[PATCH net] netrom: do some basic forms of validation on incoming frames

[PATCH net] netrom: do some basic forms of validation on incoming frames

[Greg Kroah-Hartman]

There is a lack of much validation of frame size coming from a
netrom-based device.  While these devices are "trusted" doing some
sanity checks is good to at least keep the fuzzing tools happy when they
stumble across this ancient protocol and light up with a range of bug
reports.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Simon Horman <horms@kernel.org>
Cc: linux-hams@vger.kernel.org
Assisted-by: gregkh_clanker_2000
Reviewed-by: Yizhe Zhuang <yizhe@darknavy.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/netrom/af_netrom.c | 6 ++++++
 net/netrom/nr_route.c  | 6 +++---
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/netrom/af_netrom.c b/net/netrom/af_netrom.c
index b816c56124ab..b605891bf86e 100644
--- a/net/netrom/af_netrom.c
+++ b/net/netrom/af_netrom.c
@@ -885,6 +885,9 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
 	 *	skb->data points to the netrom frame start
 	 */
 
+	if (skb->len < NR_NETWORK_LEN + NR_TRANSPORT_LEN)
+		return 0;
+
 	src  = (ax25_address *)(skb->data + 0);
 	dest = (ax25_address *)(skb->data + 7);
 
@@ -963,6 +966,9 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
 
 	sk = nr_find_listener(dest);
 
+	if (skb->len < NR_NETWORK_LEN + NR_TRANSPORT_LEN + 1 + AX25_ADDR_LEN)
+		return 0;
+
 	user = (ax25_address *)(skb->data + 21);
 
 	if (sk == NULL || sk_acceptq_is_full(sk) ||
diff --git a/net/netrom/nr_route.c b/net/netrom/nr_route.c
index 9cc29ae85b06..bf60f5682a4f 100644
--- a/net/netrom/nr_route.c
+++ b/net/netrom/nr_route.c
@@ -755,10 +755,10 @@ int nr_route_frame(struct sk_buff *skb, ax25_cb *ax25)
 	struct sk_buff *nskb, *oskb;
 
 	/*
-	 * Reject malformed packets early. Check that it contains at least 2
-	 * addresses and 1 byte more for Time-To-Live
+	 * Reject malformed packets early. Check that it contains at least
+	 * the network and transport headers (20 bytes).
 	 */
-	if (skb->len < 2 * sizeof(ax25_address) + 1)
+	if (skb->len < NR_NETWORK_LEN + NR_TRANSPORT_LEN)
 		return 0;
 
 	nr_src  = (ax25_address *)(skb->data + 0);
-- 
2.53.0

Re: [PATCH net] netrom: do some basic forms of validation on incoming frames

[Simon Horman]

On Tue, Apr 07, 2026 at 10:45:31AM +0200, Greg Kroah-Hartman wrote:
> There is a lack of much validation of frame size coming from a
> netrom-based device.  While these devices are "trusted" doing some
> sanity checks is good to at least keep the fuzzing tools happy when they
> stumble across this ancient protocol and light up with a range of bug
> reports.
> 
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: Simon Horman <horms@kernel.org>
> Cc: linux-hams@vger.kernel.org
> Assisted-by: gregkh_clanker_2000
> Reviewed-by: Yizhe Zhuang <yizhe@darknavy.com>
> Cc: stable <stable@kernel.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Hi Greg 2000!

I expect that checking skb->len isn't sufficient here
and pskb_may_pull needs to be used to ensure that
the data is also available in the linear section of the skb.

Also, although I'm all for incremental enhancements,
I do suspect that similar problems exist in the call
chain of these functions.

...