[PATCH 3.16 209/245] do_last(): fetch directory ->i_mode and ->i_uid before it's too late

Ben Hutchings
2020-04-23T23:12:41Z

From: Al Viro <viro@zeniv.linux.org.uk>

commit d0cb50185ae942b03c4327be322055d622dc79f6 upstream.

may_create_in_sticky() call is done when we already have dropped the
reference to dir.

Fixes: 30aba6656f61e (namei: allow restricted O_CREAT of FIFOs and regular files)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/namei.c | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -843,7 +843,8 @@ static int may_linkat(struct path *link)
* may_create_in_sticky - Check whether an O_CREAT open in a sticky directory
* should be allowed, or not, on files that already
* exist.
- * @dir: the sticky parent directory
+ * @dir_mode: mode bits of directory
+ * @dir_uid: owner of directory
* @inode: the inode of the file to open
*
* Block an O_CREAT open of a FIFO (or a regular file) when:
@@ -859,18 +860,18 @@ static int may_linkat(struct path *link)
*
* Returns 0 if the open is allowed, -ve on error.
*/
-static int may_create_in_sticky(struct dentry * const dir,
+static int may_create_in_sticky(umode_t dir_mode, kuid_t dir_uid,
struct inode * const inode)
{
if ((!sysctl_protected_fifos && S_ISFIFO(inode->i_mode)) ||
(!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
- likely(!(dir->d_inode->i_mode & S_ISVTX)) ||
- uid_eq(inode->i_uid, dir->d_inode->i_uid) ||
+ likely(!(dir_mode & S_ISVTX)) ||
+ uid_eq(inode->i_uid, dir_uid) ||
uid_eq(current_fsuid(), inode->i_uid))
return 0;

- if (likely(dir->d_inode->i_mode & 0002) ||
- (dir->d_inode->i_mode & 0020 &&
+ if (likely(dir_mode & 0002) ||
+ (dir_mode & 0020 &&
((sysctl_protected_fifos >= 2 && S_ISFIFO(inode->i_mode)) ||
(sysctl_protected_regular >= 2 && S_ISREG(inode->i_mode))))) {
return -EACCES;
@@ -2944,6 +2945,8 @@ static int do_last(struct nameidata *nd,
int *opened, struct filename *name)
{
struct dentry *dir = nd->path.dentry;
+ kuid_t dir_uid = dir->d_inode->i_uid;
+ umode_t dir_mode = dir->d_inode->i_mode;
int open_flag = op->open_flag;
bool will_truncate = (open_flag & O_TRUNC) != 0;
bool got_write = false;
@@ -3102,7 +3105,7 @@ finish_open:
error = -EISDIR;
if (d_is_dir(nd->path.dentry))
goto out;
- error = may_create_in_sticky(dir,
+ error = may_create_in_sticky(dir_mode, dir_uid,
d_backing_inode(nd->path.dentry));
if (unlikely(error))
goto out;