linux-kernel - Re: [PATCH v4 18/45] instrumented.h: add KMSAN support

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Message-ID: <CANpmjNOPJL7WAUh5CUZOYO8hY-dHTHMUMJzd9OGbmWES+smtrQ@mail.gmail.com>
Date:   Tue, 12 Jul 2022 15:51:31 +0200
From:   Marco Elver <elver@...gle.com>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 18/45] instrumented.h: add KMSAN support

On Fri, 1 Jul 2022 at 16:24, Alexander Potapenko <glider@...gle.com> wrote:
>
> 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@...gle.com>

Reviewed-by: Marco Elver <elver@...gle.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_copy,
> +                       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 unknown.
> +        */
> +
> +       /* 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 <= left)
> +               return;
> +
> +       ua_flags = user_access_save();
> +       if ((u64)to < TASK_SIZE) {
> +               /* This is a user memory access, check it. */
> +               kmsan_internal_check_memory((void *)from, to_copy - left, 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().

> +               user_access_restore(ua_flags);
> +               return;
> +       }
> +       /* Otherwise this is a kernel memory access. This happens when a compat
> +        * syscall passes an argument allocated on the kernel stack to a real
> +        * syscall.
> +        * Don't check anything, just copy the shadow of the copied bytes.
> +        */
> +       kmsan_internal_memmove_metadata((void *)to, (void *)from,
> +                                       to_copy - left);
> +       user_access_restore(ua_flags);
> +}
> +EXPORT_SYMBOL(kmsan_copy_to_user);