bpf: Check negative offsets in __bpf_skb_min_len()

skb_network_offset() and skb_transport_offset() can be negative when
they are called after we pull the transport header, for example, when
we use eBPF sockmap at the point of ->sk_data_ready().

__bpf_skb_min_len() uses an unsigned int to get these offsets, this
leads to a very large number which then causes bpf_skb_change_tail()
failed unexpectedly.

Fix this by using a signed int to get these offsets and ensure the
minimum is at least zero.

Fixes: 5293efe62df8 ("bpf: add bpf_skb_change_tail helper")
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20241213034057.246437-2-xiyou.wangcong@gmail.com

diff --git a/net/core/filter.c b/net/core/filter.c
index 21131ec25f24a7f4a6575d1c87c78939f98c24ba..834614071727ab92cee759dc788ec2ee6f92284b 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3734,13 +3734,22 @@ static const struct bpf_func_proto bpf_skb_adjust_room_proto = {
 
 static u32 __bpf_skb_min_len(const struct sk_buff *skb)
 {
-       u32 min_len = skb_network_offset(skb);
+       int offset = skb_network_offset(skb);
+       u32 min_len = 0;
 
-       if (skb_transport_header_was_set(skb))
-               min_len = skb_transport_offset(skb);
-       if (skb->ip_summed == CHECKSUM_PARTIAL)
-               min_len = skb_checksum_start_offset(skb) +
-                         skb->csum_offset + sizeof(__sum16);
+       if (offset > 0)
+               min_len = offset;
+       if (skb_transport_header_was_set(skb)) {
+               offset = skb_transport_offset(skb);
+               if (offset > 0)
+                       min_len = offset;
+       }
+       if (skb->ip_summed == CHECKSUM_PARTIAL) {
+               offset = skb_checksum_start_offset(skb) +
+                        skb->csum_offset + sizeof(__sum16);
+               if (offset > 0)
+                       min_len = offset;
+       }
        return min_len;
 }