[PATCH net] Drop packets with invalid headers to prevent KMSAN infoleak

[Daniel Yang <danielyangkang@gmail.com>]

KMSAN detects uninitialized memory stored to memory by
bpf_clone_redirect(). Adding a check to the transmission path to find
malformed headers prevents this issue. Specifically, we check if the length
of the data stored in skb is less than the minimum device header length. If
so, drop the packet since the skb cannot contain a valid device header.
Also check if mac_header_len(skb) is outside the range provided of valid
device header lengths.

Testing this patch with syzbot removes the bug.

Macro added to not affect normal builds.

Fixes: 88264981f208 ("Merge tag 'sched_ext-for-6.12' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext")
Reported-by: syzbot+346474e3bf0b26bd3090@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=346474e3bf0b26bd3090
Signed-off-by: Daniel Yang <danielyangkang@gmail.com>
---
v1: Enclosed in macro to not affect normal builds

 net/core/filter.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/core/filter.c b/net/core/filter.c
index cd3524cb3..9c5786f9c 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2191,6 +2191,14 @@ static int __bpf_redirect_common(struct sk_buff *skb, struct net_device *dev,
 		return -ERANGE;
 	}
 
+#if IS_ENABLED(CONFIG_KMSAN)
+	if (unlikely(skb->len < dev->min_header_len ||
+		     skb_mac_header_len(skb) < dev->min_header_len ||
+		     skb_mac_header_len(skb) > dev->hard_header_len)) {
+		kfree_skb(skb);
+		return -ERANGE;
+	}
+#endif
 	bpf_push_mac_rcsum(skb);
 	return flags & BPF_F_INGRESS ?
 	       __bpf_rx_skb(dev, skb) : __bpf_tx_skb(dev, skb);
-- 
2.39.2