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Yinhao et al. recently reported:
Our fuzzing tool was able to create a BPF program which triggered
the below BUG condition inside pskb_expand_head.
[ 23.016047][T10006] kernel BUG at net/core/skbuff.c:2232!
[...]
[ 23.017301][T10006] RIP: 0010:pskb_expand_head+0x1519/0x1530
[...]
[ 23.021249][T10006] Call Trace:
[ 23.021387][T10006] <TASK>
[ 23.021507][T10006] ? __pfx_pskb_expand_head+0x10/0x10
[ 23.021725][T10006] __bpf_skb_change_head+0x22a/0x520
[ 23.021939][T10006] bpf_skb_change_head+0x34/0x1b0
[ 23.022143][T10006] ___bpf_prog_run+0xf70/0xb670
[ 23.022342][T10006] __bpf_prog_run32+0xed/0x140
[...]
The problem is that in __bpf_skb_change_head() we need to reject a
negative head_room as otherwise this propagates all the way to the
pskb_expand_head() from skb_cow(). For example, if the BPF test infra
passes a skb with gso_skb:1 to the BPF helper with a negative head_room
of -22, then this gets passed into skb_cow(). __skb_cow() in this
example calculates a delta of -86 which gets aligned to -64, and then
triggers BUG_ON(nhead < 0). Thus, reject malformed negative input.
Fixes: 3a0af8fd61 ("bpf: BPF for lightweight tunnel infrastructure")
Reported-by: Yinhao Hu <dddddd@hust.edu.cn>
Reported-by: Kaiyan Mei <M202472210@hust.edu.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Dongliang Mu <dzm91@hust.edu.cn>
Link: https://patch.msgid.link/20251023125532.182262-1-daniel@iogearbox.net
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