On Thu, Oct 15, 2020 at 07:12:25AM +0200, Greg KH wrote:

On Thu, Oct 15, 2020 at 05:47:12AM +0530, Anmol Karn wrote:

quoted

In rose_send_frame(), when comparing two ax.25 addresses, it assigns rose_call to 
either global ROSE callsign or default port, but when the former block triggers and 
rose_call is assigned by (ax25_address *)neigh->dev->dev_addr, a NULL pointer is 
dereferenced by 'neigh' when dereferencing 'dev'.

- net/rose/rose_link.c
This bug seems to get triggered in this line:

rose_call = (ax25_address *)neigh->dev->dev_addr;

Prevent it by checking NULL condition for neigh->dev before comparing addressed for 
rose_call initialization.

Reported-by: syzbot+a1c743815982d9496393@syzkaller.appspotmail.com 
Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 
Signed-off-by: Anmol Karn <redacted>
---
I am bit sceptical about the error return code, please suggest if anything else is 
appropriate in place of '-ENODEV'.

 net/rose/rose_link.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
index f6102e6f5161..92ea6a31d575 100644
--- a/net/rose/rose_link.c
+++ b/net/rose/rose_link.c

@@ -97,6 +97,9 @@ static int rose_send_frame(struct sk_buff *skb, struct rose_neigh *neigh)
 	ax25_address *rose_call;
 	ax25_cb *ax25s;
 
+	if (!neigh->dev)
+		return -ENODEV;

How can ->dev not be set at this point in time?  Shouldn't that be
fixed, because it could change right after you check this, right?

thanks,

greg k-h

Hello Sir,

Thanks for the review,
After following the call trace i thought, if neigh->dev is NULL it should
be checked, but I will figure out what is going on with the crash reproducer,
and I think rose_loopback_timer() is the place where problem started. 

Also, I have created a diff for checking neigh->dev before assigning ROSE callsign
, please give your suggestions on this.

diff --git a/net/rose/rose_link.c b/net/rose/rose_link.c
index f6102e6f5161..2ddd5e559442 100644
--- a/net/rose/rose_link.c
+++ b/net/rose/rose_link.c

@@ -97,10 +97,14 @@ static int rose_send_frame(struct sk_buff *skb, struct rose_neigh *neigh)
        ax25_address *rose_call;
        ax25_cb *ax25s;
 
-       if (ax25cmp(&rose_callsign, &null_ax25_address) == 0)
-               rose_call = (ax25_address *)neigh->dev->dev_addr;
-       else
-               rose_call = &rose_callsign;
+       if (neigh->dev) {
+               if (ax25cmp(&rose_callsign, &null_ax25_address) == 0)
+                       rose_call = (ax25_address *)neigh->dev->dev_addr;
+               else
+                       rose_call = &rose_callsign;
+       } else {
+               return -ENODEV;
+       }
 
        ax25s = neigh->ax25;
        neigh->ax25 = ax25_send_frame(skb, 260, rose_call, &neigh->callsign, neigh->digipeat, neigh->dev);
 

Thanks,
Anmol
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