Re: possible deadlock in sidtab_sid2str_put

[Stephen Smalley]

On 1/28/20 8:39 AM, Paul Moore wrote:
> On Tue, Jan 28, 2020 at 7:50 AM syzbot
> <syzbot+61cba5033e2072d61806@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > =====================================================
> > 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?

No, we should just use spin_lock_irqsave/unlock_irqrestore() IMHO, but 
that then means we need to re-evaluate the performance gain of this change.