Vulnerability CVE-2024-35941
Published: 2024-05-19

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

net: skbuff: add overflow debug check to pull/push helpers

syzbot managed to trigger following splat:
BUG: KASAN: use-after-free in __skb_flow_dissect+0x4a3b/0x5e50
Read of size 1 at addr ffff888208a4000e by task a.out/2313
[..]
__skb_flow_dissect+0x4a3b/0x5e50
__skb_get_hash+0xb4/0x400
ip_tunnel_xmit+0x77e/0x26f0
ipip_tunnel_xmit+0x298/0x410
..

Analysis shows that the skb has a valid ->head, but bogus ->data
pointer.

skb->data gets its bogus value via the neigh layer, which does:

1556 __skb_pull(skb, skb_network_offset(skb));

... and the skb was already dodgy at this point:

skb_network_offset(skb) returns a negative value due to an
earlier overflow of skb->network_header (u16). __skb_pull thus
"adjusts" skb->data by a huge offset, pointing outside skb->head
area.

Allow debug builds to splat when we try to pull/push more than
INT_MAX bytes.

After this, the syzkaller reproducer yields a more precise splat
before the flow dissector attempts to read off skb->data memory:

WARNING: CPU: 5 PID: 2313 at include/linux/skbuff.h:2653 neigh_connected_output+0x28e/0x400
ip_finish_output2+0xb25/0xed0
iptunnel_xmit+0x4ff/0x870
ipgre_xmit+0x78e/0xbb0

 References:
https://git.kernel.org/stable/c/8af60bb2b215f478b886f1d6d302fefa7f0b917d
https://git.kernel.org/stable/c/1b2b26595bb09febf14c5444c873ac4ec90a5a77
https://git.kernel.org/stable/c/fff05b2b004d9a8a2416d08647f3dc9068e357c8
https://git.kernel.org/stable/c/219eee9c0d16f1b754a8b85275854ab17df0850a
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