UBIFS: prepare to fix a horrid bug

Al Viro pointed me to the fact that '->readdir()' and '->llseek()' have no
mutual exclusion, which means the 'ubifs_dir_llseek()' can be run while we are
in the middle of 'ubifs_readdir()'.

First of all, this means that 'file->private_data' can be freed while
'ubifs_readdir()' uses it.  But this particular patch does not fix the problem.
This patch is only a preparation, and the fix will follow next.

In this patch we make 'ubifs_readdir()' stop using 'file->f_pos' directly,
because 'file->f_pos' can be changed by '->llseek()' at any point. This may
lead 'ubifs_readdir()' to returning inconsistent data: directory entry names
may correspond to incorrect file positions.

So here we introduce a local variable 'pos', read 'file->f_pose' once at very
the beginning, and then stick to 'pos'. The result of this is that when
'ubifs_dir_llseek()' changes 'file->f_pos' while we are in the middle of
'ubifs_readdir()', the latter "wins".

Cc: stable@vger.kernel.org
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index de08c92..8e587af 100644 (file)
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -349,15 +349,16 @@ static unsigned int vfs_dent_type(uint8_t type)
 static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
 {
        int err, over = 0;
+       loff_t pos = file->f_pos;
        struct qstr nm;
        union ubifs_key key;
        struct ubifs_dent_node *dent;
        struct inode *dir = file_inode(file);
        struct ubifs_info *c = dir->i_sb->s_fs_info;
 
-       dbg_gen("dir ino %lu, f_pos %#llx", dir->i_ino, file->f_pos);
+       dbg_gen("dir ino %lu, f_pos %#llx", dir->i_ino, pos);
 
-       if (file->f_pos > UBIFS_S_KEY_HASH_MASK || file->f_pos == 2)
+       if (pos > UBIFS_S_KEY_HASH_MASK || pos == 2)
                /*
                 * The directory was seek'ed to a senseless position or there
                 * are no more entries.
@@ -365,15 +366,15 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
                return 0;
 
        /* File positions 0 and 1 correspond to "." and ".." */
-       if (file->f_pos == 0) {
+       if (pos == 0) {
                ubifs_assert(!file->private_data);
                over = filldir(dirent, ".", 1, 0, dir->i_ino, DT_DIR);
                if (over)
                        return 0;
-               file->f_pos = 1;
+               file->f_pos = pos = 1;
        }
 
-       if (file->f_pos == 1) {
+       if (pos == 1) {
                ubifs_assert(!file->private_data);
                over = filldir(dirent, "..", 2, 1,
                               parent_ino(file->f_path.dentry), DT_DIR);
@@ -389,7 +390,7 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
                        goto out;
                }
 
-               file->f_pos = key_hash_flash(c, &dent->key);
+               file->f_pos = pos = key_hash_flash(c, &dent->key);
                file->private_data = dent;
        }
 
@@ -397,17 +398,16 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
        if (!dent) {
                /*
                 * The directory was seek'ed to and is now readdir'ed.
-                * Find the entry corresponding to @file->f_pos or the
-                * closest one.
+                * Find the entry corresponding to @pos or the closest one.
                 */
-               dent_key_init_hash(c, &key, dir->i_ino, file->f_pos);
+               dent_key_init_hash(c, &key, dir->i_ino, pos);
                nm.name = NULL;
                dent = ubifs_tnc_next_ent(c, &key, &nm);
                if (IS_ERR(dent)) {
                        err = PTR_ERR(dent);
                        goto out;
                }
-               file->f_pos = key_hash_flash(c, &dent->key);
+               file->f_pos = pos = key_hash_flash(c, &dent->key);
                file->private_data = dent;
        }
 
@@ -419,7 +419,7 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
                             ubifs_inode(dir)->creat_sqnum);
 
                nm.len = le16_to_cpu(dent->nlen);
-               over = filldir(dirent, dent->name, nm.len, file->f_pos,
+               over = filldir(dirent, dent->name, nm.len, pos,
                               le64_to_cpu(dent->inum),
                               vfs_dent_type(dent->type));
                if (over)
@@ -435,7 +435,7 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
                }
 
                kfree(file->private_data);
-               file->f_pos = key_hash_flash(c, &dent->key);
+               file->f_pos = pos = key_hash_flash(c, &dent->key);
                file->private_data = dent;
                cond_resched();
        }