Hi Eric,
On 10.06.22 15:59, Eric Dumazet wrote:
> On Fri, Jun 10, 2022 at 4:08 AM Soenke Huster <soenke.huster@xxxxxxxxx> 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@xxxxxxxxx>
>> ---
>> 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);
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
