Re: [PATCH v2] Bluetooth: RFCOMM: Use skb_trim to trim checksum

[Eric Dumazet <edumazet@google.com>]

On Fri, Jun 10, 2022 at 8:35 AM Sönke Huster <soenke.huster@eknoes.de> wrote:
>
> Hi Eric,
>
> On 10.06.22 15:59, Eric Dumazet wrote:
> > On Fri, Jun 10, 2022 at 4:08 AM Soenke Huster <soenke.huster@eknoes.de> wrote:
> >>
> >> As skb->tail might be zero, it can underflow. This leads to a page
> >> fault: skb_tail_pointer simply adds skb->tail (which is now MAX_UINT)
> >> to skb->head.
> >>
> >>     BUG: unable to handle page fault for address: ffffed1021de29ff
> >>     #PF: supervisor read access in kernel mode
> >>     #PF: error_code(0x0000) - not-present page
> >>     RIP: 0010:rfcomm_run+0x831/0x4040 (net/bluetooth/rfcomm/core.c:1751)
> >>
> >> By using skb_trim instead of the direct manipulation, skb->tail
> >> is reset. Thus, the correct pointer to the checksum is used.
> >>
> >> Signed-off-by: Soenke Huster <soenke.huster@eknoes.de>
> >> ---
> >> v2: Clarified how the bug triggers, minimize code change
> >>
> >>  net/bluetooth/rfcomm/core.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> >> index 7324764384b6..443b55edb3ab 100644
> >> --- a/net/bluetooth/rfcomm/core.c
> >> +++ b/net/bluetooth/rfcomm/core.c
> >> @@ -1747,7 +1747,7 @@ static struct rfcomm_session *rfcomm_recv_frame(struct rfcomm_session *s,
> >>         type = __get_type(hdr->ctrl);
> >>
> >>         /* Trim FCS */
> >> -       skb->len--; skb->tail--;
> >> +       skb_trim(skb, skb->len - 1);
> >>         fcs = *(u8 *)skb_tail_pointer(skb);
> >>
> >>         if (__check_fcs(skb->data, type, fcs)) {
> >> --
> >> 2.36.1
> >>
> >
> > Again, I do not see how skb->tail could possibly zero at this point.
> >
> > If it was, skb with illegal layout has been queued in the first place,
> > we need to fix the producer, not the consumer.
> >
>
> Sorry, I thought that might be a right place as there is not much code in the kernel
> that manipulates ->tail directly.
>
> > A driver missed an skb_put() perhaps.
> >
>
> I am using the (I guess quite unused) virtio_bt driver, and figured out that the following
> fixes the bug:
>
> --- a/drivers/bluetooth/virtio_bt.c
> +++ b/drivers/bluetooth/virtio_bt.c
> @@ -219,7 +219,7 @@ static void virtbt_rx_work(struct work_struct *work)
>         if (!skb)
>                 return;
>
> -       skb->len = len;
> +       skb_put(skb, len);

Removing skb->len=len seems about right.
But skb_put() should be done earlier.

We are approaching the skb producer :)

Now you have to find/check who added this illegal skb in the virt queue.

Maybe virtbt_add_inbuf() ?

Also there is kernel info leak I think.

diff --git a/drivers/bluetooth/virtio_bt.c b/drivers/bluetooth/virtio_bt.c
index 67c21263f9e0f250f0719b8e7f1fe15b0eba5ee0..c9b832c447ee451f027430b284d7bb246f6ecb24
100644
--- a/drivers/bluetooth/virtio_bt.c
+++ b/drivers/bluetooth/virtio_bt.c
@@ -37,6 +37,9 @@ static int virtbt_add_inbuf(struct virtio_bluetooth *vbt)
        if (!skb)
                return -ENOMEM;

+       skb_put(skb, 1000);
+       memset(skb->data, 0, 1000);
+
        sg_init_one(sg, skb->data, 1000);

        err = virtqueue_add_inbuf(vq, sg, 1, skb, GFP_KERNEL);


>         virtbt_rx_handle(vbt, skb);
>
>         if (virtbt_add_inbuf(vbt) < 0)
>
> I guess this is the root cause? I just used Bluetooth for a while in the VM
> and no error occurred, everything worked fine.
>
> > Can you please dump the skb here  ?
> >
> > diff --git a/net/bluetooth/rfcomm/core.c b/net/bluetooth/rfcomm/core.c
> > index 7324764384b6773074032ad671777bf86bd3360e..358ccb4fe7214aea0bb4084188c7658316fe0ff7
> > 100644
> > --- a/net/bluetooth/rfcomm/core.c
> > +++ b/net/bluetooth/rfcomm/core.c
> > @@ -1746,6 +1746,11 @@ static struct rfcomm_session
> > *rfcomm_recv_frame(struct rfcomm_session *s,
> >         dlci = __get_dlci(hdr->addr);
> >         type = __get_type(hdr->ctrl);
> >
> > +       if (!skb->tail) {
> > +               DO_ONCE_LITE(skb_dump(KERN_ERR, skb, false));
> > +               kfree_skb(skb);
> > +               return s;
> > +       }
> >         /* Trim FCS */
> >         skb->len--; skb->tail--;
> >         fcs = *(u8 *)skb_tail_pointer(skb);
>
> If it might still help:
>
> skb len=4 headroom=9 headlen=4 tailroom=1728
> mac=(-1,-1) net=(0,-1) trans=-1
> shinfo(txflags=0 nr_frags=0 gso(size=0 type=0 segs=0))
> csum(0x0 ip_summed=0 complete_sw=0 valid=0 level=0)
> hash(0x0 sw=0 l4=0) proto=0x0000 pkttype=0 iif=0
> skb linear:   00000000: 03 3f 01 1c
>