linux-kernel - Re: [syzbot] [hams?] KASAN: slab-use-after-free Read in nr_add

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Message-ID: <68f472b4.050a0220.1186a4.0526.GAE@google.com>
Date: Sat, 18 Oct 2025 22:10:12 -0700
From: syzbot <syzbot+2860e75836a08b172755@...kaller.appspotmail.com>
To: listout@...tout.xyz
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org, 
	kuba@...nel.org, linux-hams@...r.kernel.org, linux-kernel@...r.kernel.org, 
	listout@...tout.xyz, netdev@...r.kernel.org, pabeni@...hat.com, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [hams?] KASAN: slab-use-after-free Read in nr_add_node

> On 18.10.2025 13:37, syzbot wrote:
>> syzbot has found a reproducer for the following issue on:
>> 
>> HEAD commit:    f406055cb18c Merge tag 'arm64-fixes' of git://git.kernel.o..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=11cf767c580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=f3e7b5a3627a90dd
>> dashboard link: https://syzkaller.appspot.com/bug?extid=2860e75836a08b172755
>> compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=155f1de2580000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12b6b52f980000
>> 
>> Downloadable assets:
>> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-f406055c.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/a4db2a99bfb1/vmlinux-f406055c.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/91d1ca420bac/bzImage-f406055c.xz
>> 
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+2860e75836a08b172755@...kaller.appspotmail.com
>> 
>> ==================================================================
>> BUG: KASAN: slab-use-after-free in nr_add_node+0x25db/0x2c00 net/netrom/nr_route.c:248
>> Read of size 4 at addr ffff888054b8cc30 by task syz.3.7839/22393
>
> #syz test linux-next

want either no args or 2 args (repo, branch), got 1

>
> diff --git a/net/netrom/nr_route.c b/net/netrom/nr_route.c
> index b94cb2ffbaf8..5fa7d9febbbb 100644
> --- a/net/netrom/nr_route.c
> +++ b/net/netrom/nr_route.c
> @@ -245,7 +245,9 @@ static int __must_check nr_add_node(ax25_address *nr, const char *mnemonic,
>  				nr_node->routes[2].neighbour->count--;
>  				nr_neigh_put(nr_node->routes[2].neighbour);
>  
> -				if (nr_node->routes[2].neighbour->count == 0 && !nr_node->routes[2].neighbour->locked)
> +				if (nr_node->routes[2].neighbour &&
> +				    nr_node->routes[2].neighbour->count == 0 &&
> +				    !nr_node->routes[2].neighbour->locked)
>  					nr_remove_neigh(nr_node->routes[2].neighbour);
>  
>  				nr_node->routes[2].quality   = quality;
>
> -- 
> Regards,
> listout