Messages in this thread | | | Subject | Re: [PATCH net] net: fix use-after-free when UDP GRO with shared fraglist | From | Yunsheng Lin <> | Date | Wed, 21 Apr 2021 17:42:12 +0800 |
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On 2021/4/19 8:35, Dongseok Yi wrote: > On Sat, Apr 17, 2021 at 11:44:35AM +0800, Yunsheng Lin wrote: >> >> On 2021/1/6 11:32, Dongseok Yi wrote: >>> On 2021-01-06 12:07, Willem de Bruijn wrote: >>>> >>>> On Tue, Jan 5, 2021 at 8:29 PM Dongseok Yi <dseok.yi@samsung.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@samsung.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@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. >>>>> >>>>> 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. >> >> Hi, Dongseok >> I understand there is liner head data shared between the frag_list skb in the >> cloned skb(cloned by pf_packet?) and original skb, which should not be shared >> when skb_segment_list() converts the frag_list skb into regular packet. >> >> But both skb->users of original and cloned skb is one(skb_shinfo(skb)->dataref >> is one for both skb too), and skb->users of each fraglist skb is two because both >> original and cloned skb is linking to the same fraglist pointer, and there is >> "skb_shinfo(skb)->frag_list = NULL" for original skb in the begin of skb_segment_list(), >> if kfree_skb() is called with original skb, the fraglist skb will not be freed. >> If kfree_skb is called with original skb,cloned skb and each fraglist skb here, the >> reference counter for three of them seem right here, so why is there a refcount_t >> warning in the commit log? am I missing something obvious here? >> >> Sorry for bringing up this thread again. > > A skb which detects use-after-free was not a part of frag_list. Please > check the commit msg once again.
I checked the commit msg again, but still have not figured it out yet:)
So I tried to see if I understand the skb'reference counting correctly:
skb->user is used to reference counting the "struct sk_buff", and skb_shinfo(skb)->dataref is used to reference counting head data.
skb_clone(): allocate a sperate "struct sk_buff" but share the head data with the original skb, so skb_shinfo()->dataref need incrmenting.
pskb_expand_head(): allocate a sperate head data(which includes the space for skb_shinfo(skb)), since the original head data and the new head data' skb_shinfo()->frag_list both point to the same fraglist skb, so each fraglist_skb's skb->users need incrmenting, and original head data's skb_shinfo() need decrmenting.
So after pf_packet called skb_clone() and pskb_expand_head(), we have:
old skb new skb | | | | old head data new head data \ / \ / \ / \ / \ / fraglist_skb1 -> fraglist_skb2 -> fraglist_skb3 .....
So both old and new skb' skb->user is one, both old and new head data's skb_shinfo()->dataref is one, and both old and new head data' skb_shinfo()->frag_list points to fraglist_skb1, and each fraglist_skb's skb->user is two.
Each fraglist_skb points to a head data, and its skb_shinfo()->dataref is one too.
Suppose old skb is called with skb_segment_list(), without this patch, we have:
new skb | | new head data / / / / / old skb -> fraglist_skb1 -> fraglist_skb2 -> fraglist_skb3 ..... | | old head data
And old skb and each fraglist_skb become a regular packet, so freeing the old skb, new skb and each fraglist_skb here do not seems to have any reference counting problem, because each fraglist_skb's skb->user is two, right?
> > Both sk_receive_queue of PF_PACKET and PF_INET (or PF_INET6) can have > a link for the same frag_skbs chain.
Does "frag_skbs chain" means fraglist_skb1? It seems only new head data's skb_shinfo()->frag_list points to fraglist_skb1
If a new skb (*not frags*) is > queued to one of the sk_receive_queue, multiple ptypes can see and > release this. It causes use-after-free.
Does "a new skb" mean each fraglist_skb after skb_segment_list()? Or other new incoming skb?
I am not so familiar with the PF_PACKET and PF_INET, so still have hard time figuring how the reference counting goes wrong here:)
> >> >>>> >>>> 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. >>>> > > > > . >
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