Subject: [PATCH] net: Fix the gso BUG_ON that treat the skb which head_frag is true as non head_frag

From: dracoding <[email-masked]>
Date: Wed, 15 May 2024 22:43:13 +0800

From: Fred Li <[email-masked]>

The crashed kernel version is 5.16.20, and I have not test this patch
because I dont find a way to reproduce it, and the mailine may be
has the same problem.

When using bpf based NAT, hits a kernel BUG_ON at function skb_segment(),
BUG_ON(skb_headlen(list_skb) > len). The bpf calls the bpf_skb_adjust_room
to decrease the gso_size, and then call bpf_redirect send packet out.

call stack:
...
   [exception RIP: skb_segment+3016]
    RIP: ffffffffb97df2a8  RSP: ffffa3f2cce08728  RFLAGS: 00010293
    RAX: 000000000000007d  RBX: 00000000fffff7b3  RCX: 0000000000000011
    RDX: 0000000000000000  RSI: ffff895ea32c76c0  RDI: 00000000000008c1
    RBP: ffffa3f2cce087f8   R8: 000000000000088f   R9: 0000000000000011
    R10: 000000000000090c  R11: ffff895e47e68000  R12: ffff895eb2022f00
    R13: 000000000000004b  R14: ffff895ecdaf2000  R15: ffff895eb2023f00
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #9 [ffffa3f2cce08720] skb_segment at ffffffffb97ded63
#10 [ffffa3f2cce08800] tcp_gso_segment at ffffffffb98d0320
#11 [ffffa3f2cce08860] tcp4_gso_segment at ffffffffb98d07a3
#12 [ffffa3f2cce08880] inet_gso_segment at ffffffffb98e6de0
#13 [ffffa3f2cce088e0] skb_mac_gso_segment at ffffffffb97f3741
#14 [ffffa3f2cce08918] skb_udp_tunnel_segment at ffffffffb98daa59
#15 [ffffa3f2cce08980] udp4_ufo_fragment at ffffffffb98db471
#16 [ffffa3f2cce089b0] inet_gso_segment at ffffffffb98e6de0
#17 [ffffa3f2cce08a10] skb_mac_gso_segment at ffffffffb97f3741
#18 [ffffa3f2cce08a48] __skb_gso_segment at ffffffffb97f388e
#19 [ffffa3f2cce08a78] validate_xmit_skb at ffffffffb97f3d6e
#20 [ffffa3f2cce08ab8] __dev_queue_xmit at ffffffffb97f4614
#21 [ffffa3f2cce08b50] dev_queue_xmit at ffffffffb97f5030
#22 [ffffa3f2cce08b60] __bpf_redirect at ffffffffb98199a8
#23 [ffffa3f2cce08b88] skb_do_redirect at ffffffffb98205cd
...

The skb has the following properties:
    doffset = 66
    list_skb = skb_shinfo(skb)->frag_list
    list_skb->head_frag = true
    skb->len = 2441 && skb->data_len = 2250
    skb_shinfo(skb)->nr_frags = 17
    skb_shinfo(skb)->gso_size = 75
    skb_shinfo(skb)->frags[0...16].bv_len = 125
    list_skb->len = 125
    list_skb->data_len = 0

3962 struct sk_buff *skb_segment(struct sk_buff *head_skb,
3963                             netdev_features_t features)
3964 {
3965         struct sk_buff *segs = NULL;
3966         struct sk_buff *tail = NULL;
...
4181                 while (pos < offset + len) {
4182                         if (i >= nfrags) {
4183                                 i = 0;
4184                                 nfrags = skb_shinfo(list_skb)->nr_frags;
4185                                 frag = skb_shinfo(list_skb)->frags;
4186                                 frag_skb = list_skb;

After segment the head_skb's last frag, the (pos == offset+len), so break the
while at line 4181, run into this BUG_ON(), not segment the head_frag frag_list
skb.

Since commit 13acc94eff122(net: permit skb_segment on head_frag frag_list skb),
it is allowed to segment the head_frag frag_list skb.

In commit 3dcbdb134f32 (net: gso: Fix skb_segment splat when splitting gso_size
mangled skb having linear-headed frag_list), it is cleared the NETIF_F_SG if it
has non head_frag skb. It is not cleared the NETIF_F_SG only with one head_frag
frag_list skb.

Signed-off-by: Fred Li <[email-masked]>
---
 net/core/skbuff.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 001152c..d805a47 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4070,7 +4070,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 
 		hsize = skb_headlen(head_skb) - offset;
 
-		if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
+		if (hsize <= 0 && i >= nfrags && !list_skb->head_frag && skb_headlen(list_skb) &&
 		    (skb_headlen(list_skb) == len || sg)) {
 			BUG_ON(skb_headlen(list_skb) > len);
 
-- 
1.8.3.1