[PATCH] fuse: mark all aliases of an inode stale in unlink

[Anand Avati <avati@redhat.com>]

Problem:
	create("filename") => 0
	link("filename", "linkname") => 0
	unlink("filename") => 0
	link("linkname", "filename") => -ENOENT ### BUG ###

This is a test case link section of the Poix compliance test suite from
ntfs-3g project.

Cause:
	fuse_unlink() clears i_nlink of the inode. If the inode has
hardlinks (and hence aliases) they kept fresh (within the entry
timeout bounds.) A link() call happening right after unlink now checks
for i_nlink == 0 (since patch aae8a97d3ec30788790d1720b71d76fd8eb44b73)
and fails the syscall for security reasons.

Fix:
	Instead of marking the just-unlinked entry as stale, mark all
the aliases as stale so that a revalidation of the alias is done during
path resolution.

Signed-off-by: Anand Avati <avati@redhat.com>
---
 fs/fuse/dir.c |   21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 2066328..880e9dd 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -103,6 +103,25 @@ void fuse_invalidate_entry_cache(struct dentry *entry)
 }
 
 /*
+ * Mark all dentry aliases of an inode as stale. It is necessary to mark
+ * all aliases as stale because a link() on an inode performed right
+ * after an unlink on one of the hardlinks will pick the inode without
+ * a revalidate and find i_nlink == 0. This ends up getting checked as
+ * a security vulnerability and fail -ENOENT. Marking all aliases as stale
+ * will instead force lookup the inode freshly at the time of link()
+ */
+void fuse_invalidate_all_aliases(struct inode *inode)
+{
+	struct dentry *alias;
+
+	spin_lock(&inode->i_lock);
+	list_for_each_entry(alias, &inode->i_dentry, d_alias) {
+		fuse_invalidate_entry_cache(alias);
+	}
+	spin_unlock(&inode->i_lock);
+}
+
+/*
  * Same as fuse_invalidate_entry_cache(), but also try to remove the
  * dentry from the hash
  */
@@ -653,7 +672,7 @@ static int fuse_unlink(struct inode *dir, struct dentry *entry)
 		clear_nlink(inode);
 		fuse_invalidate_attr(inode);
 		fuse_invalidate_attr(dir);
-		fuse_invalidate_entry_cache(entry);
+		fuse_invalidate_all_aliases(inode);
 	} else if (err == -EINTR)
 		fuse_invalidate_entry(entry);
 	return err;
-- 
1.7.6.4