Re: [PATCH net] net: fix use-after-free when UDP GRO with shared fraglist
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2021-01-04 23:50:47
Also in:
lkml
On Mon, Jan 4, 2021 at 4:00 AM Dongseok Yi [off-list ref] wrote:
skbs in frag_list could be shared by pskb_expand_head() from BPF.
Can you elaborate on the BPF connection?
While tcpdump, sk_receive_queue of PF_PACKET has the original frag_list. But the same frag_list is queued to PF_INET (or PF_INET6) as the fraglist chain made by skb_segment_list(). If the new skb (not frag_list) is queued to one of the sk_receive_queue, multiple ptypes can see this. The skb could be released by ptypes and it causes use-after-free.
If I understand correctly, a udp-gro-list skb makes it up the receive
path with one or more active packet sockets.
The packet socket will call skb_clone after accepting the filter. This
replaces the head_skb, but shares the skb_shinfo and thus frag_list.
udp_rcv_segment later converts the udp-gro-list skb to a list of
regular packets to pass these one-by-one to udp_queue_rcv_one_skb.
Now all the frags are fully fledged packets, with headers pushed
before the payload. This does not change their refcount anymore than
the skb_clone in pf_packet did. This should be 1.
Eventually udp_recvmsg will call skb_consume_udp on each packet.
The packet socket eventually also frees its cloned head_skb, which triggers
kfree_skb_list(shinfo->frag_list)
kfree_skb
skb_unref
refcount_dec_and_test(&skb->users)
quoted hunk ↗ jump to hunk
[ 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 <redacted> --- net/core/skbuff.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-)diff --git a/net/core/skbuff.c b/net/core/skbuff.c index f62cae3..1dcbda8 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)) {
I must be missing something still. This does not square with my understanding that the two sockets are operating on clones, with each frag_list skb having skb->users == 1. Unless the packet socket patch previously also triggered an skb_unclone/pskb_expand_head, as that call skb_clone_fraglist, which calls skb_get on each frag_list skb.
+ tmp = skb_clone(nskb, GFP_ATOMIC);
+ if (tmp) {
+ kfree_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