On 1/7/21 1:39 AM, Dongseok Yi wrote:
skbs in fraglist could be shared by a BPF filter loaded at TC. It triggers skb_ensure_writable -> pskb_expand_head -> skb_clone_fraglist -> skb_get on each skb in the fraglist.While tcpdump, sk_receive_queue of PF_PACKET has the original fraglist. But the same fraglist is queued to PF_INET (or PF_INET6) as the fraglist chain made by skb_segment_list. If the new skb (not fraglist) 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. [ 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...@samsung.com> Acked-by: Willem de Bruijn <will...@google.com> --- net/core/skbuff.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) v2: Expand the commit message to clarify a BPF filter loaded 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)) { + tmp = skb_clone(nskb, GFP_ATOMIC); + if (tmp) { + kfree_skb(nskb);
Should use consume_skb() to not trigger skb:kfree_skb tracepoint when looking for drops in the stack.
+ nskb = tmp; + err = skb_unclone(nskb, GFP_ATOMIC);
Could you elaborate why you also need to unclone? This looks odd here. tc layer (independent of BPF) from ingress & egress side generally assumes unshared skb, so above clone + dropping ref of nskb looks okay to make the main skb struct private for mangling attributes (e.g. mark) & should suffice. What is the exact purpose of the additional skb_unclone() in this context?
+ } 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;
