Re: [PATCH-for-6.0] net: tap: fix crash on hotplug

[Philippe Mathieu-Daudé]

Cc'ing Bin.

On 4/21/21 5:22 PM, Cole Robinson wrote:
> Attempting to hotplug a tap nic with libvirt will crash qemu:
> 
> $ sudo virsh attach-interface f32 network default
> error: Failed to attach interface
> error: Unable to read from monitor: Connection reset by peer
> 
> 0x000055875b7f3a99 in tap_send (opaque=0x55875e39eae0) at ../net/tap.c:206
> 206           if (!s->nc.peer->do_not_pad) {
> gdb$ bt
> 
> s->nc.peer may not be set at this point. This seems to be an
> expected case, as qemu_send_packet_* explicitly checks for NULL
> s->nc.peer later.
> 
> Fix it by checking for s->nc.peer here too. Padding is applied if
> s->nc.peer is not set.
> 
> https://bugzilla.redhat.com/show_bug.cgi?id=1949786
> Fixes: 969e50b61a2
> 
> Signed-off-by: Cole Robinson <crobinso@redhat.com>
> ---
> * Or should we skip padding if nc.peer is unset? I didn't dig into it
> * tap-win3.c and slirp.c may need a similar fix, but the slirp case
>   didn't crash in a simple test.
> 
>  net/tap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/tap.c b/net/tap.c
> index dd42ac6134..937559dbb8 100644
> --- a/net/tap.c
> +++ b/net/tap.c
> @@ -203,7 +203,7 @@ static void tap_send(void *opaque)
>              size -= s->host_vnet_hdr_len;
>          }
>  
> -        if (!s->nc.peer->do_not_pad) {
> +        if (!s->nc.peer || !s->nc.peer->do_not_pad) {
>              if (eth_pad_short_frame(min_pkt, &min_pktsz, buf, size)) {
>                  buf = min_pkt;
>                  size = min_pktsz;
>