netdev - Re: [PATCH net] netrom: validate header lengths in nr_rx_frame() using pskb_may

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Message-ID: <CANn89iJM3CV-_2jWMMspH52RvfWtep-3srctf47NkYUkTTboSg@mail.gmail.com>
Date: Mon, 1 Sep 2025 00:37:48 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Stanislav Fort <disclosure@...le.com>
Cc: netdev@...r.kernel.org, security@...nel.org, kuba@...nel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH net] netrom: validate header lengths in nr_rx_frame()
 using pskb_may_pull()

On Wed, Aug 27, 2025 at 7:38 AM Stanislav Fort <disclosure@...le.com> wrote:
>
> NET/ROM nr_rx_frame() dereferences the 5-byte transport header
> unconditionally. nr_route_frame() currently accepts frames as short as
> NR_NETWORK_LEN (15 bytes), which can lead to small out-of-bounds reads
> on short frames.
>
> Fix by using pskb_may_pull() in nr_rx_frame() to ensure the full
> NET/ROM network + transport header is present before accessing it, and
> guard the extra fields used by NR_CONNREQ (window, user address, and the
> optional BPQ timeout extension) with additional pskb_may_pull() checks.
>
> This aligns with recent fixes using pskb_may_pull() to validate header
> availability.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: Stanislav Fort <disclosure@...le.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Stanislav Fort <disclosure@...le.com>
> ---
>  net/netrom/af_netrom.c | 12 +++++++++++-
>  net/netrom/nr_route.c  |  2 +-
>  2 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/net/netrom/af_netrom.c b/net/netrom/af_netrom.c
> index 3331669d8e33..1fbaa161288a 100644
> --- a/net/netrom/af_netrom.c
> +++ b/net/netrom/af_netrom.c
> @@ -885,6 +885,10 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
>          *      skb->data points to the netrom frame start
>          */
>
> +       /* Ensure NET/ROM network + transport header are present */
> +       if (!pskb_may_pull(skb, NR_NETWORK_LEN + NR_TRANSPORT_LEN))
> +               return 0;
> +
>         src  = (ax25_address *)(skb->data + 0);
>         dest = (ax25_address *)(skb->data + 7);
>
> @@ -961,6 +965,12 @@ int nr_rx_frame(struct sk_buff *skb, struct net_device *dev)
>                 return 0;
>         }
>
> +       /* Ensure NR_CONNREQ fields (window + user address) are present */
> +       if (!pskb_may_pull(skb, 21 + AX25_ADDR_LEN)) {

If skb->head is reallocated by this pskb_may_pull(), dest variable
might point to a freed piece of memory

(old skb->head)

As far as netrom is concerned, I would force a full linearization of
the packet very early

It is also unclear if the bug even exists in the first place.

Can you show the stack trace leading to this function being called
from an arbitrary
provider (like a packet being fed by malicious user space)

For instance nr_rx_frame() can be called from net/netrom/nr_loopback.c
with non malicious packet.

For the remaining caller (nr_route_frame()), it is unclear to me.