Commit ace0c791 authored Feb 20, 2017 by Eric W. Biederman

proc/sysctl: Don't grab i_lock under sysctl_lock.



Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:
> This patch has locking problem. I've got lockdep splat under LTP.
>
> [ 6633.115456] ======================================================
> [ 6633.115502] [ INFO: possible circular locking dependency detected ]
> [ 6633.115553] 4.9.10-debug+ #9 Tainted: G             L
> [ 6633.115584] -------------------------------------------------------
> [ 6633.115627] ksm02/284980 is trying to acquire lock:
> [ 6633.115659]  (&sb->s_type->i_lock_key#4){+.+...}, at: [<ffffffff816bc1ce>] igrab+0x1e/0x80
> [ 6633.115834] but task is already holding lock:
> [ 6633.115882]  (sysctl_lock){+.+...}, at: [<ffffffff817e379b>] unregister_sysctl_table+0x6b/0x110
> [ 6633.116026] which lock already depends on the new lock.
> [ 6633.116026]
> [ 6633.116080]
> [ 6633.116080] the existing dependency chain (in reverse order) is:
> [ 6633.116117]
> -> #2 (sysctl_lock){+.+...}:
> -> #1 (&(&dentry->d_lockref.lock)->rlock){+.+...}:
> -> #0 (&sb->s_type->i_lock_key#4){+.+...}:
>
> d_lock nests inside i_lock
> sysctl_lock nests inside d_lock in d_compare
>
> This patch adds i_lock nesting inside sysctl_lock.

Al Viro <viro@ZenIV.linux.org.uk> replied:
> Once ->unregistering is set, you can drop sysctl_lock just fine.  So I'd
> try something like this - use rcu_read_lock() in proc_sys_prune_dcache(),
> drop sysctl_lock() before it and regain after.  Make sure that no inodes
> are added to the list ones ->unregistering has been set and use RCU list
> primitives for modifying the inode list, with sysctl_lock still used to
> serialize its modifications.
>
> Freeing struct inode is RCU-delayed (see proc_destroy_inode()), so doing
> igrab() is safe there.  Since we don't drop inode reference until after we'd
> passed beyond it in the list, list_for_each_entry_rcu() should be fine.

I agree with Al Viro's analsysis of the situtation.

Fixes: d6cffbbe ("proc/sysctl: prune stale dentries during unregistering")
Reported-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Tested-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Suggested-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

parent fea6d2a6

fs/proc/proc_sysctl.c

+18 −13

Original line number	Diff line number	Diff line
		@@ -266,21 +266,19 @@ static void proc_sys_prune_dcache(struct ctl_table_header *head)
		struct inode inode, prev = NULL;
		struct proc_inode *ei;

		list_for_each_entry(ei, &head->inodes, sysctl_inodes) {
		rcu_read_lock();
		list_for_each_entry_rcu(ei, &head->inodes, sysctl_inodes) {
		inode = igrab(&ei->vfs_inode);
		if (inode) {
		spin_unlock(&sysctl_lock);
		rcu_read_unlock();
		iput(prev);
		prev = inode;
		d_prune_aliases(inode);
		spin_lock(&sysctl_lock);
		rcu_read_lock();
		}
		}
		if (prev) {
		spin_unlock(&sysctl_lock);
		rcu_read_unlock();
		iput(prev);
		spin_lock(&sysctl_lock);
		}
		}

		/* called under sysctl_lock, will reacquire if has to wait */
		@@ -296,10 +294,10 @@ static void start_unregistering(struct ctl_table_header *p)
		p->unregistering = &wait;
		spin_unlock(&sysctl_lock);
		wait_for_completion(&wait);
		spin_lock(&sysctl_lock);
		} else {
		/* anything non-NULL; we'll never dereference it */
		p->unregistering = ERR_PTR(-EINVAL);
		spin_unlock(&sysctl_lock);
		}
		/*
		* Prune dentries for unregistered sysctls: namespaced sysctls
		@@ -310,6 +308,7 @@ static void start_unregistering(struct ctl_table_header *p)
		* do not remove from the list until nobody holds it; walking the
		* list in do_sysctl() relies on that.
		*/
		spin_lock(&sysctl_lock);
		erase_header(p);
		}

		@@ -455,11 +454,17 @@ static struct inode proc_sys_make_inode(struct super_block sb,
		inode->i_ino = get_next_ino();

		ei = PROC_I(inode);
		ei->sysctl = head;
		ei->sysctl_entry = table;

		spin_lock(&sysctl_lock);
		list_add(&ei->sysctl_inodes, &head->inodes);
		if (unlikely(head->unregistering)) {
		spin_unlock(&sysctl_lock);
		iput(inode);
		inode = NULL;
		goto out;
		}
		ei->sysctl = head;
		ei->sysctl_entry = table;
		list_add_rcu(&ei->sysctl_inodes, &head->inodes);
		head->count++;
		spin_unlock(&sysctl_lock);

		@@ -487,7 +492,7 @@ static struct inode proc_sys_make_inode(struct super_block sb,
		void proc_sys_evict_inode(struct inode inode, struct ctl_table_header head)
		{
		spin_lock(&sysctl_lock);
		list_del(&PROC_I(inode)->sysctl_inodes);
		list_del_rcu(&PROC_I(inode)->sysctl_inodes);
		if (!--head->count)
		kfree_rcu(head, rcu);
		spin_unlock(&sysctl_lock);