If skb_cow() is passed a headroom <= -NET_SKB_PAD, it will trigger a
BUG. As a result, use cases should avoid calling with a headroom that
is negative to prevent triggering this issue.

This is the same code pattern fixed in Commit 58fc7342b529 ("ipv6:
BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()").

In cipso_v4_skbuff_setattr(), len_delta can become negative, leading to
a negative headroom passed to skb_cow(). However, the BUG is not
triggerable because the condition headroom <= -NET_SKB_PAD cannot be
satisfied due to limits on the IPv4 options header size.

Avoid potential problems in the future by only using skb_cow() to grow
the skb headroom.

Signed-off-by: Will Rosenberg <redacted>
---

Notes:
    Given that IPv4 option length should not change,
    this may not be a worthwhile patch.
    
    Apologies in advance if this ends up being a waste
    of time.

 net/ipv4/cipso_ipv4.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/cipso_ipv4.c b/net/ipv4/cipso_ipv4.c
index 709021197e1c..32b951ebc0c2 100644
--- a/net/ipv4/cipso_ipv4.c
+++ b/net/ipv4/cipso_ipv4.c

@@ -2196,7 +2196,8 @@ int cipso_v4_skbuff_setattr(struct sk_buff *skb,
 	/* if we don't ensure enough headroom we could panic on the skb_push()
 	 * call below so make sure we have enough, we are also "mangling" the
 	 * packet so we should probably do a copy-on-write call anyway */
-	ret_val = skb_cow(skb, skb_headroom(skb) + len_delta);
+	ret_val = skb_cow(skb,
+			  skb_headroom(skb) + (len_delta > 0 ? len_delta : 0));
 	if (ret_val < 0)
 		return ret_val;
 

base-commit: 58bae918d73e3b6cd57d1e39fcf7c75c7dd1a8fe
-- 
2.34.1