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fuse: mark all aliases of an inode stale in unlink

PATCH In review Anand Avati 2 participants 3 messages Jan 19, 2012 – Jan 20, 2012

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Anand Avati

author
#0 lore raw

PATCH 1/1 fuse: mark all aliases of an inode stale in unlink

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 <[email protected]>
---
 fs/fuse/dir.c |   21 ++++++++++++++++++++-
 1 files changed, 20 insertions(+), 1 deletions(-)
fs/fuse/dir.c +20 -1
index 2066328..880e9dd 100644
@@ -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;
Signature · 1 line
1.7.6.4
maintainer
#1 lore raw
Quoted 16 lines · Anand Avati

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.

Quoted 4 lines

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.

This is only a partial fix: the entry may still be referenced and used:
linkat(..., AT_EMPTY_PATH). There are also several other places where i_nlink is cheked without refreshing the attributes.

So how about changing that clear_nlink() to drop_nlink()?

I don't quite understand what that comment above clear_nlink() is saying but I suspect it comes from before ->drop_inode was used. Now the inode will go away if no dentry is referencing it regardless of the value of i_nlink.

Untested patch below. Does that work for you?

Thanks,
Miklos

fs/fuse/dir.c +1 -6
index 2066328..517a7f5 100644
@@ -645,12 +645,7 @@ static int fuse_unlink(struct inode *dir, struct dentry *entry)
 	if (!err) {
 		struct inode *inode = entry->d_inode;
 
-		/*
-		 * Set nlink to zero so the inode can be cleared, if the inode
-		 * does have more links this will be discovered at the next
-		 * lookup/getattr.
-		 */
-		clear_nlink(inode);
+		drop_nlink(inode);
 		fuse_invalidate_attr(inode);
 		fuse_invalidate_attr(dir);
 		fuse_invalidate_entry_cache(entry);
author
#2 lore raw
Quoted 14 lines · Miklos Szeredi

Untested patch below. Does that work for you?

Thanks,
Miklos

fs/fuse/dir.c +0 -0
index 2066328..517a7f5 100644
@@ -645,12 +645,7 @@ static int fuse_unlink(struct inode *dir, struct dentry *entry)
  	if (!err) {
  		struct inode *inode = entry->d_inode;

fs/fuse/dir.c static int fuse_unlink(struct inode *dir, struct dentry *entry) · line 651

Quoted 10 lines
fs/fuse/dir.c +1 -6
-		/*
-		 * Set nlink to zero so the inode can be cleared, if the inode
-		 * does have more links this will be discovered at the next
-		 * lookup/getattr.
-		 */
-		clear_nlink(inode);
+		drop_nlink(inode);
  		fuse_invalidate_attr(inode);
  		fuse_invalidate_attr(dir);
  		fuse_invalidate_entry_cache(entry);

That will work for me!

Thanks,
Avati