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SubjectRe: [PATCH 1/2] fcntl: fix potential deadlocks for &fown_struct.lock
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On 2/7/21 7:44 pm, Jeff Layton wrote:
> On Fri, 2021-07-02 at 17:18 +0800, Desmond Cheong Zhi Xi wrote:
>> Syzbot reports a potential deadlock in do_fcntl:
>>
>> ========================================================
>> WARNING: possible irq lock inversion dependency detected
>> 5.12.0-syzkaller #0 Not tainted
>> --------------------------------------------------------
>> syz-executor132/8391 just changed the state of lock:
>> ffff888015967bf8 (&f->f_owner.lock){.+..}-{2:2}, at: f_getown_ex fs/fcntl.c:211 [inline]
>> ffff888015967bf8 (&f->f_owner.lock){.+..}-{2:2}, at: do_fcntl+0x8b4/0x1200 fs/fcntl.c:395
>> but this lock was taken by another, HARDIRQ-safe lock in the past:
>> (&dev->event_lock){-...}-{2:2}
>>
>> and interrupts could create inverse lock ordering between them.
>>
>> other info that might help us debug this:
>> Chain exists of:
>> &dev->event_lock --> &new->fa_lock --> &f->f_owner.lock
>>
>> Possible interrupt unsafe locking scenario:
>>
>> CPU0 CPU1
>> ---- ----
>> lock(&f->f_owner.lock);
>> local_irq_disable();
>> lock(&dev->event_lock);
>> lock(&new->fa_lock);
>> <Interrupt>
>> lock(&dev->event_lock);
>>
>> *** DEADLOCK ***
>>
>> This happens because there is a lock hierarchy of
>> &dev->event_lock --> &new->fa_lock --> &f->f_owner.lock
>> from the following call chain:
>>
>> input_inject_event():
>> spin_lock_irqsave(&dev->event_lock,...);
>> input_handle_event():
>> input_pass_values():
>> input_to_handler():
>> evdev_events():
>> evdev_pass_values():
>> spin_lock(&client->buffer_lock);
>> __pass_event():
>> kill_fasync():
>> kill_fasync_rcu():
>> read_lock(&fa->fa_lock);
>> send_sigio():
>> read_lock_irqsave(&fown->lock,...);
>>
>> However, since &dev->event_lock is HARDIRQ-safe, interrupts have to be
>> disabled while grabbing &f->f_owner.lock, otherwise we invert the lock
>> hierarchy.
>>
>> Hence, we replace calls to read_lock/read_unlock on &f->f_owner.lock,
>> with read_lock_irq/read_unlock_irq.
>>
>
> Patches look reasonable overall, but why does this one use read_lock_irq
> and the other one use read_lock_irqsave? Don't we need to *_irqsasve in
> both patches?
>
>

My thinking was that the functions f_getown_ex and f_getowner_uids are
only called from do_fcntl, and f_getown is only called from do_fnctl and
sock_ioctl. do_fnctl itself is only called from syscalls.

For sock_ioctl, the chain is
compat_sock_ioctl():
compat_sock_ioctl_trans():
sock_ioctl()

For both paths, it doesn't seem that interrupts are disabled, so I used
the *irq variants.

But of course, I might be very mistaken on this, and I'd be happy to
make the change to *_irqsave.

Also, on further inspection, if these calls should be changed to
*_irqsave, then I believe the call to write_lock_irq in f_modown (called
from do_fcntl() --> f_setown() --> __f_setown() --> f_modown()) should
also be changed to *_irqsave.

There's also a call to write_lock_irq(&fa->fa_lock) in
fasync_remove_entry and fasync_insert_entry. Whether these should be
changed as well isn't as clear to me, but since it's safe to do, perhaps
it makes sense to use *_irqsave for them too. Thoughts?

>> Reported-and-tested-by: syzbot+e6d5398a02c516ce5e70@syzkaller.appspotmail.com
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>> ---
>> fs/fcntl.c | 13 +++++++------
>> 1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/fcntl.c b/fs/fcntl.c
>> index dfc72f15be7f..cf9e81dfa615 100644
>> --- a/fs/fcntl.c
>> +++ b/fs/fcntl.c
>> @@ -150,7 +150,8 @@ void f_delown(struct file *filp)
>> pid_t f_getown(struct file *filp)
>> {
>> pid_t pid = 0;
>> - read_lock(&filp->f_owner.lock);
>> +
>> + read_lock_irq(&filp->f_owner.lock);
>> rcu_read_lock();
>> if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type)) {
>> pid = pid_vnr(filp->f_owner.pid);
>> @@ -158,7 +159,7 @@ pid_t f_getown(struct file *filp)
>> pid = -pid;
>> }
>> rcu_read_unlock();
>> - read_unlock(&filp->f_owner.lock);
>> + read_unlock_irq(&filp->f_owner.lock);
>> return pid;
>> }
>>
>> @@ -208,7 +209,7 @@ static int f_getown_ex(struct file *filp, unsigned long arg)
>> struct f_owner_ex owner = {};
>> int ret = 0;
>>
>> - read_lock(&filp->f_owner.lock);
>> + read_lock_irq(&filp->f_owner.lock);
>> rcu_read_lock();
>> if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type))
>> owner.pid = pid_vnr(filp->f_owner.pid);
>> @@ -231,7 +232,7 @@ static int f_getown_ex(struct file *filp, unsigned long arg)
>> ret = -EINVAL;
>> break;
>> }
>> - read_unlock(&filp->f_owner.lock);
>> + read_unlock_irq(&filp->f_owner.lock);
>>
>> if (!ret) {
>> ret = copy_to_user(owner_p, &owner, sizeof(owner));
>> @@ -249,10 +250,10 @@ static int f_getowner_uids(struct file *filp, unsigned long arg)
>> uid_t src[2];
>> int err;
>>
>> - read_lock(&filp->f_owner.lock);
>> + read_lock_irq(&filp->f_owner.lock);
>> src[0] = from_kuid(user_ns, filp->f_owner.uid);
>> src[1] = from_kuid(user_ns, filp->f_owner.euid);
>> - read_unlock(&filp->f_owner.lock);
>> + read_unlock_irq(&filp->f_owner.lock);
>>
>> err = put_user(src[0], &dst[0]);
>> err |= put_user(src[1], &dst[1]);
>

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