Forwarded: Re: [syzbot] [hams?] KASAN: slab-use-after-free Read in nr_add_node

[syzbot <syzbot+2860e75836a08b172755@syzkaller.appspotmail.com>]

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Subject: Re: [syzbot] [hams?] KASAN: slab-use-after-free Read in nr_add_node
Author: listout@listout.xyz

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@syzkaller.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 1c64efcb083c48c85227cb4d72ab137feef2cdac

diff --git a/net/netrom/nr_route.c b/net/netrom/nr_route.c
index b94cb2ffbaf8..aa72c4db2be4 100644
--- a/net/netrom/nr_route.c
+++ b/net/netrom/nr_route.c
@@ -242,11 +242,12 @@ static int __must_check nr_add_node(ax25_address *nr, const char *mnemonic,
 		} else {
 			/* It must be better than the worst */
 			if (quality > nr_node->routes[2].quality) {
-				nr_node->routes[2].neighbour->count--;
-				nr_neigh_put(nr_node->routes[2].neighbour);
+				struct nr_neigh *old_neighbour = nr_node->routes[2].neighbour;
+				old_neighbour->count--;
+				nr_neigh_put(old_neighbour);
 
-				if (nr_node->routes[2].neighbour->count == 0 && !nr_node->routes[2].neighbour->locked)
-					nr_remove_neigh(nr_node->routes[2].neighbour);
+				if (old_neighbour->count == 0 && !old_neighbour->locked)
+					nr_remove_neigh(old_neighbour);
 
 				nr_node->routes[2].quality   = quality;
 				nr_node->routes[2].obs_count = obs_count;

-- 
Regards,
listout