netdev - Re: [syzbot] [netfilter?] KASAN: slab-use-after-free Read in nft_array_get

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Message-ID: <aXoRvAottF2YRD3a@strlen.de>
Date: Wed, 28 Jan 2026 14:40:12 +0100
From: Florian Westphal <fw@...len.de>
To: syzbot <syzbot+d417922a3e7935517ef6@...kaller.appspotmail.com>
Cc: coreteam@...filter.org, davem@...emloft.net, edumazet@...gle.com,
	horms@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org,
	pabeni@...hat.com, pablo@...filter.org, phil@....cc,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [netfilter?] KASAN: slab-use-after-free Read in
 nft_array_get_cmp

syzbot <syzbot+d417922a3e7935517ef6@...kaller.appspotmail.com> wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    62777c8015f3 Merge branch 'net-stmmac-rk-simplify-per-soc-..
> git tree:       net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=103e49b2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=aeae47237b696a30
> dashboard link: https://syzkaller.appspot.com/bug?extid=d417922a3e7935517ef6
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15e50160580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=155ce322580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/1eb82c60e767/disk-62777c80.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/e421b2bad029/vmlinux-62777c80.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/3bc19b11eaeb/bzImage-62777c80.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d417922a3e7935517ef6@...kaller.appspotmail.com
> 
> ==================================================================
> BUG: KASAN: slab-use-after-free in nft_set_ext include/net/netfilter/nf_tables.h:795 [inline]
> BUG: KASAN: slab-use-after-free in nft_set_ext_key include/net/netfilter/nf_tables.h:800 [inline]
> BUG: KASAN: slab-use-after-free in nft_array_get_cmp+0x1f6/0x2a0 net/netfilter/nft_set_rbtree.c:133
> Read of size 1 at addr ffff888058618b19 by task syz.3.79/6217
> 
> CPU: 1 UID: 0 PID: 6217 Comm: syz.3.79 Not tainted syzkaller #0 PREEMPT(full) 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/13/2026
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  print_address_description mm/kasan/report.c:378 [inline]
>  print_report+0xba/0x230 mm/kasan/report.c:482
>  kasan_report+0x117/0x150 mm/kasan/report.c:595
>  nft_set_ext include/net/netfilter/nf_tables.h:795 [inline]
>  nft_set_ext_key include/net/netfilter/nf_tables.h:800 [inline]
>  nft_array_get_cmp+0x1f6/0x2a0 net/netfilter/nft_set_rbtree.c:133
>  __inline_bsearch include/linux/bsearch.h:15 [inline]
>  bsearch+0x50/0xc0 lib/bsearch.c:33
>  nft_rbtree_get+0x16b/0x400 net/netfilter/nft_set_rbtree.c:169

Use after free, its possible for rbtree insert to evict elements
due to expiry, but still return -EEXIST.

We can thus end up with removed elements but without a call to
->commit(), hence we have a stale binary blob referencing free'd
elements.