path: root/drivers/tty/pty.c
diff options
authorHerton R. Krzesinski <herton@redhat.com>2016-01-14 17:56:58 -0200
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-02-06 23:45:46 -0800
commit1f55c718c290616889c04946864a13ef30f64929 (patch)
treed578b6dee5f2eb8ebe8f73ccba2b79a31252c52c /drivers/tty/pty.c
parent2831c89f42dcde440cfdccb9fee9f42d54bbc1ef (diff)
pty: make sure super_block is still valid in final /dev/tty close
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>
Diffstat (limited to 'drivers/tty/pty.c')
1 files changed, 6 insertions, 3 deletions
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 3b5cde833ee5..2348fa613707 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -688,7 +688,7 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
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);

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