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

From: Pavel Skripkin
Date: Tue Aug 03 2021 - 12:31:59 EST

On 8/3/21 7:12 PM, Pavel Skripkin wrote:
On 8/3/21 7:07 PM, syzbot wrote:

syzbot found the following issue on:

HEAD commit: 1187c8c4642d net: phy: mscc: make some arrays static const..
git tree: net-next
console output:
kernel config:
dashboard link:
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@xxxxxxxxxxxxxxxxxxxxxxxxx

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]: 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

+ 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:

ofi = fib_find_info(fi);
if (ofi) {
fi->fib_dead = 1;

/ *fib_treeref is 0 here */

return ofi;

refcount_set(&fi->fib_treeref, 1);

diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index f29feb7772da..38d1fc4d0be1 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -1543,6 +1543,8 @@ struct fib_info *fib_create_info(struct fib_config *cfg,

+ refcount_set(&fi->fib_treeref, 1);
ofi = fib_find_info(fi);
if (ofi) {
fi->fib_dead = 1;
@@ -1551,7 +1553,6 @@ struct fib_info *fib_create_info(struct fib_config *cfg,
return ofi;

- refcount_set(&fi->fib_treeref, 1);
refcount_set(&fi->fib_clntref, 1);


With regards,
Pavel Skripkin