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

[Eric Dumazet <edumazet@google.com>]

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.

A driver missed an skb_put() perhaps.

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);