Re: possible deadlock in send_sigurg

[Christian Brauner]

On Fri, Apr 03, 2020 at 11:11:35AM +0200, Oleg Nesterov wrote:
> On 04/02, syzbot wrote:
> >
> >                       lock_acquire+0x1f2/0x8f0 kernel/locking/lockdep.c:4923
> >                       __raw_spin_lock include/linux/spinlock_api_smp.h:142 [inline]
> >                       _raw_spin_lock+0x2a/0x40 kernel/locking/spinlock.c:151
> >                       spin_lock include/linux/spinlock.h:353 [inline]
> >                       proc_pid_make_inode+0x1f9/0x3c0 fs/proc/base.c:1880
> 
> Yes, spin_lock(wait_pidfd.lock) is not safe...
> 
> Eric, at first glance the fix is simple.
> 
> Oleg.
> 
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c

Um, when did this lock get added to proc/base.c in the first place and
why has it been abused for this?
People just recently complained loudly about this in the
cred_guard_mutex thread that abusing locks for things they weren't
intended for is a bad idea...

> index 74f948a6b621..9ec8c114aa60 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1839,9 +1839,9 @@ void proc_pid_evict_inode(struct proc_inode *ei)
>  	struct pid *pid = ei->pid;
>  
>  	if (S_ISDIR(ei->vfs_inode.i_mode)) {
> -		spin_lock(&pid->wait_pidfd.lock);
> +		spin_lock_irq(&pid->wait_pidfd.lock);
>  		hlist_del_init_rcu(&ei->sibling_inodes);
> -		spin_unlock(&pid->wait_pidfd.lock);
> +		spin_unlock_irq(&pid->wait_pidfd.lock);
>  	}
>  
>  	put_pid(pid);
> @@ -1877,9 +1877,9 @@ struct inode *proc_pid_make_inode(struct super_block * sb,
>  	/* Let the pid remember us for quick removal */
>  	ei->pid = pid;
>  	if (S_ISDIR(mode)) {
> -		spin_lock(&pid->wait_pidfd.lock);
> +		spin_lock_irq(&pid->wait_pidfd.lock);
>  		hlist_add_head_rcu(&ei->sibling_inodes, &pid->inodes);
> -		spin_unlock(&pid->wait_pidfd.lock);
> +		spin_unlock_irq(&pid->wait_pidfd.lock);
>  	}
>  
>  	task_dump_owner(task, 0, &inode->i_uid, &inode->i_gid);
> diff --git a/fs/proc/inode.c b/fs/proc/inode.c
> index 1e730ea1dcd6..6b7ee76e1b36 100644
> --- a/fs/proc/inode.c
> +++ b/fs/proc/inode.c
> @@ -123,9 +123,9 @@ void proc_invalidate_siblings_dcache(struct hlist_head *inodes, spinlock_t *lock
>  		if (!node)
>  			break;
>  		ei = hlist_entry(node, struct proc_inode, sibling_inodes);
> -		spin_lock(lock);
> +		spin_lock_irq(lock);
>  		hlist_del_init_rcu(&ei->sibling_inodes);
> -		spin_unlock(lock);
> +		spin_unlock_irq(lock);
>  
>  		inode = &ei->vfs_inode;
>  		sb = inode->i_sb;
>