On Mon, Jan 4, 2021 at 4:00 AM Dongseok Yi <dseok...@samsung.com> 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)
>
> [ 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>
> ---
> 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
>
