[PATCH] knfsd: close a race-opportunity in d_splice_alias

There is a possible race in d_splice_alias.  Though __d_find_alias(inode, 1)
will only return a dentry with DCACHE_DISCONNECTED set, it is possible for it
to get cleared before the BUG_ON, and it is is not possible to lock against
that.

There are a couple of problems here.  Firstly, the code doesn't match the
comment.  The comment describes a 'disconnected' dentry as being IS_ROOT as
well as DCACHE_DISCONNECTED, however there is not testing of IS_ROOT anythere.

A dentry is marked DCACHE_DISCONNECTED when allocated with d_alloc_anon, and
remains DCACHE_DISCONNECTED while a path is built up towards the root.  So a
dentry can have a valid name and a valid parent and even grandparent, but will
still be DCACHE_DISCONNECTED until a path to the root is created.  Once the
path to the root is complete, everything in the path gets DCACHE_DISCONNECTED
cleared.  So the fact that DCACHE_DISCONNECTED isn't enough to say that a
dentry is free to be spliced in with a given name.  This can only be allowed
if the dentry does not yet have a name, so the IS_ROOT test is needed too.

However even adding that test to __d_find_alias isn't enough.  As
d_splice_alias drops dcache_lock before calling d_move to perform the splice,
it could race with another thread calling d_splice_alias to splice the inode
in with a different name in a different part of the tree (in the case where a
file has hard links).  So that splicing code is only really safe for
directories (as we know that directories only have one link).  For
directories, the caller of d_splice_alias will be holding i_mutex on the
(unique) parent so there is no room for a race.

A consequence of this is that a non-directory will never benefit from being
spliced into a pre-exisiting dentry, but that isn't a problem.  It is
perfectly OK for a non-directory to have multiple dentries, some anonymous,
some not.  And the comment for d_splice_alias says that it only happens for
directories anyway.
Signed-off-by: Neil Brown <neilb@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/dcache.c

@@ -291,9 +291,9 @@ struct dentry * dget_locked(struct dentry *dentry)
  * it can be unhashed only if it has no children, or if it is the root
  * of a filesystem.
  *
- * If the inode has a DCACHE_DISCONNECTED alias, then prefer
+ * If the inode has an IS_ROOT, DCACHE_DISCONNECTED alias, then prefer
  * any other hashed alias over that one unless @want_discon is set,
- * in which case only return a DCACHE_DISCONNECTED alias.
+ * in which case only return an IS_ROOT, DCACHE_DISCONNECTED alias.
  */
 
 static struct dentry * __d_find_alias(struct inode *inode, int want_discon)
@@ -309,7 +309,8 @@ static struct dentry * __d_find_alias(struct inode *inode, int want_discon)
 		prefetch(next);
 		alias = list_entry(tmp, struct dentry, d_alias);
  		if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
-			if (alias->d_flags & DCACHE_DISCONNECTED)
+			if (IS_ROOT(alias) &&
+			    (alias->d_flags & DCACHE_DISCONNECTED))
 				discon_alias = alias;
 			else if (!want_discon) {
 				__dget_locked(alias);
@@ -1004,7 +1005,7 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 {
 	struct dentry *new = NULL;
 
-	if (inode) {
+	if (inode && S_ISDIR(inode->i_mode)) {
 		spin_lock(&dcache_lock);
 		new = __d_find_alias(inode, 1);
 		if (new) {