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Thu, 15 Oct 2020 07:10:20 -0700 (PDT) Date: Thu, 15 Oct 2020 19:40:12 +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: <20201015141012.GB77038@Thinkpad> References: <20201015001712.72976-1-anmol.karan123@gmail.com> <20201015051225.GA404970@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201015051225.GA404970@kroah.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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; + } ax25s = neigh->ax25; neigh->ax25 = ax25_send_frame(skb, 260, rose_call, &neigh->callsign, neigh->digipeat, neigh->dev); Thanks, Anmol