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    Subject[PATCH AUTOSEL 6.0 20/73] bpf: make sure skb->len != 0 when redirecting to a tunneling device
    Date
    From: Stanislav Fomichev <sdf@google.com>

    [ Upstream commit 07ec7b502800ba9f7b8b15cb01dd6556bb41aaca ]

    syzkaller managed to trigger another case where skb->len == 0
    when we enter __dev_queue_xmit:

    WARNING: CPU: 0 PID: 2470 at include/linux/skbuff.h:2576 skb_assert_len include/linux/skbuff.h:2576 [inline]
    WARNING: CPU: 0 PID: 2470 at include/linux/skbuff.h:2576 __dev_queue_xmit+0x2069/0x35e0 net/core/dev.c:4295

    Call Trace:
    dev_queue_xmit+0x17/0x20 net/core/dev.c:4406
    __bpf_tx_skb net/core/filter.c:2115 [inline]
    __bpf_redirect_no_mac net/core/filter.c:2140 [inline]
    __bpf_redirect+0x5fb/0xda0 net/core/filter.c:2163
    ____bpf_clone_redirect net/core/filter.c:2447 [inline]
    bpf_clone_redirect+0x247/0x390 net/core/filter.c:2419
    bpf_prog_48159a89cb4a9a16+0x59/0x5e
    bpf_dispatcher_nop_func include/linux/bpf.h:897 [inline]
    __bpf_prog_run include/linux/filter.h:596 [inline]
    bpf_prog_run include/linux/filter.h:603 [inline]
    bpf_test_run+0x46c/0x890 net/bpf/test_run.c:402
    bpf_prog_test_run_skb+0xbdc/0x14c0 net/bpf/test_run.c:1170
    bpf_prog_test_run+0x345/0x3c0 kernel/bpf/syscall.c:3648
    __sys_bpf+0x43a/0x6c0 kernel/bpf/syscall.c:5005
    __do_sys_bpf kernel/bpf/syscall.c:5091 [inline]
    __se_sys_bpf kernel/bpf/syscall.c:5089 [inline]
    __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5089
    do_syscall_64+0x54/0x70 arch/x86/entry/common.c:48
    entry_SYSCALL_64_after_hwframe+0x61/0xc6

    The reproducer doesn't really reproduce outside of syzkaller
    environment, so I'm taking a guess here. It looks like we
    do generate correct ETH_HLEN-sized packet, but we redirect
    the packet to the tunneling device. Before we do so, we
    __skb_pull l2 header and arrive again at skb->len == 0.
    Doesn't seem like we can do anything better than having
    an explicit check after __skb_pull?

    Cc: Eric Dumazet <edumazet@google.com>
    Reported-by: syzbot+f635e86ec3fa0a37e019@syzkaller.appspotmail.com
    Signed-off-by: Stanislav Fomichev <sdf@google.com>
    Link: https://lore.kernel.org/r/20221027225537.353077-1-sdf@google.com
    Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/core/filter.c | 4 ++++
    1 file changed, 4 insertions(+)

    diff --git a/net/core/filter.c b/net/core/filter.c
    index c191db80ce93..e7c8d76c22db 100644
    --- a/net/core/filter.c
    +++ b/net/core/filter.c
    @@ -2125,6 +2125,10 @@ static int __bpf_redirect_no_mac(struct sk_buff *skb, struct net_device *dev,

    if (mlen) {
    __skb_pull(skb, mlen);
    + if (unlikely(!skb->len)) {
    + kfree_skb(skb);
    + return -ERANGE;
    + }

    /* At ingress, the mac header has already been pulled once.
    * At egress, skb_pospull_rcsum has to be done in case that
    --
    2.35.1
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