Re: [syzbot] net-next boot error: WARNING: refcount bug in fib_create_info

On Tue, 3 Aug 2021 14:04:35 -0700
Jakub Kicinski [off-list ref] wrote:

On Tue, 3 Aug 2021 19:31:50 +0300 Pavel Skripkin wrote:

quoted

On 8/3/21 7:12 PM, Pavel Skripkin wrote:

quoted

On 8/3/21 7:07 PM, syzbot wrote:  

quoted

Hello,

syzbot found the following issue on:

HEAD commit:    1187c8c4642d net: phy: mscc: make some arrays
static const.. git tree:       net-next
console output:
https://syzkaller.appspot.com/x/log.txt?x=140e7b3e300000 kernel
config:
https://syzkaller.appspot.com/x/.config?x=f9bb42efdc6f1d7
dashboard link:
https://syzkaller.appspot.com/bug?extid=c5ac86461673ef58847c
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld
(GNU Binutils for Debian) 2.35.1

IMPORTANT: if you fix the issue, please add the following tag to
the commit: Reported-by:
syzbot+c5ac86461673ef58847c@syzkaller.appspotmail.com

FS-Cache: Netfs 'afs' registered for caching
Btrfs loaded, crc32c=crc32c-intel, assert=on, zoned=yes
Key type big_key registered
Key type encrypted registered
AppArmor: AppArmor sha1 policy hashing enabled
ima: No TPM chip found, activating TPM-bypass!
Loading compiled-in module X.509 certificates
Loaded X.509 cert 'Build time autogenerated kernel key:
f850c787ad998c396ae089c083b940ff0a9abb77' ima: Allocated hash
algorithm: sha256 ima: No architecture policies found
evm: Initialising EVM extended attributes:
evm: security.selinux (disabled)
evm: security.SMACK64 (disabled)
evm: security.SMACK64EXEC (disabled)
evm: security.SMACK64TRANSMUTE (disabled)
evm: security.SMACK64MMAP (disabled)
evm: security.apparmor
evm: security.ima
evm: security.capability
evm: HMAC attrs: 0x1
PM:   Magic number: 1:990:690
printk: console [netcon0] enabled
netconsole: network logging started
gtp: GTP module loaded (pdp ctx size 104 bytes)
rdma_rxe: loaded
cfg80211: Loading compiled-in X.509 certificates for regulatory
database cfg80211: Loaded X.509 cert 'sforshee:
00b28ddf47aef9cea7' ALSA device list:
   #0: Dummy 1
   #1: Loopback 1
   #2: Virtual MIDI Card 1
md: Waiting for all devices to be available before autodetect
md: If you don't use raid, use raid=noautodetect
md: Autodetecting RAID arrays.
md: autorun ...
md: ... autorun DONE.
EXT4-fs (sda1): mounted filesystem without journal. Opts:
(null). Quota mode: none. VFS: Mounted root (ext4 filesystem)
readonly on device 8:1. devtmpfs: mounted
Freeing unused kernel image (initmem) memory: 4476K
Write protecting the kernel read-only data: 169984k
Freeing unused kernel image (text/rodata gap) memory: 2012K
Freeing unused kernel image (rodata/data gap) memory: 1516K
Run /sbin/init as init process
systemd[1]: systemd 232 running in system mode. (+PAM +AUDIT
+SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP
+GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD
+IDN) systemd[1]: Detected virtualization kvm. systemd[1]:
Detected architecture x86-64. systemd[1]: Set hostname to
<syzkaller>. ------------[ cut here ]------------ refcount_t:
addition on 0; use-after-free. WARNING: CPU: 1 PID: 1 at
lib/refcount.c:25 refcount_warn_saturate+0x169/0x1e0
lib/refcount.c:25 Modules linked in: CPU: 1 PID: 1 Comm: systemd
Not tainted 5.14.0-rc3-syzkaller #0 Hardware name: Google Google
Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
Code: 09 31 ff 89 de e8 d7 fa 9e fd 84 db 0f 85 36 ff ff ff e8
8a f4 9e fd 48 c7 c7 c0 81 e3 89 c6 05 70 51 81 09 01 e8 48 f8
13 05 <0f> 0b e9 17 ff ff ff e8 6b f4 9e fd 0f b6 1d 55 51 81 09
31 ff 89 RSP: 0018:ffffc90000c66ab0 EFLAGS: 00010286 RAX:
0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff88813fe48000 RSI: ffffffff815d7b25 RDI:
fffff5200018cd48 RBP: 0000000000000002 R08: 0000000000000000
R09: 0000000000000001 R10: ffffffff815d195e R11:
0000000000000000 R12: 0000000000000004 R13: 0000000000000001
R14: 0000000000000000 R15: ffff888027722e00 FS:
00007f8c1c5d0500(0000) GS:ffff8880b9d00000(0000)
knlGS:0000000000000000 CS:  0010 DS: 0000 ES: 0000 CR0:
0000000080050033 CR2: 000055ed0ced4368 CR3: 0000000026bac000
CR4: 00000000001506e0 DR0: 0000000000000000 DR1:
0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000
DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace:
__refcount_add include/linux/refcount.h:199 [inline]
__refcount_inc include/linux/refcount.h:250 [inline]
refcount_inc include/linux/refcount.h:267 [inline]
fib_create_info+0x36af/0x4910 net/ipv4/fib_semantics.c:1554  

Missed refcount_set(), I think

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index f29feb7772da..bb9949f6bb70 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c

@@ -1428,6 +1428,7 @@ struct fib_info *fib_create_info(struct

fib_config *cfg,
   	}

   	fib_info_cnt++;
+	refcount_set(&fi->fib_treeref, 1);
   	fi->fib_net = net;
   	fi->fib_protocol = cfg->fc_protocol;
   	fi->fib_scope = cfg->fc_scope;

Oops, it's already fixed in -next, so

#syz fix: ipv4: Fix refcount warning for new fib_info


BTW: there is one more bug with refcounts:

link_it:
	ofi = fib_find_info(fi);
	if (ofi) {
		fi->fib_dead = 1;
		free_fib_info(fi);
		refcount_inc(&ofi->fib_treeref);

		^^^^^^^^^^^^^^^^^^^^^^^
		/ *fib_treeref is 0 here */

Why 0? ofi is an existing object it's already initialized.

Yep, I see now, sorry for misinformation :(



With regards,
Pavel Skripkin