Re: possible deadlock in sidtab_sid2str_put

[Paul Moore <paul@paul-moore.com>]

On Tue, Jan 28, 2020 at 7:50 AM syzbot
<syzbot+61cba5033e2072d61806@syzkaller.appspotmail.com> wrote:
>
> syzbot has found a reproducer for the following crash on:
>
> HEAD commit:    b0be0eff Merge tag 'x86-pti-2020-01-28' of git://git.kerne..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1432aebee00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=9784e57c96a92f20
> dashboard link: https://syzkaller.appspot.com/bug?extid=61cba5033e2072d61806
> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10088e95e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13fa605ee00000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+61cba5033e2072d61806@syzkaller.appspotmail.com
>
> =====================================================
> WARNING: SOFTIRQ-safe -> SOFTIRQ-unsafe lock order detected
> 5.5.0-syzkaller #0 Not tainted
> -----------------------------------------------------
> syz-executor305/10624 [HC0[0]:SC0[2]:HE1:SE0] is trying to acquire:
> ffff888098c14098 (&(&s->cache_lock)->rlock){+.+.}, at: spin_lock include/linux/spinlock.h:338 [inline]
> ffff888098c14098 (&(&s->cache_lock)->rlock){+.+.}, at: sidtab_sid2str_put.part.0+0x36/0x880 security/selinux/ss/sidtab.c:533
>
> and this task is already holding:
> ffffffff89865770 (&(&nf_conntrack_locks[i])->rlock){+.-.}, at: spin_lock include/linux/spinlock.h:338 [inline]
> ffffffff89865770 (&(&nf_conntrack_locks[i])->rlock){+.-.}, at: nf_conntrack_lock+0x17/0x70 net/netfilter/nf_conntrack_core.c:91
> which would create a new lock dependency:
>  (&(&nf_conntrack_locks[i])->rlock){+.-.} -> (&(&s->cache_lock)->rlock){+.+.}
>
> but this new dependency connects a SOFTIRQ-irq-safe lock:
>  (&(&nf_conntrack_locks[i])->rlock){+.-.}
>
> ... which became SOFTIRQ-irq-safe at:
>   lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4484
>   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>   _raw_spin_lock+0x2f/0x40 kernel/locking/spinlock.c:151
>   spin_lock include/linux/spinlock.h:338 [inline]
>   nf_conntrack_lock+0x17/0x70 net/netfilter/nf_conntrack_core.c:91

...

> to a SOFTIRQ-irq-unsafe lock:
>  (&(&s->cache_lock)->rlock){+.+.}
>
> ... which became SOFTIRQ-irq-unsafe at:
> ...
>   lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4484
>   __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
>   _raw_spin_lock+0x2f/0x40 kernel/locking/spinlock.c:151
>   spin_lock include/linux/spinlock.h:338 [inline]
>   sidtab_sid2str_put.part.0+0x36/0x880 security/selinux/ss/sidtab.c:533
>   sidtab_sid2str_put+0xa0/0xc0 security/selinux/ss/sidtab.c:566
>   sidtab_entry_to_string security/selinux/ss/services.c:1279 [inline]
>   sidtab_entry_to_string+0xf2/0x110 security/selinux/ss/services.c:1266
>   security_sid_to_context_core+0x2c6/0x3c0 security/selinux/ss/services.c:1361
>   security_sid_to_context+0x34/0x40 security/selinux/ss/services.c:1384
>   avc_audit_post_callback+0x102/0x790 security/selinux/avc.c:709
>   common_lsm_audit+0x5ac/0x1e00 security/lsm_audit.c:466
>   slow_avc_audit+0x16a/0x1f0 security/selinux/avc.c:782
>   avc_audit security/selinux/include/avc.h:140 [inline]
>   avc_has_perm+0x543/0x610 security/selinux/avc.c:1185
>   inode_has_perm+0x1a8/0x230 security/selinux/hooks.c:1631
>   selinux_mmap_file+0x10a/0x1d0 security/selinux/hooks.c:3701
>   security_mmap_file+0xa4/0x1e0 security/security.c:1482
>   vm_mmap_pgoff+0xf0/0x230 mm/util.c:502

...

> other info that might help us debug this:
>
>  Possible interrupt unsafe locking scenario:
>
>        CPU0                    CPU1
>        ----                    ----
>   lock(&(&s->cache_lock)->rlock);
>                                local_irq_disable();
>                                lock(&(&nf_conntrack_locks[i])->rlock);
>                                lock(&(&s->cache_lock)->rlock);
>   <Interrupt>
>     lock(&(&nf_conntrack_locks[i])->rlock);
>
>  *** DEADLOCK ***
>
> 4 locks held by syz-executor305/10624:
>  #0: ffffffff8c1acc68 (&table[i].mutex){+.+.}, at: nfnl_lock net/netfilter/nfnetlink.c:62 [inline]
>  #0: ffffffff8c1acc68 (&table[i].mutex){+.+.}, at: nfnetlink_rcv_msg+0x9ee/0xfb0 net/netfilter/nfnetlink.c:224
>  #1: ffff8880836415d8 (nlk_cb_mutex-NETFILTER){+.+.}, at: netlink_dump+0xe7/0xfb0 net/netlink/af_netlink.c:2199
>  #2: ffffffff89865770 (&(&nf_conntrack_locks[i])->rlock){+.-.}, at: spin_lock include/linux/spinlock.h:338 [inline]
>  #2: ffffffff89865770 (&(&nf_conntrack_locks[i])->rlock){+.-.}, at: nf_conntrack_lock+0x17/0x70 net/netfilter/nf_conntrack_core.c:91
>  #3: ffffffff8b7df008 (&selinux_ss.policy_rwlock){.+.?}, at: security_sid_to_context_core+0x1ca/0x3c0 security/selinux/ss/services.c:1344

I think this is going to be tricky to fix due to the differing
contexts from which sidtab_sid2str_put() may be called.  We already
have a check for !in_task() in sidtab_sid2str_put(), do we want to add
a check for !in_serving_softirq() too?

-- 
paul moore
www.paul-moore.com