Subject: [PATCH] net: linearizing skb when downgrade gso_size

From: Fred Li <[email-masked]>
Date: Mon, 8 Jul 2024 22:31:28 +0800

Here is a patch that linearizing skb when downgrade
gso_size and sg should disabled, If there are no issues,
I will submit a formal patch shortly.

Signed-off-by: Fred Li <[email-masked]>
---
 include/linux/skbuff.h | 22 ++++++++++++++++++++++
 net/core/filter.c      | 16 ++++++++++++----
 net/core/skbuff.c      | 19 ++-----------------
 3 files changed, 36 insertions(+), 21 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 5f11f9873341..99b7fc1e826a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2400,6 +2400,28 @@ static inline unsigned int skb_headlen(const struct sk_buff *skb)
 	return skb->len - skb->data_len;
 }
 
+static inline bool skb_is_nonsg(const struct sk_buff *skb)
+{
+	struct sk_buff *list_skb = skb_shinfo(skb)->frag_list;
+	struct sk_buff *check_skb;
+	for (check_skb = list_skb; check_skb; check_skb = check_skb->next) {
+		if (skb_headlen(check_skb) && !check_skb->head_frag) {
+			/* gso_size is untrusted, and we have a frag_list with
+                         * a linear non head_frag item.
+                         *
+                         * If head_skb's headlen does not fit requested gso_size,
+                         * it means that the frag_list members do NOT terminate
+                         * on exact gso_size boundaries. Hence we cannot perform
+                         * skb_frag_t page sharing. Therefore we must fallback to
+                         * copying the frag_list skbs; we do so by disabling SG.
+                         */
+			return true;
+		}
+	}
+
+	return false;
+}
+
 static inline unsigned int __skb_pagelen(const struct sk_buff *skb)
 {
 	unsigned int i, len = 0;
diff --git a/net/core/filter.c b/net/core/filter.c
index df4578219e82..c0e6e7f28635 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3525,13 +3525,21 @@ static int bpf_skb_net_grow(struct sk_buff *skb, u32 off, u32 len_diff,
 	if (skb_is_gso(skb)) {
 		struct skb_shared_info *shinfo = skb_shinfo(skb);
 
-		/* Due to header grow, MSS needs to be downgraded. */
-		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
-			skb_decrease_gso_size(shinfo, len_diff);
-
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= gso_type;
 		shinfo->gso_segs = 0;
+
+		/* Due to header grow, MSS needs to be downgraded.
+		 * There is BUG_ON When segment the frag_list with
+		 * head_frag true so linearize skb after downgrade
+		 * the MSS.
+		 */
+		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO)) {
+			skb_decrease_gso_size(shinfo, len_diff);
+			if (skb_is_nonsg(skb))
+				return skb_linearize(skb) ? : 0;
+		}
+
 	}
 
 	return 0;
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index b1dab1b071fc..81e018185527 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4458,23 +4458,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 
 	if ((skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY) &&
 	    mss != GSO_BY_FRAGS && mss != skb_headlen(head_skb)) {
-		struct sk_buff *check_skb;
-
-		for (check_skb = list_skb; check_skb; check_skb = check_skb->next) {
-			if (skb_headlen(check_skb) && !check_skb->head_frag) {
-				/* gso_size is untrusted, and we have a frag_list with
-				 * a linear non head_frag item.
-				 *
-				 * If head_skb's headlen does not fit requested gso_size,
-				 * it means that the frag_list members do NOT terminate
-				 * on exact gso_size boundaries. Hence we cannot perform
-				 * skb_frag_t page sharing. Therefore we must fallback to
-				 * copying the frag_list skbs; we do so by disabling SG.
-				 */
-				features &= ~NETIF_F_SG;
-				break;
-			}
-		}
+		if (skb_is_nonsg(head_skb))
+			features &= ~NETIF_F_SG;
 	}
 
 	__skb_push(head_skb, doffset);
-- 
2.33.0