CVE-2023-53354

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

skbuff: skb_segment, Call zero copy functions before using skbuff frags

Commit bf5c25d60861 ("skbuff: in skbsegment, call zerocopy functions once per nskb") added the call to zero copy functions in skbsegment(). The change introduced a bug in skbsegment() because skborphan_frags() may possibly change the number of fragments or allocate new fragments altogether leaving nrfrags and frag to point to the old values. This can cause a panic with stacktrace like the one below.

[ 193.894380] BUG: kernel NULL pointer dereference, address: 00000000000000bc [ 193.895273] CPU: 13 PID: 18164 Comm: vh-net-17428 Kdump: loaded Tainted: G O 5.15.123+ #26 [ 193.903919] RIP: 0010:skbsegment+0xb0e/0x12f0 [ 194.021892] Call Trace: [ 194.027422] <TASK> [ 194.072861] tcpgsosegment+0x107/0x540 [ 194.082031] inetgsosegment+0x15c/0x3d0 [ 194.090783] skbmacgsosegment+0x9f/0x110 [ 194.095016] __skbgsosegment+0xc1/0x190 [ 194.103131] netemenqueue+0x290/0xb10 [schnetem] [ 194.107071] devqdiscenqueue+0x16/0x70 [ 194.110884] __devqueuexmit+0x63b/0xb30 [ 194.121670] bondstartxmit+0x159/0x380 [bonding] [ 194.128506] devhardstart_xmit+0xc3/0x1e0 [ 194.131787] __devqueuexmit+0x8a0/0xb30 [ 194.138225] macvlanstartxmit+0x4f/0x100 [macvlan] [ 194.141477] devhardstartxmit+0xc3/0x1e0 [ 194.144622] schdirect_xmit+0xe3/0x280 [ 194.147748] _devqueuexmit+0x54a/0xb30 [ 194.154131] tapgetuser+0x2a8/0x9c0 [tap] [ 194.157358] tapsendmsg+0x52/0x8e0 [tap] [ 194.167049] handletxzerocopy+0x14e/0x4c0 [vhostnet] [ 194.173631] handletx+0xcd/0xe0 [vhostnet] [ 194.176959] vhostworker+0x76/0xb0 [vhost] [ 194.183667] kthread+0x118/0x140 [ 194.190358] retfromfork+0x1f/0x30 [ 194.193670] </TASK>

In this case calling skborphanfrags() updated nrfrags leaving nrfrags local variable in skbsegment() stale. This resulted in the code hitting i >= nrfrags prematurely and trying to move to next fragskb using listskb pointer, which was NULL, and caused kernel panic. Move the call to zero copy functions before using frags and nr_frags.