CVE-2024-50039: In the Linux kernel, the following vulnerability has been resolved:  net/sched: accept TCA_STAB only for root qdisc  Most qdiscs maintain their backlog using qdisc_pkt_len(skb) on the assumption it is invariant between the enqueue() and dequeue() handlers.  Unfortunately syzbot can crash a host rather easily using a TBF + SFQ combination, with an STAB on SFQ [1]  We can't support TCA_STAB on arbitrary level, this would require to maintain per-qdisc storage.  [1] [   88.796496] BUG: kernel NULL pointer dereference, address: 0000000000000000 [   88.798611] #PF: supervisor read access in kernel mode [   88.799014] #PF: error_code(0x0000) - not-present page [   88.799506] PGD 0 P4D 0 [   88.799829] Oops: Oops: 0000 [#1] SMP NOPTI [   88.800569] CPU: 14 UID: 0 PID: 2053 Comm: b371744477 Not tainted 6.12.0-rc1-virtme #1117 [   88.801107] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [   88.801779] RIP: 0010:sfq_dequeue (net/sched/sch_sfq.c:272 net/sched/sch_sfq.c:499) sch_sfq [ 88.802544] Code: 0f b7 50 12 48 8d 04 d5 00 00 00 00 48 89 d6 48 29 d0 48 8b 91 c0 01 00 00 48 c1 e0 03 48 01 c2 66 83 7a 1a 00 7e c0 48 8b 3a <4c> 8b 07 4c 89 02 49 89 50 08 48 c7 47 08 00 00 00 00 48 c7 07 00 All code ========    0: 0f b7 50 12           movzwl 0x12(%rax),%edx    4: 48 8d 04 d5 00 00 00  lea    0x0(,%rdx,8),%rax    b: 00    c: 48 89 d6              mov    %rdx,%rsi    f: 48 29 d0              sub    %rdx,%rax   12: 48 8b 91 c0 01 00 00  mov    0x1c0(%rcx),%rdx   19: 48 c1 e0 03           shl    $0x3,%rax   1d: 48 01 c2              add    %rax,%rdx   20: 66 83 7a 1a 00        cmpw   $0x0,0x1a(%rdx)   25: 7e c0                 jle    0xffffffffffffffe7   27: 48 8b 3a              mov    (%rdx),%rdi   2a:* 4c 8b 07              mov    (%rdi),%r8  <-- trapping instruction   2d: 4c 89 02              mov    %r8,(%rdx)   30: 49 89 50 08           mov    %rdx,0x8(%r8)   34: 48 c7 47 08 00 00 00  movq   $0x0,0x8(%rdi)   3b: 00   3c: 48                    rex.W   3d: c7                    .byte 0xc7   3e: 07                    (bad)  ...  Code starting with the faulting instruction ===========================================    0: 4c 8b 07              mov    (%rdi),%r8    3: 4c 89 02              mov    %r8,(%rdx)    6: 49 89 50 08           mov    %rdx,0x8(%r8)    a: 48 c7 47 08 00 00 00  movq   $0x0,0x8(%rdi)   11: 00   12: 48                    rex.W   13: c7                    .byte 0xc7   14: 07                    (bad)  ... [   88.803721] RSP: 0018:ffff9a1f892b7d58 EFLAGS: 00000206 [   88.804032] RAX: 0000000000000000 RBX: ffff9a1f8420c800 RCX: ffff9a1f8420c800 [   88.804560] RDX: ffff9a1f81bc1440 RSI: 0000000000000000 RDI: 0000000000000000 [   88.805056] RBP: ffffffffc04bb0e0 R08: 0000000000000001 R09: 00000000ff7f9a1f [   88.805473] R10: 000000000001001b R11: 0000000000009a1f R12: 0000000000000140 [   88.806194] R13: 0000000000000001 R14: ffff9a1f886df400 R15: ffff9a1f886df4ac [   88.806734] FS:  00007f445601a740(0000) GS:ffff9a2e7fd80000(0000) knlGS:0000000000000000 [   88.807225] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [   88.807672] CR2: 0000000000000000 CR3: 000000050cc46000 CR4: 00000000000006f0 [   88.808165] Call Trace: [   88.808459]  <TASK> [   88.808710] ? __die (arch/x86/kernel/dumpstack.c:421 arch/x86/kernel/dumpstack.c:434) [   88.809261] ? page_fault_oops (arch/x86/mm/fault.c:715) [   88.809561] ? exc_page_fault (./arch/x86/include/asm/irqflags.h:26 ./arch/x86/include/asm/irqflags.h:87 ./arch/x86/include/asm/irqflags.h:147 arch/x86/mm/fault.c:1489 arch/x86/mm/fault.c:1539) [   88.809806] ? asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:623) [   88.810074] ? sfq_dequeue (net/sched/sch_sfq.c:272 net/sched/sch_sfq.c:499) sch_sfq [   88.810411] sfq_reset (net/sched/sch_sfq.c:525) sch_sfq [   88.810671] qdisc_reset (./include/linux/skbuff.h:2135 ./include/linux/skbuff.h:2441 ./include/linux/skbuff.h:3304 ./include/linux/skbuff.h:3310 net/sched/sch_g ---truncated---