[PATCH v5] net: gro: fix double aggregation of flush-marked skbs

[Shiming Cheng (成诗明)]

On Mon, 2026-07-06 at 12:36 -0400, Willem de Bruijn wrote:
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> 
> [PATCH net v5]
> 
> Shiming Cheng wrote:
> > The skb_gro_receive_list() function is missing a critical safety
> > check
> > that exists in the skb_gro_receive() implementation. Specifically,
> > it
> > does not validate NAPI_GRO_CB(skb)->flush before allowing packet
> > aggregation
> 
> In v3 I requested referring to the commit that fixed this in
> skb_gro_receive: commit 0ab03f353d36 ("net-gro: Fix GRO flush when
> receiving a GSO packet."). That explains the issue well.

updated in PATCH V6.

> > 
> > This allows already-GRO'd packets with existing frag_list to be
> > re-aggregated into a new GRO session, corrupting the frag_list
> > chain
> > structure. When skb_segment() attempts to unpack these malformed
> > packets,
> > it encounters invalid state and triggers a kernel panic.
> > 
> > Scenario (Tethering/Device forwarding):
> >   1. Driver: Generated aggregated packet P1 via LRO with frag_list
> >   2. Dev A: Receives aggregated fraglist packet and flush flag set
> >   3. Dev A: Re-enters GRO, skb_gro_receive_list() is called
> >   4. Missing flush check allows re-aggregation despite flush flag
> >   5. Frag_list chain becomes corrupted (loops or dangling refs)
> >   6. Dev B: TX path calls skb_segment(), crashes on corrupted
> > frag_list
> > 
> > Root cause in skb_segment():
> >   The check at line ~4891:
> >     if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
> >         (skb_headlen(list_skb) == len || sg)) {
> > 
> >   When frag_list is corrupted by double aggregation, when list_skb
> > is
> >   a NULL pointer from skb->next, skb_headlen(list_skb) dereference
> >   NULL/corrupted pointers occurs.
> > 
> > Call Trace:
> >  skb_headlen(NULL skb)
> >  skb_segment
> >  tcp_gso_segment
> >  tcp4_gso_segment
> >  inet_gso_segment
> >  skb_mac_gso_segment
> >  __skb_gso_segment
> >  skb_gso_segment
> >  validate_xmit_skb
> >  validate_xmit_skb_list
> >  sch_direct_xmit
> >  qdisc_restart
> >  __qdisc_run
> >  qdisc_run
> >  net_tx_action
> > 
> > Fix: Add NAPI_GRO_CB(skb)->flush validation to the early-return
> > check in
> > skb_gro_receive_list(), matching the defensive programming pattern
> > of
> > skb_gro_receive().
> > 
> > Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
> > Cc: stable at vger.kernel.org
> > Signed-off-by: Shiming Cheng <shiming.cheng at mediatek.com>
> > ---
> >  net/core/gro.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/core/gro.c b/net/core/gro.c
> > index 35f2f708f010..b1573d98f3a5 100644
> > --- a/net/core/gro.c
> > +++ b/net/core/gro.c
> > @@ -229,7 +229,14 @@ int skb_gro_receive(struct sk_buff *p, struct
> > sk_buff *skb)
> > 
> >  int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
> >  {
> > -     if (unlikely(p->len + skb->len >= 65536))
> > +     /*
> > +      * Packets marked with NAPI_GRO_CB(skb)->flush have already
> > gone
> > +      * through GRO/LRO processing and must not be aggregated
> > again.
> > +      * Re-entering frag_list GRO may corrupt the frag_list chain
> > and
> > +      * later crash during GSO segmentaiont.
> > +      */
> 
> Such a verbose comment is not needed. Code would be overwhelmed by
> comments if done everywhere.
> 

updated comment in PATCH V6.

> > +     if (unlikely(p->len + skb->len >= 65536 ||
> > +                  NAPI_GRO_CB(skb)->flush))
> >               return -E2BIG;
> > 
> >       if (!pskb_may_pull(skb, skb_gro_offset(skb))) {
> > --
> > 2.45.2
> > 
> 
>