On Fri, Oct 30, 2020 at 04:24:13PM +0530, Anmol Karn wrote: > 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+a1c743815982d9496...@syzkaller.appspotmail.com > > > > > Link: > > > > > https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 > > > > > > > > > > Signed-off-by: Anmol Karn <anmol.karan...@gmail.com> > > > > > --- > > > > > 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.
That seems better, does it solve the syzbot test? thanks, greg k-h
