[PATCH net-next v2 2/8] net: skbuff: don't BUG() on a bad frag_list layout in skb_segment()

[Josef Bacik <josef@toxicpanda.com>]

skb_segment()'s frag_list walk assumes the GRO-shaped layout it expects
and BUG()s when the layout doesn't match.  Anybody who can get a
malformed GSO skb to a segmentation point gets to crash the box.  Most
recently commit d5dc1e69fd72 ("inet: frags: strip GSO state from
fragments before reassembly") fixed one that an unprivileged user could
trigger with two writes to a tap device in their own user namespace.
commit 3382a1ed7f77 ("net: fix udp gso skb_segment after pull from
frag_list") fixed another.

skb_segment() already has an error path for a bad layout: the
too-many-frags check sets -EINVAL and frees the partial segment list.
Take that path for the four layout checks, with a
DEBUG_NET_WARN_ON_ONCE() for debug kernels.  The packet gets dropped,
which is what should happen to a packet we can't segment.

The one check that runs after skb_clone() and before the clone is
linked into the segment list frees the clone itself.

Assisted-by: LLM
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 net/core/skbuff.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d226fe77d484..a6821ab13969 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4916,7 +4916,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 
 		if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
 		    (skb_headlen(list_skb) == len || sg)) {
-			BUG_ON(skb_headlen(list_skb) > len);
+			if (unlikely(skb_headlen(list_skb) > len)) {
+				DEBUG_NET_WARN_ON_ONCE(1);
+				err = -EINVAL;
+				goto err;
+			}
 
 			nskb = skb_clone(list_skb, GFP_ATOMIC);
 			if (unlikely(!nskb))
@@ -4929,7 +4933,12 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 			pos += skb_headlen(list_skb);
 
 			while (pos < offset + len) {
-				BUG_ON(i >= nfrags);
+				if (unlikely(i >= nfrags)) {
+					DEBUG_NET_WARN_ON_ONCE(1);
+					kfree_skb(nskb);
+					err = -EINVAL;
+					goto err;
+				}
 
 				size = skb_frag_size(frag);
 				if (pos + size > offset + len)
@@ -5036,9 +5045,17 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 				skb_shinfo(nskb)->flags |= skb_shinfo(frag_skb)->flags & SKBFL_SHARED_FRAG;
 
 				if (!skb_headlen(list_skb)) {
-					BUG_ON(!nfrags);
+					if (unlikely(!nfrags)) {
+						DEBUG_NET_WARN_ON_ONCE(1);
+						err = -EINVAL;
+						goto err;
+					}
 				} else {
-					BUG_ON(!list_skb->head_frag);
+					if (unlikely(!list_skb->head_frag)) {
+						DEBUG_NET_WARN_ON_ONCE(1);
+						err = -EINVAL;
+						goto err;
+					}
 
 					/* to make room for head_frag. */
 					i--;

-- 
2.55.0