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pty: make sure super_block is still valid in final /dev/tty close

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Considering current pty code and multiple devpts instances, it's possible
to umount a devpts file system while a program still has /dev/tty opened
pointing to a previosuly closed pty pair in that instance. In the case all
ptmx and pts/N files are closed, umount can be done. If the program closes
/dev/tty after umount is done, devpts_kill_index will use now an invalid
super_block, which was already destroyed in the umount operation after
running ->kill_sb. This is another "use after free" type of issue, but now
related to the allocated super_block instance.

To avoid the problem (warning at ida_remove and potential crashes) for
this specific case, I added two functions in devpts which grabs additional
references to the super_block, which pty code now uses so it makes sure
the super block structure is still valid until pty shutdown is done.
I also moved the additional inode references to the same functions, which
also covered similar case with inode being freed before /dev/tty final
close/shutdown.

Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
Cc: stable@vger.kernel.org # 2.6.29+
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Herton R. Krzesinski
2016-01-14 17:56:58 -02:00
committed by Greg Kroah-Hartman
parent 2831c89f42
commit 1f55c718c2
3 changed files with 30 additions and 3 deletions
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9
drivers/tty/pty.c
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@@ -688,7 +688,7 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
else
ptmx_inode = tty->link->driver_data;
devpts_kill_index(ptmx_inode, tty->index);
iput(ptmx_inode); /* drop reference we acquired at ptmx_open */
devpts_del_ref(ptmx_inode);
}
static const struct tty_operations ptm_unix98_ops = {
@@ -785,9 +785,12 @@ static int ptmx_open(struct inode *inode, struct file *filp)
* still have /dev/tty opened pointing to the master/slave pair (ptmx
* is closed/released before /dev/tty), we must make sure that the inode
* is still valid when we call the final pty_unix98_shutdown, thus we
* hold an additional reference to the ptmx inode
* hold an additional reference to the ptmx inode. For the same /dev/tty
* last close case, we also need to make sure the super_block isn't
* destroyed (devpts instance unmounted), before /dev/tty is closed and
* on its release devpts_kill_index is called.
*/
ihold(inode);
devpts_add_ref(inode);
tty_add_file(tty, filp);