Commit 1f55c718 authored by Herton R. Krzesinski's avatar Herton R. Krzesinski Committed by Greg Kroah-Hartman
Browse files

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: default avatarHerton R. Krzesinski <herton@redhat.com>
Cc: stable@vger.kernel.org # 2.6.29+
Reviewed-by: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 2831c89f

drivers/tty/pty.c

+6 −3
Original line number Diff line number Diff line
@@ -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);

fs/devpts/inode.c

+20 −0
Original line number Diff line number Diff line
@@ -575,6 +575,26 @@ void devpts_kill_index(struct inode *ptmx_inode, int idx) 
	mutex_unlock(&allocated_ptys_lock);

}



/*

 * pty code needs to hold extra references in case of last /dev/tty close

 */



void devpts_add_ref(struct inode *ptmx_inode)

{

	struct super_block *sb = pts_sb_from_inode(ptmx_inode);



	atomic_inc(&sb->s_active);

	ihold(ptmx_inode);

}



void devpts_del_ref(struct inode *ptmx_inode)

{

	struct super_block *sb = pts_sb_from_inode(ptmx_inode);



	iput(ptmx_inode);

	deactivate_super(sb);

}



/**

 * devpts_pty_new -- create a new inode in /dev/pts/

 * @ptmx_inode: inode of the master

include/linux/devpts_fs.h

+4 −0
Original line number Diff line number Diff line
@@ -19,6 +19,8 @@ 


int devpts_new_index(struct inode *ptmx_inode);

void devpts_kill_index(struct inode *ptmx_inode, int idx);

void devpts_add_ref(struct inode *ptmx_inode);

void devpts_del_ref(struct inode *ptmx_inode);

/* mknod in devpts */

struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,

		void *priv);
@@ -32,6 +34,8 @@ void devpts_pty_kill(struct inode *inode); 
/* Dummy stubs in the no-pty case */

static inline int devpts_new_index(struct inode *ptmx_inode) { return -EINVAL; }

static inline void devpts_kill_index(struct inode *ptmx_inode, int idx) { }

static inline void devpts_add_ref(struct inode *ptmx_inode) { }

static inline void devpts_del_ref(struct inode *ptmx_inode) { }

static inline struct inode *devpts_pty_new(struct inode *ptmx_inode,

		dev_t device, int index, void *priv)

{

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