CVE-2014-9710 (C, published 2015-05-27T10:59Z)
Description: value: The Btrfs implementation in the Linux kernel before 3.19 does not ensure that the visible xattr state is consistent with a requested replacement, which allows local users to bypass intended ACL settings and gain privileges via standard filesystem operations (1) during an xattr-replacement time window, related to a race condition, or (2) after an xattr-replacement attempt that fails because the data does not fit.
Severity: MEDIUM (CVSS v3 base score: )
Weakness: CWE-362 \u2014 Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.
Repository: https://github.com/torvalds/linux
Files changed:
fs/btrfs/ctree.c
fs/btrfs/ctree.h
fs/btrfs/dir-item.c
fs/btrfs/xattr.c
Commit message: Btrfs: make xattr replace operations atomic

Replacing a xattr consists of doing a lookup for its existing value, delete
the current value from the respective leaf, release the search path and then
finally insert the new value. This leaves a time window where readers (getxattr,
listxattrs) won't see any value for the xattr. Xattrs are used to store ACLs,
so this has security implications.

This change also fixes 2 other existing issues which were:

*) Deleting the old xattr value without verifying first if the new xattr will
   fit in the existing leaf item (in case multiple xattrs are packed in the
   same item due to name hash collision);

*) Returning -EEXIST when the flag XATTR_CREATE is given and the xattr doesn't
   exist but we have have an existing item that packs muliple xattrs with
   the same name hash as the input xattr. In this case we should return ENOSPC.

A test case for xfstests follows soon.

Thanks to Alexandre Oliva for reporting the non-atomicity of the xattr replace
implementation.

Reported-by: Alexandre Oliva <oliva@gnu.org>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>


Vulnerable code:
	if (ret < 0)
static struct btrfs_dir_item *btrfs_match_dir_item_name(struct btrfs_root *root,
			      struct btrfs_path *path,
			      const char *name, int name_len);

static struct btrfs_dir_item *btrfs_match_dir_item_name(struct btrfs_root *root,
			      struct btrfs_path *path,
			      const char *name, int name_len)
	struct btrfs_dir_item *di;
		di = btrfs_lookup_xattr(trans, root, path, btrfs_ino(inode), name,
					name_len, -1);
		if (IS_ERR(di)) {
			ret = PTR_ERR(di);
			goto out;
		} else if (!di) {
		ret = btrfs_delete_one_dir_name(trans, root, path, di);
		if (ret)
			goto out;
		 * remove the attribute
		if (!value)
			goto out;
	} else {
		di = btrfs_lookup_xattr(NULL, root, path, btrfs_ino(inode),
					name, name_len, 0);
		if (IS_ERR(di)) {
			ret = PTR_ERR(di);
		if (!di && !value)
			goto out;
		btrfs_release_path(path);
again:
	ret = btrfs_insert_xattr_item(trans, root, path, btrfs_ino(inode),
				      name, name_len, value, size);
	/*
	 * If we're setting an xattr to a new value but the new value is say
	 * exactly BTRFS_MAX_XATTR_SIZE, we could end up with EOVERFLOW getting
	 * back from split_leaf.  This is because it thinks we'll be extending
	 * the existing item size, but we're asking for enough space to add the
	 * item itself.  So if we get EOVERFLOW just set ret to EEXIST and let
	 * the rest of the function figure it out.
	 */
	if (ret == -EOVERFLOW)
	if (ret == -EEXIST) {
		if (flags & XATTR_CREATE)
			goto out;
		 * We can't use the path we already have since we won't have the
		 * proper locking for a delete, so release the path and
		 * re-lookup to delete the thing.
		btrfs_release_path(path);
		di = btrfs_lookup_xattr(trans, root, path, btrfs_ino(inode),
					name, name_len, -1);
		if (IS_ERR(di)) {
			ret = PTR_ERR(di);
			goto out;
		} else if (!di) {
			/* Shouldn't happen but just in case... */
			btrfs_release_path(path);
			goto again;
		ret = btrfs_delete_one_dir_name(trans, root, path, di);
		if (ret)
			goto out;
		 * We have a value to set, so go back and try to insert it now.
		if (value) {
			btrfs_release_path(path);
			goto again;
		}

