UBIFS: 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()'.

This means that 'file->private_data' can be freed while 'ubifs_readdir()' uses
it, and this is a very bad bug: not only 'ubifs_readdir()' can return garbage,
but this may corrupt memory and lead to all kinds of problems like crashes an
security holes.

This patch fixes the problem by using the 'file->f_version' field, which
'->llseek()' always unconditionally sets to zero. We set it to 1 in
'ubifs_readdir()' and whenever we detect that it became 0, we know there was a
seek and it is time to clear the state saved in 'file->private_data'.

I tested this patch by writing a user-space program which runds readdir and
seek in parallell. I could easily crash the kernel without these patches, but
could not crash it with these patches.

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>

1 files changed, 27 insertions, 3 deletions

diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index 8e587afd7732..605af512aec2 100644
--- a/fs/ubifs/dir.c
+++ b/fs/ubifs/dir.c
@@ -365,6 +365,24 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
                  */
                 return 0;
 
+        if (file->f_version == 0) {
+                /*
+                 * The file was seek'ed, which means that @file->private_data
+                 * is now invalid. This may also be just the first
+                 * 'ubifs_readdir()' invocation, in which case
+                 * @file->private_data is NULL, and the below code is
+                 * basically a no-op.
+                 */
+                kfree(file->private_data);
+                file->private_data = NULL;
+        }
+
+        /*
+         * 'generic_file_llseek()' unconditionally sets @file->f_version to
+         * zero, and we use this for detecting whether the file was seek'ed.
+         */
+        file->f_version = 1;
+
         /* File positions 0 and 1 correspond to "." and ".." */
         if (pos == 0) {
                 ubifs_assert(!file->private_data);
@@ -438,6 +456,14 @@ static int ubifs_readdir(struct file *file, void *dirent, filldir_t filldir)
                 file->f_pos = pos = key_hash_flash(c, &dent->key);
                 file->private_data = dent;
                 cond_resched();
+
+                if (file->f_version == 0)
+                        /*
+                         * The file was seek'ed meanwhile, lets return and start
+                         * reading direntries from the new position on the next
+                         * invocation.
+                         */
+                        return 0;
         }
 
 out:
@@ -448,15 +474,13 @@ out:
 
         kfree(file->private_data);
         file->private_data = NULL;
+        /* 2 is a special value indicating that there are no more direntries */
         file->f_pos = 2;
         return 0;
 }
 
-/* If a directory is seeked, we have to free saved readdir() state */
 static loff_t ubifs_dir_llseek(struct file *file, loff_t offset, int whence)
 {
-        kfree(file->private_data);
-        file->private_data = NULL;
         return generic_file_llseek(file, offset, whence);
 }