Subject: [PATCH v2 1/2] net: Fix skb_segment when splitting gso_size mangled skb having linear-headed frag_list whose head_frag=true

From: Fred Li <[email-masked]>
Date: Wed, 26 Jun 2024 14:55:54 +0800

When using calico bpf based NAT, hits a kernel BUG_ON at function
skb_segment(), line 4560. Performing NAT translation when accessing
a Service IP across subnets, the calico will encap vxlan and calls the
bpf_skb_adjust_room to decrease the gso_size, and then call bpf_redirect
send packets out.

4438 struct sk_buff *skb_segment(struct sk_buff *head_skb,
4439                             netdev_features_t features)
4440 {
4441     struct sk_buff *segs = NULL;
4442     struct sk_buff *tail = NULL;
...
4558         if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
4559             (skb_headlen(list_skb) == len || sg)) {
4560                 BUG_ON(skb_headlen(list_skb) > len);
4561
4562                 nskb = skb_clone(list_skb, GFP_ATOMIC);
4563                 if (unlikely(!nskb))
4564                     goto err;

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
...

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

When slicing the frag_list skb, there three cases:
1. Only *non* head_frag
    sg will be false, only when skb_headlen(list_skb)==len is satisfied,
    it will enter the branch at line 4560, and there will be no crash.
2. Mixed head_frag
    sg will be false, Only when skb_headlen(list_skb)==len is satisfied,
    it will enter the branch at line 4560, and there will be no crash.
3. Only frag_list with head_frag=true
    sg is true, three cases below:
    (1) skb_headlen(list_skb)==len is satisfied, it will enter the branch
       at line 4560, and there will be no crash.
    (2) skb_headlen(list_skb)<len is satisfied, it will enter the branch
       at line 4560, and there will be no crash.
    (3) skb_headlen(list_skb)>len is satisfied, it will be crash.

Applying this patch, three cases will be:
1. Only *non* head_frag
    sg will be false. No difference with before.
2. Mixed head_frag
    sg will be false. No difference with before.
3. Only frag_list with head_frag=true
    sg is true, there also three cases:
    (1) skb_headlen(list_skb)==len is satisfied, no difference with before.
    (2) skb_headlen(list_skb)<len is satisfied, will be revert to copying
        in this case.
    (3) skb_headlen(list_skb)>len is satisfied, will be revert to copying
        in this case.

Since commit 13acc94eff122("net: permit skb_segment on head_frag frag_list
skb"), it is allowed to segment the 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 f0a9ef1aeaa2..b1dab1b071fc 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4556,7 +4556,7 @@ 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) &&
-		    (skb_headlen(list_skb) == len || sg)) {
+		    (skb_headlen(list_skb) == len)) {
 			BUG_ON(skb_headlen(list_skb) > len);
 
 			nskb = skb_clone(list_skb, GFP_ATOMIC);
-- 
2.33.0