Re: [PATCH net] can: raw: fix ro->uniq use-after-free in raw_rcv()

[Oliver Hartkopp]

Hello Sam,

many thanks for your investigation and for the provided fix.
Excellent work!

Btw. you also suggested a different solution with synchronize_rcu():

diff --git a/net/can/raw.c b/net/can/raw.c
index eee244ffc31e..5bb9a84f2471 100644
--- a/net/can/raw.c
+++ b/net/can/raw.c
@@ -431,6 +431,13 @@ static int raw_release(struct socket *sock)
     if (ro->count > 1)
         kfree(ro->filter);

+    /*
+     * Wait for any in-flight raw_rcv() calls to finish before freeing
+     * ro->uniq.  can_rx_unregister() scheduled deletion via call_rcu(),
+     * but RCU readers (raw_rcv in softirq) may still be active.
+     */
+    synchronize_rcu();
+
     ro->ifindex = 0;
     ro->bound = 0;
     ro->dev = NULL;


Can you tell why you preferred the destructor solution now?

And if I see it correctly the UAF problem might also show up with the
kfree(ro->filter) statement we can see at the beginning of the above patch.

So either free_percpu(ro->uniq) and kfree(ro->filter) should be handled 
after the finalized synchronize_rcu() process, right?

Many thanks and best regards,
Oliver


On 08.04.26 16:30, Sam P wrote:
> raw_release() unregisters raw CAN receive filters via can_rx_unregister(),
> but receiver deletion is deferred with call_rcu(). This leaves a window
> where raw_rcv() may still be running in an RCU read-side critical section
> after raw_release() frees ro->uniq, leading to a use-after-free of the
> percpu uniq storage.
>
> Move free_percpu(ro->uniq) out of raw_release() and into a raw-specific
> socket destructor. can_rx_unregister() takes an extra reference to the
> socket and only drops it from the RCU callback, so freeing uniq from
> sk_destruct ensures the percpu area is not released until the relevant
> callbacks have drained.
>
> Fixes: 514ac99c64b2 ("can: fix multiple delivery of a single CAN frame 
> for overlapping CAN filters")
> Cc: stable@xxxxxxxxxxxxxxx # v4.1+
> Assisted-by: Bynario AI
> Signed-off-by: Samuel Page <sam@xxxxxxxx>
>
> ---
>   net/can/raw.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/net/can/raw.c b/net/can/raw.c
> index eee244ffc31e..f042c4316890 100644
> --- a/net/can/raw.c
> +++ b/net/can/raw.c
> @@ -361,6 +361,14 @@ static int raw_notifier(struct notifier_block *nb, 
> unsigned long msg,
>       return NOTIFY_DONE;
>   }
>
> +static void raw_sock_destruct(struct sock *sk)
> +{
> +    struct raw_sock *ro = raw_sk(sk);
> +
> +    free_percpu(ro->uniq);
> +    can_sock_destruct(sk);
> +}
> +
>   static int raw_init(struct sock *sk)
>   {
>       struct raw_sock *ro = raw_sk(sk);
> @@ -387,6 +395,8 @@ static int raw_init(struct sock *sk)
>       if (unlikely(!ro->uniq))
>           return -ENOMEM;
>
> +    sk->sk_destruct = raw_sock_destruct;
> +
>       /* set notifier */
>       spin_lock(&raw_notifier_lock);
>       list_add_tail(&ro->notifier, &raw_notifier_list);
> @@ -436,7 +446,6 @@ static int raw_release(struct socket *sock)
>       ro->bound = 0;
>       ro->dev = NULL;
>       ro->count = 0;
> -    free_percpu(ro->uniq);
>
>       sock_orphan(sk);
>       sock->sk = NULL;