Re: [syzbot] [net?] KMSAN: uninit-value in __llc_lookup_established

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: syzbot <syzbot+b5ad66046b913bc04c6f@syzkaller.appspotmail.com>
Date: Tue, 05 Dec 2023 10:25:31 -0800
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    1c41041124bd Merge tag 'i3c/for-6.7' of git://git.kernel.o..
> git tree:       upstream
> console+strace: https://syzkaller.appspot.com/x/log.txt?x=10429eeb680000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=956549bd1d1e9efd
> dashboard link: https://syzkaller.appspot.com/bug?extid=b5ad66046b913bc04c6f
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12b6a00f680000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=121471ef680000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/20fd86e677f1/disk-1c410411.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/ccd39cb0b7b6/vmlinux-1c410411.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/156fc60f97bc/bzImage-1c410411.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+b5ad66046b913bc04c6f@syzkaller.appspotmail.com
> 
> syz-executor994 uses obsolete (PF_INET,SOCK_PACKET)
> =====================================================
> BUG: KMSAN: uninit-value in __llc_lookup_established+0xe9d/0xf90
>  __llc_lookup_established+0xe9d/0xf90
>  __llc_lookup net/llc/llc_conn.c:611 [inline]
>  llc_conn_handler+0x4bd/0x1360 net/llc/llc_conn.c:791
>  llc_rcv+0xfbb/0x14a0 net/llc/llc_input.c:206
>  __netif_receive_skb_one_core net/core/dev.c:5527 [inline]
>  __netif_receive_skb+0x1a6/0x5a0 net/core/dev.c:5641
>  netif_receive_skb_internal net/core/dev.c:5727 [inline]
>  netif_receive_skb+0x58/0x660 net/core/dev.c:5786
>  tun_rx_batched+0x3ee/0x980 drivers/net/tun.c:1555
>  tun_get_user+0x53af/0x66d0 drivers/net/tun.c:2002
>  tun_chr_write_iter+0x3af/0x5d0 drivers/net/tun.c:2048
>  call_write_iter include/linux/fs.h:2020 [inline]
>  new_sync_write fs/read_write.c:491 [inline]
>  vfs_write+0x8ef/0x1490 fs/read_write.c:584
>  ksys_write+0x20f/0x4c0 fs/read_write.c:637
>  __do_sys_write fs/read_write.c:649 [inline]
>  __se_sys_write fs/read_write.c:646 [inline]
>  __x64_sys_write+0x93/0xd0 fs/read_write.c:646
>  do_syscall_x64 arch/x86/entry/common.c:51 [inline]
>  do_syscall_64+0x44/0x110 arch/x86/entry/common.c:82
>  entry_SYSCALL_64_after_hwframe+0x63/0x6b
> 
> Local variable daddr created at:
>  llc_conn_handler+0x53/0x1360 net/llc/llc_conn.c:783
>  llc_rcv+0xfbb/0x14a0 net/llc/llc_input.c:206
> 
> CPU: 1 PID: 5004 Comm: syz-executor994 Not tainted 6.6.0-syzkaller-14500-g1c41041124bd #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/09/2023
> =====================================================

llc_pdu_decode_[sd]a() initialises addr.mac only for ETH_P_802_2.
Otherwise, llc_estab_match() reads an uninit value form both [sd]addr.

#syz test git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 1c41041124bd

diff --git a/net/llc/llc_conn.c b/net/llc/llc_conn.c
index 0a3f5e0bec00..730561e186ce 100644
--- a/net/llc/llc_conn.c
+++ b/net/llc/llc_conn.c
@@ -780,7 +780,7 @@ static struct sock *llc_create_incoming_sock(struct sock *sk,
 
 void llc_conn_handler(struct llc_sap *sap, struct sk_buff *skb)
 {
-	struct llc_addr saddr, daddr;
+	struct llc_addr saddr = {}, daddr = {};
 	struct sock *sk;
 
 	llc_pdu_decode_sa(skb, saddr.mac);