CVE-2021-47557

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

net/sched: sch_ets: don't peek at classes beyond 'nbands'

when the number of DRR classes decreases, the round-robin active list can contain elements that have already been freed in etsqdiscchange(). As a consequence, it's possible to see a NULL dereference crash, caused by the attempt to call cl->qdisc->ops->peek(cl->qdisc) when cl->qdisc is NULL:

BUG: kernel NULL pointer dereference, address: 0000000000000018 #PF: supervisor read access in kernel mode #PF: errorcode(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP NOPTI CPU: 1 PID: 910 Comm: mausezahn Not tainted 5.16.0-rc1+ #475 Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014 RIP: 0010:etsqdiscdequeue+0x129/0x2c0 [schets] Code: c5 01 41 39 ad e4 02 00 00 0f 87 18 ff ff ff 49 8b 85 c0 02 00 00 49 39 c4 0f 84 ba 00 00 00 49 8b ad c0 02 00 00 48 8b 7d 10 <48> 8b 47 18 48 8b 40 38 0f ae e8 ff d0 48 89 c3 48 85 c0 0f 84 9d RSP: 0000:ffffbb36c0b5fdd8 EFLAGS: 00010287 RAX: ffff956678efed30 RBX: 0000000000000000 RCX: 0000000000000000 RDX: 0000000000000002 RSI: ffffffff9b938dc9 RDI: 0000000000000000 RBP: ffff956678efed30 R08: e2f3207fe360129c R09: 0000000000000000 R10: 0000000000000001 R11: 0000000000000001 R12: ffff956678efeac0 R13: ffff956678efe800 R14: ffff956611545000 R15: ffff95667ac8f100 FS: 00007f2aa9120740(0000) GS:ffff95667b800000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000018 CR3: 000000011070c000 CR4: 0000000000350ee0 Call Trace: <TASK> qdiscpeekdequeued+0x29/0x70 [schets] tbfdequeue+0x22/0x260 [schtbf] _qdiscrun+0x7f/0x630 nettxaction+0x290/0x4c0 _dosoftirq+0xee/0x4f8 irqexitrcu+0xf4/0x130 sysvecapictimerinterrupt+0x52/0xc0 asmsysvecapictimerinterrupt+0x12/0x20 RIP: 0033:0x7f2aa7fc9ad4 Code: b9 ff ff 48 8b 54 24 18 48 83 c4 08 48 89 ee 48 89 df 5b 5d e9 ed fc ff ff 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 f3 0f 1e fa <53> 48 83 ec 10 48 8b 05 10 64 33 00 48 8b 00 48 85 c0 0f 85 84 00 RSP: 002b:00007ffe5d33fab8 EFLAGS: 00000202 RAX: 0000000000000002 RBX: 0000561f72c31460 RCX: 0000561f72c31720 RDX: 0000000000000002 RSI: 0000561f72c31722 RDI: 0000561f72c31720 RBP: 000000000000002a R08: 00007ffe5d33fa40 R09: 0000000000000014 R10: 0000000000000000 R11: 0000000000000246 R12: 0000561f7187e380 R13: 0000000000000000 R14: 0000000000000000 R15: 0000561f72c31460 </TASK> Modules linked in: schets schtbf dummy rfkill iTCOwdt intelraplmsr iTCOvendorsupport intelraplcommon joydev virtioballoon lpcich i2ci801 i2csmbus pcspkr iptables xfs libcrc32c crct10difpclmul crc32pclmul crc32cintel ahci libahci ghashclmulniintel serioraw libata virtioblk virtioconsole virtionet netfailover failover sunrpc dmmirror dmregionhash dmlog dm_mod CR2: 0000000000000018

Ensuring that 'alist' was never zeroed [1] was not sufficient, we need to remove from the active list those elements that are no more SP nor DRR.

[1] https://lore.kernel.org/netdev/60d274838bf09777f0371253416e8af71360bc08.1633609148.git.dcaratti@redhat.com/

v3: fix race between etsqdiscchange() and etsqdiscdequeue() delisting DRR classes beyond 'nbands' in etsqdiscchange() with the qdisc lock acquired, thanks to Cong Wang.

v2: when a NULL qdisc is found in the DRR active list, try to dequeue skb from the next list item.