[02/10] audit: Fix possible spurious -ENOSPC error
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Message ID 20180710100217.12866-3-jack@suse.cz
State New
Headers show

Commit Message

Jan Kara July 10, 2018, 10:02 a.m. UTC
When an inode is tagged with a tree, tag_chunk() checks whether there is
audit_tree_group mark attached to the inode and adds one if not. However
nothing protects another tag_chunk() to add the mark between we've
checked and try to add the fsnotify mark thus resulting in an error from
fsnotify_add_mark() and consequently an ENOSPC error from tag_chunk().

Fix the problem by holding mark_mutex over the whole check-insert code
sequence.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 kernel/audit_tree.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

Comments

Paul Moore July 27, 2018, 4:47 a.m. UTC | #1
On Tue, Jul 10, 2018 at 6:02 AM Jan Kara <jack@suse.cz> wrote:
> When an inode is tagged with a tree, tag_chunk() checks whether there is
> audit_tree_group mark attached to the inode and adds one if not. However
> nothing protects another tag_chunk() to add the mark between we've
> checked and try to add the fsnotify mark thus resulting in an error from
> fsnotify_add_mark() and consequently an ENOSPC error from tag_chunk().
>
> Fix the problem by holding mark_mutex over the whole check-insert code
> sequence.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  kernel/audit_tree.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)

...

> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index 1c82eb6674c4..de8d344d91b1 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -342,25 +342,29 @@ static void untag_chunk(struct node *p)
>         spin_lock(&hash_lock);
>  }
>
> +/* Call with group->mark_mutex held, releases it */

Stuff like that always makes me nervous.  Could we defer releasing the
mutex to the caller, after create_chunk() returns?  It looks like
fsnotify_destroy_mark() allows a single level of nesting so it should
be okay, yes?

--
paul moore
www.paul-moore.com
Jan Kara Sept. 4, 2018, 10 a.m. UTC | #2
On Fri 27-07-18 00:47:10, Paul Moore wrote:
> On Tue, Jul 10, 2018 at 6:02 AM Jan Kara <jack@suse.cz> wrote:
> > When an inode is tagged with a tree, tag_chunk() checks whether there is
> > audit_tree_group mark attached to the inode and adds one if not. However
> > nothing protects another tag_chunk() to add the mark between we've
> > checked and try to add the fsnotify mark thus resulting in an error from
> > fsnotify_add_mark() and consequently an ENOSPC error from tag_chunk().
> >
> > Fix the problem by holding mark_mutex over the whole check-insert code
> > sequence.
> >
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  kernel/audit_tree.c | 26 ++++++++++++++++----------
> >  1 file changed, 16 insertions(+), 10 deletions(-)
> 
> ...
> 
> > diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> > index 1c82eb6674c4..de8d344d91b1 100644
> > --- a/kernel/audit_tree.c
> > +++ b/kernel/audit_tree.c
> > @@ -342,25 +342,29 @@ static void untag_chunk(struct node *p)
> >         spin_lock(&hash_lock);
> >  }
> >
> > +/* Call with group->mark_mutex held, releases it */
> 
> Stuff like that always makes me nervous.

Yes, I also prefer to avoid stuff like this.

> Could we defer releasing the mutex to the caller, after create_chunk()
> returns?  It looks like fsnotify_destroy_mark() allows a single level of
> nesting so it should be okay, yes?

This won't work. fsnotify_destroy_mark() would try to acquire the same
mutex and block indefinitely (the nesting depth is there just for lockdep
so that you can possibly nest mark_mutexes of two different group's). And
even if we do more work and use separate fsnotify_detach_mark() and
fsnotify_free_mark() calls instead of fsnotify_destroy_mark(), the problem
is still there as fsnotify_free_mark() must not be called under mark_mutex
(as it can acquire it in some cases).

So as much as I don't like functions that release locks they didn't take I
don't see how to avoid that here without creating even bigger mess.

