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

From: Pavel Skripkin <[email-masked]>
Date: Wed, 4 Aug 2021 08:05:36 +0300

On Tue, 3 Aug 2021 14:04:35 -0700
Jakub Kicinski <[email-masked]> wrote:

> On Tue, 3 Aug 2021 19:31:50 +0300 Pavel Skripkin wrote:
> > On 8/3/21 7:12 PM, Pavel Skripkin wrote:
> > > On 8/3/21 7:07 PM, syzbot wrote:  
> > >> 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:
> > >> [email-masked]
> > >> 
> > >> 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