[PATCH v3 net 1/3] flow_dissector: avoid u16 truncation of skb->len when computing thoff

[Eric Dumazet <edumazet@kernel.org>]

__skb_flow_dissect() computes key_control->thoff using:

  key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);

min_t(u16, ...) casts both arguments to u16 before comparing them.
Whenever skb->len (or hlen) modulo 65536 is smaller than nhoff, the
truncated length wins and thoff is set to a bogus small value, even
when the dissection succeeded. For example, an IPv4/TCP frame with
skb->len == 65540 and nhoff == 34 gets thoff == 4, so callers such as
skb_probe_transport_header() point the transport header inside the
Ethernet header.

Such skbs are not exotic:
- At the time of commit d0c081b49137 ("flow_dissector: properly cap
  thoff field"), AF_PACKET with PACKET_VNET_HDR could already build GSO
  skbs larger than 64KB (MTU checks are skipped for GSO, and
  alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0
  pages on top of the linear part).
- BIG TCP now makes skbs larger than 64KB common.
- The following patch makes tun_get_user() dissect IFF_TAP frames
  before eth_type_trans() pulls the Ethernet header, so a GSO frame
  carrying a 65522..65535 byte L3 packet will be dissected with
  skb->len in [65536, 65549].

Compare as u32 instead. If the resulting offset cannot be represented
in the u16 key_control->thoff, cap it to U16_MAX and report the
dissection as failed rather than silently returning a wrong transport
offset. thoff is still set on failure, as some callers (such as
eth_get_headlen()) use it regardless of the return value.

Fixes: d0c081b49137 ("flow_dissector: properly cap thoff field")
Assisted-by: LLM
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/core/flow_dissector.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 8aa4f9b4df81016a3d9a97e0d561f6dfc51aa88d..27d8a01bc92306ff043389b4fde2d24af97d3106 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1071,6 +1071,7 @@ bool __skb_flow_dissect(const struct net *net,
 	int mpls_lse = 0;
 	int num_hdrs = 0;
 	u8 ip_proto = 0;
+	u32 thoff;
 	bool ret;
 
 	if (!data) {
@@ -1692,7 +1693,13 @@ bool __skb_flow_dissect(const struct net *net,
 	ret = true;
 
 out:
-	key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
+	thoff = min_t(u32, nhoff, skb ? skb->len : hlen);
+	if (unlikely(thoff > U16_MAX)) {
+		/* Cannot be represented in key_control->thoff. */
+		thoff = U16_MAX;
+		ret = false;
+	}
+	key_control->thoff = thoff;
 	key_basic->n_proto = proto;
 	key_basic->ip_proto = ip_proto;
 
-- 
2.56.0.rc1.315.gc6ed9934b7-goog