copy zero bytes. No need to check. */
> > +       if (!to_copy)
> > +               return;
> > +       /* Or maybe copy_to_user() failed to copy anything. */
> > +       if (to_copy <=3D left)
> > +               return;
> > +
> > +       ua_flags =3D user_access_save();
> > +       if ((u64)to < TASK_SIZE) {
> > +               /* This is a user memory access, check it. */
> > +               kmsan_internal_check_memory((void *)from, to_copy - lef=
t, to,
> > +                                           REASON_COPY_TO_USER);
>
> This could just do "} else {" and the stuff below, and would result in
> simpler code with no explicit "return" and no duplicated
> user_access_restore().

Sounds good, will do.


--=20
Alexander Potapenko
Software Engineer

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