Re: [syzbot] general protection fault in nft_set_elem_expr_alloc

On Wed, Jun 02, 2021 at 09:37:26AM -0700, syzbot wrote:

Hello,

syzbot found the following issue on:

HEAD commit:    6850ec97 Merge branch 'mptcp-fixes-for-5-13'
git tree:       net
console output: https://syzkaller.appspot.com/x/log.txt?x=1355504dd00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=770708ea7cfd4916
dashboard link: https://syzkaller.appspot.com/bug?extid=ce96ca2b1d0b37c6422d
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1502d517d00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12bbbe13d00000

The issue was bisected to:

commit 05abe4456fa376040f6cc3cc6830d2e328723478
Author: Pablo Neira Ayuso [off-list ref]
Date:   Wed May 20 13:44:37 2020 +0000

    netfilter: nf_tables: allow to register flowtable with no devices

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=10fa1387d00000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=12fa1387d00000
console output: https://syzkaller.appspot.com/x/log.txt?x=14fa1387d00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+ce96ca2b1d0b37c6422d@syzkaller.appspotmail.com
Fixes: 05abe4456fa3 ("netfilter: nf_tables: allow to register flowtable with no devices")

general protection fault, probably for non-canonical address 0xdffffc000000000e: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000070-0x0000000000000077]
CPU: 1 PID: 8438 Comm: syz-executor343 Not tainted 5.13.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:nft_set_elem_expr_alloc+0x17e/0x280 net/netfilter/nf_tables_api.c:5321
Code: 48 c1 ea 03 80 3c 02 00 0f 85 09 01 00 00 49 8b 9d c0 00 00 00 48 b8 00 00 00 00 00 fc ff df 48 8d 7b 70 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 d9 00 00 00 48 8b 5b 70 48 85 db 74 21 e8 9a bd

It's a real bug. Bisect is not correct though.

I'll post a patch to fix it. Thanks.