[RFC] should we allow BPF to transmit empty skbs

Some layers in tx path do not expect skb being empty (skb->len == 0)

syzbot reported a crash [1] in fq_codel.

But I expect many drivers would also crash later.


Sure the immediate fq_codel crash could be 'fixed', but I would rather
add some sanity checks in net/core/filter.c

Thanks.

diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
index bb0cd6d3d2c2749d54e26368fb2558beedea85c9..73688b0ec83c473322669ca6a331bf3f3aefb293 100644
--- a/net/sched/sch_fq_codel.c
+++ b/net/sched/sch_fq_codel.c

@@ -203,7 +203,14 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
        codel_set_enqueue_time(skb);
        flow = &q->flows[idx];
        flow_queue_add(flow, skb);
-       q->backlogs[idx] += qdisc_pkt_len(skb);
+
+       /* fq_codel_drop() depends on qdisc_pkt_len(skb) being not zero. */
+       pkt_len = qdisc_pkt_len(skb);
+       if (unlikely(!pkt_len)) {
+               pkt_len = 1;
+               qdisc_skb_cb(skb)->pkt_len = pkt_len;
+       }
+       q->backlogs[idx] += pkt_len;
        qdisc_qstats_backlog_inc(sch, skb);
 
        if (list_empty(&flow->flowchain)) {

@@ -220,8 +227,6 @@ static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
        prev_backlog = sch->qstats.backlog;
        prev_qlen = sch->q.qlen;
 
-       /* save this packet length as it might be dropped by fq_codel_drop() */
-       pkt_len = qdisc_pkt_len(skb);
        /* fq_codel_drop() is quite expensive, as it performs a linear search
         * in q->backlogs[] to find a fat flow.
         * So instead of dropping a single packet, drop half of its backlog

[1]
general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 1 PID: 6542 Comm: syz-executor965 Not tainted 5.15.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:dequeue_head net/sched/sch_fq_codel.c:120 [inline]
RIP: 0010:fq_codel_drop net/sched/sch_fq_codel.c:168 [inline]
RIP: 0010:fq_codel_enqueue+0x83e/0x10c0 net/sched/sch_fq_codel.c:230
Code: f8 e2 25 fa 45 39 ec 0f 83 cb 00 00 00 e8 1a dc 25 fa 48 8b 44 24 10 80 38 00 0f 85 9a 06 00 00 49 8b 07 48 89 c2 48 c1 ea 03 <42> 80 3c 32 00 0f 85 6e 06 00 00 48 8b 10 48 8d 78 28 49 89 17 48
RSP: 0018:ffffc90001187310 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff87504776 RDI: 0000000000000003
RBP: ffffc900011874e0 R08: 0000000000000400 R09: 0000000000000001
R10: ffffffff875046d6 R11: 0000000000000000 R12: 0000000000000400
R13: 0000000000000000 R14: dffffc0000000000 R15: ffff888071660000
FS:  0000555556b21300(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f9c09885040 CR3: 0000000021c77000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 dev_qdisc_enqueue+0x40/0x300 net/core/dev.c:3771
 __dev_xmit_skb net/core/dev.c:3855 [inline]
 __dev_queue_xmit+0x1f0e/0x36e0 net/core/dev.c:4170
 __bpf_tx_skb net/core/filter.c:2114 [inline]
 __bpf_redirect_no_mac net/core/filter.c:2139 [inline]
 __bpf_redirect+0x5ba/0xd20 net/core/filter.c:2162
 ____bpf_clone_redirect net/core/filter.c:2429 [inline]
 bpf_clone_redirect+0x2ae/0x420 net/core/filter.c:2401
 ___bpf_prog_run+0x3592/0x77d0 kernel/bpf/core.c:1548
 __bpf_prog_run512+0x91/0xd0 kernel/bpf/core.c:1776
 bpf_dispatcher_nop_func include/linux/bpf.h:718 [inline]
 __bpf_prog_run include/linux/filter.h:624 [inline]
 bpf_prog_run include/linux/filter.h:631 [inline]
 bpf_test_run+0x37c/0xa20 net/bpf/test_run.c:119
 bpf_prog_test_run_skb+0xa7c/0x1cb0 net/bpf/test_run.c:662
 bpf_prog_test_run kernel/bpf/syscall.c:3307 [inline]
 __sys_bpf+0x2137/0x5df0 kernel/bpf/syscall.c:4605
 __do_sys_bpf kernel/bpf/syscall.c:4691 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:4689 [inline]
 __x64_sys_bpf+0x75/0xb0 kernel/bpf/syscall.c:4689
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

On 10/29/21 5:22 PM, Eric Dumazet wrote:

Some layers in tx path do not expect skb being empty (skb->len == 0)

syzbot reported a crash [1] in fq_codel.

But I expect many drivers would also crash later.

Sure the immediate fq_codel crash could be 'fixed', but I would rather
add some sanity checks in net/core/filter.c

Makes sense, we shouldn't have to add this to fq_codel fast path, but rather
a sanity check for bpf_clone_redirect().

I wonder if it's only related to bpf_prog_test_run() infra or if it could also
have been generated via stack?

Thanks,
Daniel

On Fri, Oct 29, 2021 at 8:39 AM Daniel Borkmann [off-list ref] wrote:

On 10/29/21 5:22 PM, Eric Dumazet wrote:

quoted

Some layers in tx path do not expect skb being empty (skb->len == 0)

syzbot reported a crash [1] in fq_codel.

But I expect many drivers would also crash later.

Sure the immediate fq_codel crash could be 'fixed', but I would rather
add some sanity checks in net/core/filter.c

Makes sense, we shouldn't have to add this to fq_codel fast path, but rather
a sanity check for bpf_clone_redirect().

I wonder if it's only related to bpf_prog_test_run() infra or if it could also
have been generated via stack?

probably bpf_prog_test_run_skb only.
I would only add size !=0 check there.

On 10/29/21 8:41 AM, Alexei Starovoitov wrote:

On Fri, Oct 29, 2021 at 8:39 AM Daniel Borkmann [off-list ref] wrote:

quoted

On 10/29/21 5:22 PM, Eric Dumazet wrote:

quoted

Some layers in tx path do not expect skb being empty (skb->len == 0)

syzbot reported a crash [1] in fq_codel.

But I expect many drivers would also crash later.

Sure the immediate fq_codel crash could be 'fixed', but I would rather
add some sanity checks in net/core/filter.c

Makes sense, we shouldn't have to add this to fq_codel fast path, but rather
a sanity check for bpf_clone_redirect().

I wonder if it's only related to bpf_prog_test_run() infra or if it could also
have been generated via stack?

probably bpf_prog_test_run_skb only.
I would only add size !=0 check there.

We have a C repro, I will release the syzbot bug so that it can be shared with you.

Thanks.