Re: [Linux-kernel-mentees] [PATCH v4 net] rose: Fix Null pointer dereference in rose_send_frame()

[Jakub Kicinski]

On Wed, 11 Nov 2020 22:29:54 +0530 Anmol Karn wrote:
> rose_send_frame() dereferences `neigh->dev` when called from
> rose_transmit_clear_request(), and the first occurrence of the
> `neigh` is in rose_loopback_timer() as `rose_loopback_neigh`,
> and it is initialized in rose_add_loopback_neigh() as NULL.
> i.e when `rose_loopback_neigh` used in rose_loopback_timer()
> its `->dev` was still NULL and rose_loopback_timer() was calling
> rose_rx_call_request() without checking for NULL.
> 
> - net/rose/rose_link.c
> This bug seems to get triggered in this line:
> 
> rose_call = (ax25_address *)neigh->dev->dev_addr;
> 
> Fix it by adding NULL checking for `rose_loopback_neigh->dev`
> in rose_loopback_timer().
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzbot+a1c743815982d9496...@syzkaller.appspotmail.com
> Tested-by: syzbot+a1c743815982d9496...@syzkaller.appspotmail.com
> Link: 
> https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3
> Signed-off-by: Anmol Karn <anmol.karan...@gmail.com>

> diff --git a/net/rose/rose_loopback.c b/net/rose/rose_loopback.c
> index 7b094275ea8b..6a71b6947d92 100644
> --- a/net/rose/rose_loopback.c
> +++ b/net/rose/rose_loopback.c
> @@ -96,10 +96,12 @@ static void rose_loopback_timer(struct timer_list *unused)
>               }
> 
>               if (frametype == ROSE_CALL_REQUEST) {
> -                     if ((dev = rose_dev_get(dest)) != NULL) {
> +                     dev = rose_dev_get(dest);
> +                     if (rose_loopback_neigh->dev && dev) {
>                               if (rose_rx_call_request(skb, dev, 
> rose_loopback_neigh, lci_o) == 0)
>                                       kfree_skb(skb);
>                       } else {
> +                             dev_put(dev);
>                               kfree_skb(skb);
>                       }
>               } else {

This is still not correct. With this code dev_put() could be called with
NULL, which would cause a crash.

There is also a dev_put() missing if rose_rx_call_request() returns 0.

I think that this is the correct code:

diff --git a/net/rose/rose_loopback.c b/net/rose/rose_loopback.c
index 7b094275ea8b..ff252ef73592 100644
--- a/net/rose/rose_loopback.c
+++ b/net/rose/rose_loopback.c
@@ -96,11 +96,22 @@ static void rose_loopback_timer(struct timer_list *unused)
                }
 
                if (frametype == ROSE_CALL_REQUEST) {
-                       if ((dev = rose_dev_get(dest)) != NULL) {
-                               if (rose_rx_call_request(skb, dev, 
rose_loopback_neigh, lci_o) == 0)
-                                       kfree_skb(skb);
-                       } else {
+                       if (!rose_loopback_neigh->dev) {
                                kfree_skb(skb);
+                               continue;
+                       }
+
+                       dev = rose_dev_get(dest);
+                       if (!dev) {
+                               kfree_skb(skb);
+                               continue;
+                       }
+
+                       if (rose_rx_call_request(skb, dev, rose_loopback_neigh,
+                                                lci_o) == 0) {
+                               dev_put(dev);
+                               kfree_skb(skb);
                        }
                } else {
                        kfree_skb(skb);

Please test this and resubmit it if it works.