[51/88] UBIFS: fix a horrid bug

Steven Rostedt
2013-08-13T16:00:15Z

[-- Attachment #1: 0051-UBIFS-fix-a-horrid-bug.patch --]
[-- Type: text/plain, Size: 3349 bytes --]

3.6.11.7-rc1 stable review patch.
If anyone has any objections, please let me know.

------------------

From: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

[ Upstream commit 605c912bb843c024b1ed173dc427cd5c08e5d54d ]

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>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
fs/ubifs/dir.c | 30 +++++++++++++++++++++++++++---
1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
index 63799a7..1f61701 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 origin)
{
- kfree(file->private_data);
- file->private_data = NULL;
return generic_file_llseek(file, offset, origin);
}

--
1.7.10.4