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Date:   Tue, 3 Aug 2021 14:04:35 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     syzbot <syzbot+c5ac86461673ef58847c@...kaller.appspotmail.com>,
        davem@...emloft.net, dsahern@...nel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, yoshfuji@...ux-ipv6.org
Subject: Re: [syzbot] net-next boot error: WARNING: refcount bug in
 fib_create_info

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: syzbot+c5ac86461673ef58847c@...kaller.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.

> 		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,
>   	}
> 
>   link_it:
> +	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);
>   	spin_lock_bh(&fib_info_lock);
>   	hlist_add_head(&fi->fib_hash,
> 
> 
> Thoughts?

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