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Fri, 30 Oct 2020 03:54:20 -0700 (PDT) Date: Fri, 30 Oct 2020 16:24:13 +0530 From: Anmol Karn To: Greg KH Cc: ralf@linux-mips.org, davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org, syzkaller-bugs@googlegroups.com, linux-kernel@vger.kernel.org, syzbot+a1c743815982d9496393@syzkaller.appspotmail.com, linux-hams@vger.kernel.org, linux-kernel-mentees@lists.linuxfoundation.org Subject: Re: [Linux-kernel-mentees] [PATCH] net: rose: Fix Null pointer dereference in rose_send_frame() Message-ID: <20201030105413.GA32091@Thinkpad> References: <20201015001712.72976-1-anmol.karan123@gmail.com> <20201015051225.GA404970@kroah.com> <20201015141012.GB77038@Thinkpad> <20201015155051.GB66528@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201015155051.GB66528@kroah.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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@syzkaller.appspotmail.com > > > > Link: https://syzkaller.appspot.com/bug?id=9d2a7ca8c7f2e4b682c97578dfa3f236258300b3 > > > > Signed-off-by: Anmol Karn > > > > --- > > > > 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. Thanks, Anmol