[PATCH 1/2] net: fix OOB read in qdisc_pkt_len_segs_init() on negative transport offset

[zjamg <ndaugoing@gmail.com>]

In qdisc_pkt_len_segs_init(), the header length up to the transport layer
is computed via skb_transport_offset() or skb_inner_transport_offset():

	/* mac layer + network layer */
	if (!skb->encapsulation) {
		if (unlikely(!skb_transport_header_was_set(skb)))
			return SKB_NOT_DROPPED_YET;
		hdr_len = skb_transport_offset(skb);
	} else {
		hdr_len = skb_inner_transport_offset(skb);
	}

Both offset accessors return a signed int, but hdr_len is declared as
unsigned int.

If a packet undergoes header operations (such as skb_vlan_untag() pulling
stacked VLAN tags without updating inner_transport_header, or other header
stripping that advances skb->data past the transport header), the offset
becomes negative.

When stored into unsigned int hdr_len, negative values are converted to
large positive numbers near UINT_MAX (e.g. (unsigned int)-16 is
0xFFFFFFF0). Subsequently:

1. pskb_may_pull(skb, hdr_len + sizeof(struct tcphdr)) evaluates
   0xFFFFFFF0 + 20, which overflows 32-bit unsigned arithmetic to 4.
   Since the packet length exceeds 4 bytes, pskb_may_pull() returns true,
   bypassing the bounds check.

2. th = (const struct tcphdr *)(skb->data + hdr_len) zero-extends hdr_len
   on 64-bit architectures, creating a wild pointer pointing ~4 GiB past
   skb->data into unmapped memory.

3. __tcp_hdrlen(th) dereferences th->doff, triggering an immediate kernel
   crash (page fault / KASAN wild-memory-access panic).

A similar unsigned overflow occurs for UDP (SKB_GSO_UDP_L4).

Fix this by declaring hdr_len as int to match the return types of
skb_transport_offset() and skb_inner_transport_offset(), and explicitly
rejecting negative offsets by returning SKB_DROP_REASON_SKB_BAD_GSO.

Fixes: 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()")
Cc: stable@vger.kernel.org
Signed-off-by: zjamg <ndaugoing@gmail.com>
---
 net/core/dev.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index ecfbd72d5d1a..f2dbf99181ec 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4159,8 +4159,9 @@ EXPORT_SYMBOL_GPL(validate_xmit_skb_list);
 static enum skb_drop_reason qdisc_pkt_len_segs_init(struct sk_buff *skb)
 {
 	struct skb_shared_info *shinfo = skb_shinfo(skb);
-	unsigned int hdr_len, tlen;
+	unsigned int tlen;
 	u16 gso_segs;
+	int hdr_len;
 
 	qdisc_skb_cb(skb)->pkt_len = skb->len;
 	if (!shinfo->gso_size) {
@@ -4182,6 +4183,8 @@ static enum skb_drop_reason qdisc_pkt_len_segs_init(struct sk_buff *skb)
 	} else {
 		hdr_len = skb_inner_transport_offset(skb);
 	}
+	if (unlikely(hdr_len < 0))
+		return SKB_DROP_REASON_SKB_BAD_GSO;
 	/* + transport layer */
 	if (likely(shinfo->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))) {
 		const struct tcphdr *th;
-- 
2.53.0