On Thu, Oct 15, 2020 at 05:50:51PM +0200, Greg KH wrote:

quoted

On Thu, Oct 15, 2020 at 07:40:12PM +0530, Anmol Karn wrote:

quoted

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

quoted

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;
+       }

The point I am trying to make is that if someone else is setting ->dev
to NULL in some other thread/context/whatever, while this is running,
checking for it like this will not work.

What is the lifetime rules of that pointer?  Who initializes it, and who
sets it to NULL.  Figure that out first please to determine how to check
for this properly.

thanks,

greg k-h

Hello All,

I investigated further on this,

Here is some things i noticed:

When I followed the call trace,

[ 84.241331][ C3] Call Trace:
[ 84.241331][ C3] rose_transmit_clear_request ($SOURCE/net/rose/rose_link.c:255)
[ 84.241331][ C3] ? lockdep_hardirqs_on ($SOURCE/kernel/locking/lockdep.c:4161)
[ 84.241331][ C3] rose_rx_call_request ($SOURCE/net/rose/af_rose.c:999)
[ 84.241331][ C3] ? rose_release ($SOURCE/net/rose/af_rose.c:970)
[ 84.241331][ C3] rose_loopback_timer ($SOURCE/net/rose/rose_loopback.c:100)
[ 84.241331][ C3] ? rose_transmit_link ($SOURCE/net/rose/rose_loopback.c:60)

in the rose_send_frame() it dereferenced `neigh->dev` when called from 
rose_transmit_clear_request(), and the first occurance of the `neigh`
is in rose_loopback_timer() as `rose_loopback_neigh`, and it is initialized 
in rose_add_loopback_neighh() as NULL.

- net/rose/rose_route.c:381

void rose_add_loopback_neigh(void)
{
	struct rose_neigh *sn;

	rose_loopback_neigh = kmalloc(sizeof(struct rose_neigh), GFP_KERNEL);
	if (!rose_loopback_neigh)
		return;
	sn = rose_loopback_neigh;

	sn->callsign  = null_ax25_address;
	sn->digipeat  = NULL;
	sn->ax25      = NULL;
	sn->dev       = 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.


I have created the following patch to check for NULL pointer.

diff --git a/net/rose/rose_loopback.c b/net/rose/rose_loopback.c
index 7b094275ea8b..cd7774cb1d07 100644
--- a/net/rose/rose_loopback.c
+++ b/net/rose/rose_loopback.c

@@ -96,7 +96,7 @@ static void rose_loopback_timer(struct timer_list *unused)
                }
 
                if (frametype == ROSE_CALL_REQUEST) {
-                       if ((dev = rose_dev_get(dest)) != NULL) {
+                       if (rose_loopback_neigh->dev && (dev = rose_dev_get(dest)) != NULL) {
                                if (rose_rx_call_request(skb, dev, rose_loopback_neigh, lci_o) == 0)
                                        kfree_skb(skb);
                        } else {

Please, review it and give me suggestions whether i am going right or not.