Date:   Tue, 28 Mar 2017 19:18:58 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Ladi Prosek <lprosek@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        David Hildenbrand <david@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        Konstantin Neumoin <kneumoin@...tuozzo.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Minchan Kim <minchan@...nel.org>,
        "Denis V. Lunev" <den@...nvz.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] virtio_balloon: prevent uninitialized variable use

On Mon, Mar 27, 2017 at 12:02:33PM +0200, Ladi Prosek wrote:
> On Fri, Mar 24, 2017 at 9:59 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > On Fri, Mar 24, 2017 at 09:40:07PM +0100, Arnd Bergmann wrote:
> >> On Fri, Mar 24, 2017 at 9:11 PM, Ladi Prosek <lprosek@...hat.com> wrote:
> >> > On Fri, Mar 24, 2017 at 7:38 PM, David Hildenbrand <david@...hat.com> wrote:
> >> >> On 23.03.2017 16:17, Arnd Bergmann wrote:
> >> >>> The latest gcc-7.0.1 snapshot reports a new warning:
> >> >>>
> >> >>> virtio/virtio_balloon.c: In function 'update_balloon_stats':
> >> >>> virtio/virtio_balloon.c:258:26: error: 'events[2]' is used uninitialized in this function [-Werror=uninitialized]
> >> >>> virtio/virtio_balloon.c:260:26: error: 'events[3]' is used uninitialized in this function [-Werror=uninitialized]
> >> >>> virtio/virtio_balloon.c:261:56: error: 'events[18]' is used uninitialized in this function [-Werror=uninitialized]
> >> >>> virtio/virtio_balloon.c:262:56: error: 'events[17]' is used uninitialized in this function [-Werror=uninitialized]
> >> >>>
> >> >>> This seems absolutely right, so we should add an extra check to
> >> >>> prevent copying uninitialized stack data into the statistics.
> >> >>> From all I can tell, this has been broken since the statistics code
> >> >>> was originally added in 2.6.34.
> >> >>>
> >> >>> Fixes: 9564e138b1f6 ("virtio: Add memory statistics reporting to the balloon driver (V4)")
> >> >>> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> >> >>> ---
> >> >>>  drivers/virtio/virtio_balloon.c | 2 ++
> >> >>>  1 file changed, 2 insertions(+)
> >> >>>
> >> >>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> >> >>> index 4e1191508228..cd5c54e2003d 100644
> >> >>> --- a/drivers/virtio/virtio_balloon.c
> >> >>> +++ b/drivers/virtio/virtio_balloon.c
> >> >>> @@ -254,12 +254,14 @@ static void update_balloon_stats(struct virtio_balloon *vb)
> >> >>>
> >> >>>       available = si_mem_available();
> >> >>>
> >> >>> +#ifdef CONFIG_VM_EVENT_COUNTERS
> >> >>>       update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_IN,
> >> >>>                               pages_to_bytes(events[PSWPIN]));
> >> >>>       update_stat(vb, idx++, VIRTIO_BALLOON_S_SWAP_OUT,
> >> >>>                               pages_to_bytes(events[PSWPOUT]));
> >> >>>       update_stat(vb, idx++, VIRTIO_BALLOON_S_MAJFLT, events[PGMAJFAULT]);
> >> >>>       update_stat(vb, idx++, VIRTIO_BALLOON_S_MINFLT, events[PGFAULT]);
> >> >>> +#endif
> >> >>>       update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMFREE,
> >> >>>                               pages_to_bytes(i.freeram));
> >> >>>       update_stat(vb, idx++, VIRTIO_BALLOON_S_MEMTOT,
> >> >
> >> > This will leave four uninitialized slots in vb->stats if
> >> > CONFIG_VM_EVENT_COUNTERS is not defined. update_balloon_stats should
> >> > have
> >> >
> >> >   BUG_ON(idx < VIRTIO_BALLOON_S_NR);
> >> >
> >> > at the end.
> >> >
> >> > You need to make sure that vb->stats is smaller, either by using
> >> > something else than VIRTIO_BALLOON_S_NR for its size or something else
> >> > than sizeof(vb->stats) as the last argument to sg_init_one in this
> >> > file.
> >>
> >> Ah, got it. Would one of you create a fixed patch for this, or should I?
> 
> I will do it. If we go the route of returning 'idx' from
> update_balloon_stats, my patch
> 
>    virtio_balloon: don't push uninitialized buffers to stats virtqueue
> 
> needs to be redone anyway. Thanks!

Can you do it soon pls? I'd like to do a pull request.

> >> An easy way to solve it would be to preinitialize the events array
> >> and return zeroes to the host, but I don't know if that's the right
> >> solution.
> >
> >
> > I think that's out of spec.
> >
> >> Another option would be to return 'idx' from update_balloon_stats,
> >> and use that for the size:
> >>
> >> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> >> index cd5c54e2003d..bea3cfb88e53 100644
> >> --- a/drivers/virtio/virtio_balloon.c
> >> +++ b/drivers/virtio/virtio_balloon.c
> >> @@ -242,7 +242,7 @@ static inline void update_stat(struct
> >> virtio_balloon *vb, int idx,
> >>
> >>  #define pages_to_bytes(x) ((u64)(x) << PAGE_SHIFT)
> >>
> >> -static void update_balloon_stats(struct virtio_balloon *vb)
> >> +static unsigned int update_balloon_stats(struct virtio_balloon *vb)
> >>  {
> >>   unsigned long events[NR_VM_EVENT_ITEMS];
> >>   struct sysinfo i;
> >> @@ -268,6 +268,8 @@ static void update_balloon_stats(struct virtio_balloon *vb)
> >>   pages_to_bytes(i.totalram));
> >>   update_stat(vb, idx++, VIRTIO_BALLOON_S_AVAIL,
> >>   pages_to_bytes(available));
> >> +
> >> + return idx;
> >>  }
> >>
> >>  /*
> >> @@ -294,13 +296,14 @@ static void stats_handle_request(struct
> >> virtio_balloon *vb)
> >>   struct virtqueue *vq;
> >>   struct scatterlist sg;
> >>   unsigned int len;
> >> + unsigned int num_stats;
> >>
> >> - update_balloon_stats(vb);
> >> + num_stats = update_balloon_stats(vb);
> >>
> >>   vq = vb->stats_vq;
> >>   if (!virtqueue_get_buf(vq, &len))
> >>   return;
> >> - sg_init_one(&sg, vb->stats, sizeof(vb->stats));
> >> + sg_init_one(&sg, vb->stats, sizeof(vb->stats[0] * num_stats));
> >>   virtqueue_add_outbuf(vq, &sg, 1, vb, GFP_KERNEL);
> >>   virtqueue_kick(vq);
> >>  }
> >>
> >>      Arnd
> >
> > Sounds reasonable.
> >
> > --
> > MST

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