FAILED: patch "[PATCH] virtio-net: fix received length check in big packets" failed to apply to 6.1-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 0c716703965ffc5ef4311b65cb5d84a703784717
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025110954-lunacy-murkiness-7783@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 0c716703965ffc5ef4311b65cb5d84a703784717 Mon Sep 17 00:00:00 2001
From: Bui Quang Minh <minhquangbui99@gmail.com>
Date: Thu, 30 Oct 2025 21:44:38 +0700
Subject: [PATCH] virtio-net: fix received length check in big packets

Since commit 4959aebba8c0 ("virtio-net: use mtu size as buffer length
for big packets"), when guest gso is off, the allocated size for big
packets is not MAX_SKB_FRAGS * PAGE_SIZE anymore but depends on
negotiated MTU. The number of allocated frags for big packets is stored
in vi->big_packets_num_skbfrags.

Because the host announced buffer length can be malicious (e.g. the host
vhost_net driver's get_rx_bufs is modified to announce incorrect
length), we need a check in virtio_net receive path. Currently, the
check is not adapted to the new change which can lead to NULL page
pointer dereference in the below while loop when receiving length that
is larger than the allocated one.

This commit fixes the received length check corresponding to the new
change.

Fixes: 4959aebba8c0 ("virtio-net: use mtu size as buffer length for big packets")
Cc: stable@vger.kernel.org
Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Tested-by: Lei Yang <leiyang@redhat.com>
Link: https://patch.msgid.link/20251030144438.7582-1-minhquangbui99@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index e6e650bc3bc3..8855a994e12b 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -910,17 +910,6 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 		goto ok;
 	}
 
-	/*
-	 * Verify that we can indeed put this data into a skb.
-	 * This is here to handle cases when the device erroneously
-	 * tries to receive more than is possible. This is usually
-	 * the case of a broken device.
-	 */
-	if (unlikely(len > MAX_SKB_FRAGS * PAGE_SIZE)) {
-		net_dbg_ratelimited("%s: too much data\n", skb->dev->name);
-		dev_kfree_skb(skb);
-		return NULL;
-	}
 	BUG_ON(offset >= PAGE_SIZE);
 	while (len) {
 		unsigned int frag_size = min((unsigned)PAGE_SIZE - offset, len);
@@ -2112,9 +2101,19 @@ static struct sk_buff *receive_big(struct net_device *dev,
 				   struct virtnet_rq_stats *stats)
 {
 	struct page *page = buf;
-	struct sk_buff *skb =
-		page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
+	struct sk_buff *skb;
 
+	/* Make sure that len does not exceed the size allocated in
+	 * add_recvbuf_big.
+	 */
+	if (unlikely(len > (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE)) {
+		pr_debug("%s: rx error: len %u exceeds allocated size %lu\n",
+			 dev->name, len,
+			 (vi->big_packets_num_skbfrags + 1) * PAGE_SIZE);
+		goto err;
+	}
+
+	skb = page_to_skb(vi, rq, page, 0, len, PAGE_SIZE, 0);
 	u64_stats_add(&stats->bytes, len - vi->hdr_len);
 	if (unlikely(!skb))
 		goto err;