Fixed code:
	if (ret < 0 && !p->skip_release_on_error)
	unsigned int skip_release_on_error:1;
struct btrfs_dir_item *btrfs_match_dir_item_name(struct btrfs_root *root,
						 struct btrfs_path *path,
						 const char *name,
						 int name_len);
struct btrfs_dir_item *btrfs_match_dir_item_name(struct btrfs_root *root,
						 struct btrfs_path *path,
						 const char *name, int name_len)
#include "locking.h"
	struct btrfs_dir_item *di = NULL;
	path->skip_release_on_error = 1;

	if (!value) {
		di = btrfs_lookup_xattr(trans, root, path, btrfs_ino(inode),
					name, name_len, -1);
		if (!di && (flags & XATTR_REPLACE))
			ret = -ENODATA;
		else if (di)
			ret = btrfs_delete_one_dir_name(trans, root, path, di);
		goto out;
	}
	/*
	 * For a replace we can't just do the insert blindly.
	 * Do a lookup first (read-only btrfs_search_slot), and return if xattr
	 * doesn't exist. If it exists, fall down below to the insert/replace
	 * path - we can't race with a concurrent xattr delete, because the VFS
	 * locks the inode's i_mutex before calling setxattr or removexattr.
	 */
		ASSERT(mutex_is_locked(&inode->i_mutex));
		di = btrfs_lookup_xattr(NULL, root, path, btrfs_ino(inode),
					name, name_len, 0);
		if (!di) {
		di = NULL;
	}
	ret = btrfs_insert_xattr_item(trans, root, path, btrfs_ino(inode),
				      name, name_len, value, size);
	if (ret == -EOVERFLOW) {
		 * We have an existing item in a leaf, split_leaf couldn't
		 * expand it. That item might have or not a dir_item that
		 * matches our target xattr, so lets check.
		ret = 0;
		btrfs_assert_tree_locked(path->nodes[0]);
		di = btrfs_match_dir_item_name(root, path, name, name_len);
		if (!di && !(flags & XATTR_REPLACE)) {
			ret = -ENOSPC;
	} else if (ret == -EEXIST) {
		ret = 0;
		di = btrfs_match_dir_item_name(root, path, name, name_len);
		ASSERT(di); /* logic error */
	} else if (ret) {
		goto out;
	if (di && (flags & XATTR_CREATE)) {
		goto out;
	}
	if (di) {
		 * We're doing a replace, and it must be atomic, that is, at
		 * any point in time we have either the old or the new xattr
		 * value in the tree. We don't want readers (getxattr and
		 * listxattrs) to miss a value, this is specially important
		 * for ACLs.
		const int slot = path->slots[0];
		struct extent_buffer *leaf = path->nodes[0];
		const u16 old_data_len = btrfs_dir_data_len(leaf, di);
		const u32 item_size = btrfs_item_size_nr(leaf, slot);
		const u32 data_size = sizeof(*di) + name_len + size;
		struct btrfs_item *item;
		unsigned long data_ptr;
		char *ptr;

		if (size > old_data_len) {
			if (btrfs_leaf_free_space(root, leaf) <
			    (size - old_data_len)) {
				ret = -ENOSPC;
				goto out;
			}
		if (old_data_len + name_len + sizeof(*di) == item_size) {
			/* No other xattrs packed in the same leaf item. */
			if (size > old_data_len)
				btrfs_extend_item(root, path,
						  size - old_data_len);
			else if (size < old_data_len)
				btrfs_truncate_item(root, path, data_size, 1);
		} else {
			/* There are other xattrs packed in the same item. */
			ret = btrfs_delete_one_dir_name(trans, root, path, di);
			if (ret)
				goto out;
			btrfs_extend_item(root, path, data_size);
		}
		item = btrfs_item_nr(slot);
		ptr = btrfs_item_ptr(leaf, slot, char);
		ptr += btrfs_item_size(leaf, item) - data_size;
		di = (struct btrfs_dir_item *)ptr;
		btrfs_set_dir_data_len(leaf, di, size);
		data_ptr = ((unsigned long)(di + 1)) + name_len;
		write_extent_buffer(leaf, value, data_ptr, size);
		btrfs_mark_buffer_dirty(leaf);
	} else {
		 * Insert, and we had space for the xattr, so path->slots[0] is
		 * where our xattr dir_item is and btrfs_insert_xattr_item()
		 * filled it.