CVE-2026-43496: net/sched: sch_red: Replace direct dequeue call with peek and qdisc_dequeue_peeked

In the Linux kernel, the following vulnerability has been resolved:

net/sched: schred: Replace direct dequeue call with peek and qdiscdequeuepeeked

When red qdisc has children (eg qfq qdisc) whose peek() callback is qdiscpeekdequeued(), we could get a kernel panic. When the parent of such qdiscs (eg illustrated in patch #3 as tbf) wants to retrieve an skb from its child (red in this case), it will do the following: 1a. do a peek() - and when sensing there's an skb the child can offer, then - the child in this case(red) calls its child's (qfq) peek. qfq does the right thing and will return the gsoskb queue packet. Note: if there wasnt a gsoskb entry then qfq will store it there. 1b. invoke a dequeue() on the child (red). And herein lies the problem. - red will call the child's dequeue() which will essentially just try to grab something of qfq's queue.

[ 78.667668][ T363] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f] [ 78.667927][ T363] CPU: 1 UID: 0 PID: 363 Comm: ping Not tainted 7.1.0-rc1-00033-g46f74a3f7d57-dirty #790 PREEMPT(full) [ 78.668263][ T363] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011 [ 78.668486][ T363] RIP: 0010:qfqdequeue+0x446/0xc90 [schqfq] [ 78.668718][ T363] Code: 54 c0 e8 dd 90 00 f1 48 c7 c7 e0 03 54 c0 48 89 de e8 ce 90 00 f1 48 8d 7b 48 b8 ff ff 37 00 48 89 fa 48 c1 e0 2a 48 c1 ea 03 <80> 3c 02 00 74 05 e8 ef a1 e1 f1 48 8b 7b 48 48 8d 54 24 58 48 8d [ 78.669312][ T363] RSP: 0018:ffff88810de573e0 EFLAGS: 00010216 [ 78.669533][ T363] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000 [ 78.669790][ T363] RDX: 0000000000000009 RSI: 0000000000000004 RDI: 0000000000000048 [ 78.670044][ T363] RBP: ffff888110dc4000 R08: ffffffffb1b0885a R09: fffffbfff6ba9078 [ 78.670297][ T363] R10: 0000000000000003 R11: ffff888110e31c80 R12: 0000001880000000 [ 78.670560][ T363] R13: ffff888110dc4150 R14: ffff888110dc42b8 R15: 0000000000000200 [ 78.670814][ T363] FS: 00007f66a8f09c40(0000) GS:ffff888163428000(0000) knlGS:0000000000000000 [ 78.671110][ T363] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 78.671324][ T363] CR2: 000055db4c6a30a8 CR3: 000000010da67000 CR4: 0000000000750ef0 [ 78.671585][ T363] PKRU: 55555554 [ 78.671713][ T363] Call Trace: [ 78.671843][ T363] <TASK> [ 78.671936][ T363] ? pfxqfqdequeue+0x10/0x10 [schqfq] [ 78.672148][ T363] ? pfxprintk+0x10/0x10 [ 78.672322][ T363] ? srsoaliasreturnthunk+0x5/0xfbef5 [ 78.672496][ T363] ? lockdephardirqsonprepare+0xa8/0x1a0 [ 78.672706][ T363] ? srsoaliasreturnthunk+0x5/0xfbef5 [ 78.672875][ T363] ? tracehardirqson+0x19/0x1a0 [ 78.673047][ T363] reddequeue+0x65/0x270 [schred] [ 78.673217][ T363] ? srsoaliasreturnthunk+0x5/0xfbef5 [ 78.673385][ T363] tbfdequeue.cold+0xb0/0x70c [schtbf] [ 78.673566][ T363] qdiscrun+0x169/0x1900

The right thing to do in #1b is to grab the skb off gsoskb queue. This patchset fixes that issue by changing #1b to use qdiscdequeuepeeked() method instead.