CVE-2024-49949

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

net: avoid potential underflow in qdisc_pkt_len_init() with UFO

After commit 7c6d2ecbda83 ("net: be more gentle about silly gso

requests coming from user") virtio_net_hdr_to_skb() had sanity check

to detect malicious attempts from user space to cook a bad GSO packet.

Then commit cf9acc90c80ec ("net: virtio_net_hdr_to_skb: count

transport header in UFO") while fixing one issue, allowed user space

to cook a GSO packet with the following characteristic :

IPv4 SKB_GSO_UDP, gso_size=3, skb->len = 28.

When this packet arrives in qdisc_pkt_len_init(), we end up

with hdr_len = 28 (IPv4 header + UDP header), matching skb->len

Then the following sets gso_segs to 0 :

gso_segs = DIV_ROUND_UP(skb->len - hdr_len,

shinfo->gso_size);

Then later we set qdisc_skb_cb(skb)->pkt_len to back to zero :/

qdisc_skb_cb(skb)->pkt_len += (gso_segs - 1) * hdr_len;

This leads to the following crash in fq_codel [1]

qdisc_pkt_len_init() is best effort, we only want an estimation

of the bytes sent on the wire, not crashing the kernel.

This patch is fixing this particular issue, a following one

adds more sanity checks for another potential bug.

[1]

[ 70.724101] BUG: kernel NULL pointer dereference, address: 0000000000000000

[ 70.724561] #PF: supervisor read access in kernel mode

[ 70.724561] #PF: error_code(0x0000) - not-present page

[ 70.724561] PGD 10ac61067 P4D 10ac61067 PUD 107ee2067 PMD 0

[ 70.724561] Oops: Oops: 0000 [#1] SMP NOPTI

[ 70.724561] CPU: 11 UID: 0 PID: 2163 Comm: b358537762 Not tainted 6.11.0-virtme #991

[ 70.724561] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014

[ 70.724561] RIP: 0010:fq_codel_enqueue (net/sched/sch_fq_codel.c:120 net/sched/sch_fq_codel.c:168 net/sched/sch_fq_codel.c:230) sch_fq_codel

[ 70.724561] Code: 24 08 49 c1 e1 06 44 89 7c 24 18 45 31 ed 45 31 c0 31 ff 89 44 24 14 4c 03 8b 90 01 00 00 eb 04 39 ca 73 37 4d 8b 39 83 c7 01 <49> 8b 17 49 89 11 41 8b 57 28 45 8b 5f 34 49 c7 07 00 00 00 00 49

All code

========

0: 24 08 and $0x8,%al

2: 49 c1 e1 06 shl $0x6,%r9

6: 44 89 7c 24 18 mov %r15d,0x18(%rsp)

b: 45 31 ed xor %r13d,%r13d

e: 45 31 c0 xor %r8d,%r8d

11: 31 ff xor %edi,%edi

13: 89 44 24 14 mov %eax,0x14(%rsp)

17: 4c 03 8b 90 01 00 00 add 0x190(%rbx),%r9

1e: eb 04 jmp 0x24

20: 39 ca cmp %ecx,%edx

22: 73 37 jae 0x5b

24: 4d 8b 39 mov (%r9),%r15

27: 83 c7 01 add $0x1,%edi

2a:* 49 8b 17 mov (%r15),%rdx <-- trapping instruction

2d: 49 89 11 mov %rdx,(%r9)

30: 41 8b 57 28 mov 0x28(%r15),%edx

34: 45 8b 5f 34 mov 0x34(%r15),%r11d

38: 49 c7 07 00 00 00 00 movq $0x0,(%r15)

3f: 49 rex.WB

Code starting with the faulting instruction

===========================================

0: 49 8b 17 mov (%r15),%rdx

3: 49 89 11 mov %rdx,(%r9)

6: 41 8b 57 28 mov 0x28(%r15),%edx

a: 45 8b 5f 34 mov 0x34(%r15),%r11d

e: 49 c7 07 00 00 00 00 movq $0x0,(%r15)

15: 49 rex.WB

[ 70.724561] RSP: 0018:ffff95ae85e6fb90 EFLAGS: 00000202

[ 70.724561] RAX: 0000000002000000 RBX: ffff95ae841de000 RCX: 0000000000000000

[ 70.724561] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000001

[ 70.724561] RBP: ffff95ae85e6fbf8 R08: 0000000000000000 R09: ffff95b710a30000

[ 70.724561] R10: 0000000000000000 R11: bdf289445ce31881 R12: ffff95ae85e6fc58

[ 70.724561] R13: 0000000000000000 R14: 0000000000000040 R15: 0000000000000000

[ 70.724561] FS: 000000002c5c1380(0000) GS:ffff95bd7fcc0000(0000) knlGS:0000000000000000

[ 70.724561] CS: 0010 DS: 0000 ES: 0000 C

---truncated---