[BUG] net/ipv4: NULL pointer dereference in tcp_v4_rcv()

[co <co+2c72469dbbec34af@bugs.sh>]

We found a bug reachable in:

    path    net/ipv4
    crash   NULL pointer dereference in tcp_v4_rcv()
    commit  7cbfb180945c ("net/sched: sch_cake: fix autorate reconfiguration throttling")

Config, environment, the sanitizer report and a C reproducer follow.

== Notes ===============================================================
    If you patch the bug based on our artifacts, a tag would be
    appreciated:

Reported-by: co+2c72469dbbec34af@bugs.sh

    Everything in this mail is validated by the reproducer below.

    We also hold an LLM-generated root-cause analysis and a candidate
    patch. The patch passes an A/B test: the same reproducer panics the
    unpatched kernel and runs clean on the patched one. Neither has had
    human review, so both still require validation before you send or
    apply them. Available on:

        patch.diff  https://bugs.sh/b/2c72469dbbec34af/patch.diff
        report.md   https://bugs.sh/b/2c72469dbbec34af/report.md

    This is an open science project. The code and the full set of PoCs
    are not public at this moment, as we intend to disclose our findings
    in an ethical way.

    Happy to test patches. Complaints and suggestions about our work
    are welcome at:

        cedalion@bugs.sh

== Environment =========================================================
    Reproduced on  7cbfb180945c ("net/sched: sch_cake: fix autorate reconfiguration throttling")
    VM setup       https://bugs.sh/b/2c72469dbbec34af/run.sh
    config         https://bugs.sh/b/2c72469dbbec34af/config.gz
    poc            https://bugs.sh/b/2c72469dbbec34af/repro.c

== Sanitizer Report ====================================================
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000010: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000080-0x0000000000000087]
CPU: 1 UID: 0 PID: 145 Comm: exploit Not tainted 7.2.0+ #63 PREEMPTLAZY
RIP: 0010:tcp_v4_rcv+0x2637/0x3fe0  (net/ipv4/tcp_ipv4.c:1055 <- net/ipv4/tcp_ipv4.c:2333)
Call Trace:
 <IRQ>
 ip_protocol_deliver_rcu+0x66/0x310  (net/ipv4/ip_input.c:207 (discriminator 1))
 ip_local_deliver_finish+0x356/0x4f0  (net/ipv4/ip_input.c:241 (discriminator 1))
 ip_local_deliver+0x25c/0x2c0  (./include/linux/netfilter.h:325 <- ./include/linux/netfilter.h:319 <- net/ipv4/ip_input.c:262)
 ip_rcv+0x301/0x370  (./include/net/dst.h:480 (discriminator 6) <- net/ipv4/ip_input.c:492 (discriminator 6) <- ./include/linux/netfilter.h:325 (discriminator 6) <- ./include/linux/netfilter.h:319 (discriminator 6) <- net/ipv4/ip_input.c:612 (discriminator 6))
 __netif_receive_skb_one_core+0x166/0x1b0  (net/core/dev.c:6264 (discriminator 4))
 process_backlog+0x1d7/0x700  (net/core/dev.c:6377 <- net/core/dev.c:6728)
 __napi_poll+0xa1/0x560  (net/core/dev.c:7787)
 net_rx_action+0x401/0xd80  (net/core/dev.c:7850 <- net/core/dev.c:8007)
 handle_softirqs+0x1a4/0x620  (kernel/softirq.c:645)
 do_softirq.part.0+0x3b/0x60  (kernel/softirq.c:546 (discriminator 20))
 </IRQ>
 __local_bh_enable_ip+0x6d/0x70  (kernel/softirq.c:538 <- kernel/softirq.c:473)
 __dev_queue_xmit+0x83e/0x36a0  (./include/linux/bottom_half.h:33 (discriminator 1) <- ./include/linux/rcupdate.h:914 (discriminator 1) <- net/core/dev.c:4961 (discriminator 1))
 ip_finish_output2+0x8da/0x19c0  (./include/linux/netdevice.h:3461 <- ./include/net/neighbour.h:544 <- ./include/net/neighbour.h:558 <- net/ipv4/ip_output.c:236)
 ip_output+0x162/0x2c0  (./include/linux/netfilter.h:314 <- net/ipv4/ip_output.c:437)
 __ip_queue_xmit+0x7cf/0x1dc0  (net/ipv4/ip_output.c:533)
 __tcp_transmit_skb+0x303b/0x4a30  (net/ipv4/tcp_output.c:1716 (discriminator 4))
 tcp_connect+0x2a92/0x5060  (net/ipv4/tcp_output.c:1734 <- net/ipv4/tcp_output.c:4383)
 tcp_v4_connect+0x1186/0x17c0  (net/ipv4/tcp_ipv4.c:345)
 __inet_stream_connect+0x224/0xf60  (net/ipv4/af_inet.c:684)
 inet_stream_connect+0x57/0xa0  (net/ipv4/af_inet.c:755)
 __sys_connect+0x101/0x130  (net/socket.c:2183)
 __x64_sys_connect+0x72/0xb0  (net/socket.c:2189 <- net/socket.c:2186 <- net/socket.c:2186)
 do_syscall_64+0x94/0x450  (arch/x86/entry/syscall_64.c:61 <- arch/x86/entry/syscall_64.c:84)
 entry_SYSCALL_64_after_hwframe+0x76/0x7e  (arch/x86/entry/entry_64.S:121)
Kernel panic - not syncing: Fatal exception in interrupt


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