[PATCH net v3] net: fix use-after-free when UDP GRO with shared fraglist

[Dongseok Yi <dseok.yi@samsung.com>]

skbs in fraglist could be shared by a BPF filter loaded at TC. If TC
writes, it will call skb_ensure_writable -> pskb_expand_head to create
a private linear section for the head_skb. And then call
skb_clone_fraglist -> skb_get on each skb in the fraglist.

skb_segment_list overwrites part of the skb linear section of each
fragment itself. Even after skb_clone, the frag_skbs share their
linear section with their clone in PF_PACKET.

Both sk_receive_queue of PF_PACKET and PF_INET (or PF_INET6) can have
a link for the same frag_skbs chain. If a new skb (not frags) is
queued to one of the sk_receive_queue, multiple ptypes can see and
release this. It causes use-after-free.

[ 4443.426215] ------------[ cut here ]------------
[ 4443.426222] refcount_t: underflow; use-after-free.
[ 4443.426291] WARNING: CPU: 7 PID: 28161 at lib/refcount.c:190
refcount_dec_and_test_checked+0xa4/0xc8
[ 4443.426726] pstate: 60400005 (nZCv daif +PAN -UAO)
[ 4443.426732] pc : refcount_dec_and_test_checked+0xa4/0xc8
[ 4443.426737] lr : refcount_dec_and_test_checked+0xa0/0xc8
[ 4443.426808] Call trace:
[ 4443.426813]  refcount_dec_and_test_checked+0xa4/0xc8
[ 4443.426823]  skb_release_data+0x144/0x264
[ 4443.426828]  kfree_skb+0x58/0xc4
[ 4443.426832]  skb_queue_purge+0x64/0x9c
[ 4443.426844]  packet_set_ring+0x5f0/0x820
[ 4443.426849]  packet_setsockopt+0x5a4/0xcd0
[ 4443.426853]  __sys_setsockopt+0x188/0x278
[ 4443.426858]  __arm64_sys_setsockopt+0x28/0x38
[ 4443.426869]  el0_svc_common+0xf0/0x1d0
[ 4443.426873]  el0_svc_handler+0x74/0x98
[ 4443.426880]  el0_svc+0x8/0xc

Fixes: 3a1296a38d0c (net: Support GRO/GSO fraglist chaining.)
Signed-off-by: Dongseok Yi <dseok.yi@samsung.com>
Acked-by: Willem de Bruijn <willemb@google.com>
---
 net/core/skbuff.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

v2:
per Willem de Bruijn request,
expanded the commit message to clarify a BPF filter loaded.

v3:
per Daniel Borkmann request,
added further details from Willem in the commit message,
and use consume_skb instead of kfree_skb.

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f62cae3..b6f2b52 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3655,7 +3655,8 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
 	unsigned int delta_truesize = 0;
 	unsigned int delta_len = 0;
 	struct sk_buff *tail = NULL;
-	struct sk_buff *nskb;
+	struct sk_buff *nskb, *tmp;
+	int err;
 
 	skb_push(skb, -skb_network_offset(skb) + offset);
 
@@ -3665,11 +3666,28 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
 		nskb = list_skb;
 		list_skb = list_skb->next;
 
+		err = 0;
+		if (skb_shared(nskb)) {
+			tmp = skb_clone(nskb, GFP_ATOMIC);
+			if (tmp) {
+				consume_skb(nskb);
+				nskb = tmp;
+				err = skb_unclone(nskb, GFP_ATOMIC);
+			} else {
+				err = -ENOMEM;
+			}
+		}
+
 		if (!tail)
 			skb->next = nskb;
 		else
 			tail->next = nskb;
 
+		if (unlikely(err)) {
+			nskb->next = list_skb;
+			goto err_linearize;
+		}
+
 		tail = nskb;
 
 		delta_len += nskb->len;
-- 
2.7.4