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

[Anmol Karn]

On Thu, Oct 15, 2020 at 05:50:51PM +0200, Greg KH wrote:
> On Thu, Oct 15, 2020 at 07:40:12PM +0530, Anmol Karn wrote:
> > 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:
> > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx 
> > > > Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 
> > > > Signed-off-by: Anmol Karn <anmol.karan123@xxxxxxxxx>
> > > > ---
> > > > 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

Sure sir, understood.


Thanks,
Anmol