								Honza

Patch
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diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
index 1c82eb6674c4..de8d344d91b1 100644
--- a/kernel/audit_tree.c
+++ b/kernel/audit_tree.c
@@ -342,25 +342,29 @@  static void untag_chunk(struct node *p)
 	spin_lock(&hash_lock);
 }
 
+/* Call with group->mark_mutex held, releases it */
 static int create_chunk(struct inode *inode, struct audit_tree *tree)
 {
 	struct fsnotify_mark *entry;
 	struct audit_chunk *chunk = alloc_chunk(1);
-	if (!chunk)
+
+	if (!chunk) {
+		mutex_unlock(&audit_tree_group->mark_mutex);
 		return -ENOMEM;
+	}
 
 	entry = &chunk->mark;
-	if (fsnotify_add_inode_mark(entry, inode, 0)) {
+	if (fsnotify_add_inode_mark_locked(entry, inode, 0)) {
+		mutex_unlock(&audit_tree_group->mark_mutex);
 		fsnotify_put_mark(entry);
 		return -ENOSPC;
 	}
 
-	mutex_lock(&entry->group->mark_mutex);
 	spin_lock(&hash_lock);
 	if (tree->goner) {
 		spin_unlock(&hash_lock);
 		chunk->dead = 1;
-		mutex_unlock(&entry->group->mark_mutex);
+		mutex_unlock(&audit_tree_group->mark_mutex);
 		fsnotify_destroy_mark(entry, audit_tree_group);
 		fsnotify_put_mark(entry);
 		return 0;
@@ -375,7 +379,7 @@  static int create_chunk(struct inode *inode, struct audit_tree *tree)
 	}
 	insert_hash(chunk);
 	spin_unlock(&hash_lock);
-	mutex_unlock(&entry->group->mark_mutex);
+	mutex_unlock(&audit_tree_group->mark_mutex);
 	fsnotify_put_mark(entry);	/* drop initial reference */
 	return 0;
 }
@@ -389,6 +393,7 @@  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 	struct node *p;
 	int n;
 
+	mutex_lock(&audit_tree_group->mark_mutex);
 	old_entry = fsnotify_find_mark(&inode->i_fsnotify_marks,
 				       audit_tree_group);
 	if (!old_entry)
@@ -401,6 +406,7 @@  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 	for (n = 0; n < old->count; n++) {
 		if (old->owners[n].owner == tree) {
 			spin_unlock(&hash_lock);
+			mutex_unlock(&audit_tree_group->mark_mutex);
 			fsnotify_put_mark(old_entry);
 			return 0;
 		}
@@ -409,20 +415,20 @@  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 
 	chunk = alloc_chunk(old->count + 1);
 	if (!chunk) {
+		mutex_unlock(&audit_tree_group->mark_mutex);
 		fsnotify_put_mark(old_entry);
 		return -ENOMEM;
 	}
 
 	chunk_entry = &chunk->mark;
 
-	mutex_lock(&old_entry->group->mark_mutex);
 	/*
 	 * mark_mutex protects mark from getting detached and thus also from
 	 * mark->connector->obj getting NULL.
 	 */
 	if (!(old_entry->flags & FSNOTIFY_MARK_FLAG_ATTACHED)) {
 		/* old_entry is being shot, lets just lie */
-		mutex_unlock(&old_entry->group->mark_mutex);
+		mutex_unlock(&audit_tree_group->mark_mutex);
 		fsnotify_put_mark(old_entry);
 		fsnotify_put_mark(&chunk->mark);
 		return -ENOENT;
@@ -430,7 +436,7 @@  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 
 	if (fsnotify_add_mark_locked(chunk_entry, old_entry->connector->obj,
 				     FSNOTIFY_OBJ_TYPE_INODE, 1)) {
-		mutex_unlock(&old_entry->group->mark_mutex);
+		mutex_unlock(&audit_tree_group->mark_mutex);
 		fsnotify_put_mark(chunk_entry);
 		fsnotify_put_mark(old_entry);
 		return -ENOSPC;
@@ -440,7 +446,7 @@  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 	if (tree->goner) {
 		spin_unlock(&hash_lock);
 		chunk->dead = 1;
-		mutex_unlock(&old_entry->group->mark_mutex);
+		mutex_unlock(&audit_tree_group->mark_mutex);
 
 		fsnotify_destroy_mark(chunk_entry, audit_tree_group);
 
@@ -471,7 +477,7 @@  static int tag_chunk(struct inode *inode, struct audit_tree *tree)
 		list_add(&tree->same_root, &chunk->trees);
 	}
 	spin_unlock(&hash_lock);
-	mutex_unlock(&old_entry->group->mark_mutex);
+	mutex_unlock(&audit_tree_group->mark_mutex);
 	fsnotify_destroy_mark(old_entry, audit_tree_group);
 	fsnotify_put_mark(chunk_entry);	/* drop initial reference */
 	fsnotify_put_mark(old_entry); /* pair to fsnotify_find mark_entry */