do_last(): fetch directory ->i_mode and ->i_uid before it's too late

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>

diff --git a/fs/namei.c b/fs/namei.c
index d2720dc..4fb61e0 100644 (file)
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1001,7 +1001,8 @@ 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:
@@ -1017,18 +1018,18 @@ 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))))) {
                const char *operation = S_ISFIFO(inode->i_mode) ?
@@ -3201,6 +3202,8 @@ static int do_last(struct nameidata *nd,
                   struct file *file, const struct open_flags *op)
 {
        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;
@@ -3331,7 +3334,7 @@ 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;