Re: [PATCH net v5] net: erspan: set lltx to avoid sch_direct_xmit deadlock

[Ido Schimmel]

On Thu, Oct 08, 2026 at 05:38:24PM +0800, Yun Zhou wrote:
> The ERSPAN xmit path re-enters the network stack, causing nested
> acquisition of _xmit_lock on the underlay device while already holding
> the ERSPAN device's _xmit_lock, creating an ABBA deadlock. This happens
> for both IPv4 ERSPAN (erspan_xmit() -> ip_tunnel_xmit()) and IPv6 ERSPAN
> (ip6erspan_tunnel_xmit() -> ip6_tnl_xmit()):
> 
>   sch_direct_xmit [lock erspan] -> erspan_xmit -> ip_tunnel_xmit ->
>   ip_output -> __dev_queue_xmit -> sch_direct_xmit [lock underlay]
> 
>   sch_direct_xmit [lock erspan] -> ip6erspan_tunnel_xmit -> ip6_tnl_xmit ->
>   ip6_output -> __dev_queue_xmit -> sch_direct_xmit [lock underlay]
> 
> Set dev->lltx = true so HARD_TX_LOCK() skips the spinlock for ERSPAN.
> This is safe as the ERSPAN xmit path has no shared mutable state: o_seqno
> is atomic, TX stats are per-CPU u64_stats, dst_cache is per-CPU, and
> o_flags is no longer modified in the xmit path since commit 9958e69b9893
> ("gre: fix ERSPAN o_flags race/corruption in xmit and fill_info").
> GRETAP, the sibling device with identical xmit structure, already sets
> lltx.
> 
> Reported-by: syzbot+9bda1b9fbb7fbdf9b62b@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=9bda1b9fbb7fbdf9b62b
> Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
> Fixes: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Yun Zhou <yun.zhou@xxxxxxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>