Subject: Re: [Linux-kernel-mentees] [PATCH v2 net] rose: Fix Null pointer dereference in rose_send_frame()

From: Anmol Karn <[email-masked]>
Date: Sat, 7 Nov 2020 13:50:41 +0530

Hello Sir,

On Fri, Nov 06, 2020 at 01:04:27PM -0800, Saeed Mahameed wrote:
> On Thu, 2020-11-05 at 21:26 +0530, Anmol Karn wrote:
> > rose_send_frame() dereferences `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_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(). 
> > 
> > Reported-and-tested-by: 
> > [email-masked] 
> > Link: 
> > https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3
> >  
> > Signed-off-by: Anmol Karn <[email-masked]>
> 
> missing proper fixes tag.
> 
> > ---
> >  net/rose/rose_loopback.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > 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 {
> 
> check patch is not happy:
> 
> WARNING:TYPO_SPELLING: 'occurance' may be misspelled - perhaps
> 'occurrence'?
> #7: 
> rose_transmit_clear_request(), and the first occurance of the `neigh`
> 
> ERROR:ASSIGN_IN_IF: do not use assignment in if condition
> #36: FILE: net/rose/rose_loopback.c:99:
> +                       if (rose_loopback_neigh->dev && (dev =
> rose_dev_get(dest)) != NULL) {
> 
> total: 1 errors, 1 warnings, 0 checks, 8 lines checked
> 
> 

Thank you for your review will rectify these and send another version.

Thanks,
Anmol