Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH net v2] net: fix use-after-free when UDP GRO with shared fraglist | From | Daniel Borkmann <> | Date | Thu, 7 Jan 2021 13:49:26 +0100 |
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On 1/7/21 12:40 PM, Dongseok Yi wrote: > On 2021-01-07 20:05, Daniel Borkmann wrote: >> 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.yi@samsung.com> >>> Acked-by: Willem de Bruijn <willemb@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. > > I will use to consume_skb() on the next version. > >>> + 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? > > Willem de Bruijn said: > 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.
Yes.
> PF_PACKET handles untouched fraglist. To modify the payload only > for udp_rcv_segment, skb_unclone is necessary.
I don't parse this last sentence here, please elaborate in more detail on why it is necessary.
For example, if tc layer would modify mark on the skb, then __copy_skb_header() in skb_segment_list() will propagate it. If tc layer would modify payload, the skb_ensure_writable() will take care of that internally and if needed pull in parts from fraglist into linear section to make it private. The purpose of the skb_clone() above iff shared is to make the struct itself private (to safely modify its struct members). What am I missing?
>>> + } 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; >>> > >
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