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Message-ID: <CAF=yD-Lg92JdpCU8CEQnutzi4VyS67_VNfAniRU=RxDvfYMruw@mail.gmail.com>
Date: Tue, 5 Jan 2021 22:07:21 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Dongseok Yi <dseok.yi@...sung.com>
Cc: "David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Miaohe Lin <linmiaohe@...wei.com>,
Willem de Bruijn <willemb@...gle.com>,
Paolo Abeni <pabeni@...hat.com>,
Florian Westphal <fw@...len.de>,
Al Viro <viro@...iv.linux.org.uk>,
Guillaume Nault <gnault@...hat.com>,
Yunsheng Lin <linyunsheng@...wei.com>,
Steffen Klassert <steffen.klassert@...unet.com>,
Yadu Kishore <kyk.segfault@...il.com>,
Marco Elver <elver@...gle.com>,
Network Development <netdev@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, namkyu78.kim@...sung.com
Subject: Re: [PATCH net] net: fix use-after-free when UDP GRO with shared fraglist
On Tue, Jan 5, 2021 at 8:29 PM Dongseok Yi <dseok.yi@...sung.com> wrote:
>
> On 2021-01-05 06:03, Willem de Bruijn wrote:
> >
> > On Mon, Jan 4, 2021 at 4:00 AM Dongseok Yi <dseok.yi@...sung.com> wrote:
> > >
> > > skbs in frag_list could be shared by pskb_expand_head() from BPF.
> >
> > Can you elaborate on the BPF connection?
>
> With the following registered ptypes,
>
> /proc/net # cat ptype
> Type Device Function
> ALL tpacket_rcv
> 0800 ip_rcv.cfi_jt
> 0011 llc_rcv.cfi_jt
> 0004 llc_rcv.cfi_jt
> 0806 arp_rcv
> 86dd ipv6_rcv.cfi_jt
>
> BPF checks skb_ensure_writable between tpacket_rcv and ip_rcv
> (or ipv6_rcv). And it calls pskb_expand_head.
>
> [ 132.051228] pskb_expand_head+0x360/0x378
> [ 132.051237] skb_ensure_writable+0xa0/0xc4
> [ 132.051249] bpf_skb_pull_data+0x28/0x60
> [ 132.051262] bpf_prog_331d69c77ea5e964_schedcls_ingres+0x5f4/0x1000
> [ 132.051273] cls_bpf_classify+0x254/0x348
> [ 132.051284] tcf_classify+0xa4/0x180
Ah, you have a BPF program loaded at TC. That was not entirely obvious.
This program gets called after packet sockets with ptype_all, before
those with a specific protocol.
Tcpdump will have inserted a program with ptype_all, which cloned the
skb. This triggers skb_ensure_writable -> pskb_expand_head ->
skb_clone_fraglist -> skb_get.
> [ 132.051294] __netif_receive_skb_core+0x590/0xd28
> [ 132.051303] __netif_receive_skb+0x50/0x17c
> [ 132.051312] process_backlog+0x15c/0x1b8
>
> >
> > > 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)
>
> Every your understanding is right, but
>
> >
> > >
> > > [ 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@...sung.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.
>
> A cloned skb after tpacket_rcv cannot go through skb_ensure_writable
> with the original shinfo. pskb_expand_head reallocates the shinfo of
> the skb and call skb_clone_fraglist. skb_release_data in
> pskb_expand_head could not reduce skb->users of the each frag_list skb
> if skb_shinfo(skb)->dataref == 2.
>
> After the reallocation, skb_shinfo(skb)->dataref == 1 but each frag_list
> skb could have skb->users == 2.
Yes, that makes sense. skb_clone_fraglist just increments the
frag_list skb's refcounts.
skb_segment_list must create an unshared struct sk_buff before it
changes skb data to insert the protocol headers.
> >
> >
> > > + tmp = skb_clone(nskb, GFP_ATOMIC);
> > > + if (tmp) {
> > > + kfree_skb(nskb);
> > > + nskb = tmp;
> > > + err = skb_unclone(nskb, GFP_ATOMIC);
Calling clone and unclone in quick succession looks odd.
But you need the first to create a private skb and to trigger the
second to create a private copy of the linear data (as well as frags,
if any, but these are not touched). So this looks okay.
> > > + } else {
> > > + err = -ENOMEM;
> > > + }
> > > + }
> > > +
> > > if (!tail)
> > > skb->next = nskb;
> > > else
> > > tail->next = nskb;
> > >
> > > + if (unlikely(err)) {
> > > + nskb->next = list_skb;
To avoid leaking these skbs when calling kfree_skb_list(skb->next). Is
that concern new with this patch, or also needed for the existing
error case?
> > > + goto err_linearize;
> > > + }
> > > +
> > > tail = nskb;
> > >
> > > delta_len += nskb->len;
> > > --
> > > 2.7.4
> > >
>
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