linux-stable.git/drivers/tty/tty_ldisc.c, branch linux-4.9.y

tty: Don't block on IO when ldisc change is pending

2019-12-21T09:41:25+00:00

[ Upstream commit c96cf923a98d1b094df9f0cf97a83e118817e31b ]

There might be situations where tty_ldisc_lock() has blocked, but there
is already IO on tty and it prevents line discipline changes.
It might theoretically turn into dead-lock.

Basically, provide more priority to pending tty_ldisc_lock() than to
servicing reads/writes over tty.

User-visible issue was reported by Mikulas where on pa-risc with
Debian 5 reboot took either 80 seconds, 3 minutes or 3:25 after proper
locking in tty_reopen().

Cc: Jiri Slaby 
Reported-by: Mikulas Patocka 
Signed-off-by: Dmitry Safonov 
Signed-off-by: Greg Kroah-Hartman 
Signed-off-by: Sasha Levin

tty: ldisc: add sysctl to prevent autoloading of ldiscs

2019-04-17T06:36:44+00:00

commit 7c0cca7c847e6e019d67b7d793efbbe3b947d004 upstream.

By default, the kernel will automatically load the module of any line
dicipline that is asked for.  As this sometimes isn't the safest thing
to do, provide a sysctl to disable this feature.

By default, we set this to 'y' as that is the historical way that Linux
has worked, and we do not want to break working systems.  But in the
future, perhaps this can default to 'n' to prevent this functionality.

Signed-off-by: Greg Kroah-Hartman 
Reviewed-by: Theodore Ts'o 
Signed-off-by: Greg Kroah-Hartman

tty: Use __GFP_NOFAIL for tty_ldisc_get()

2018-05-01T22:13:06+00:00

commit bcdd0ca8cb8730573afebcaae4138f8f4c8eaa20 upstream.

syzbot is reporting crashes triggered by memory allocation fault injection
at tty_ldisc_get() [1]. As an attempt to handle OOM in a graceful way, we
have tried commit 5362544bebe85071 ("tty: don't panic on OOM in
tty_set_ldisc()"). But we reverted that attempt by commit a8983d01f9b7d600
("Revert "tty: don't panic on OOM in tty_set_ldisc()"") due to reproducible
crash. We should spend resource for finding and fixing race condition bugs
rather than complicate error paths for 2 * sizeof(void *) bytes allocation
failure.

[1] https://syzkaller.appspot.com/bug?id=489d33fa386453859ead58ff5171d43772b13aa3

Signed-off-by: Tetsuo Handa 
Reported-by: syzbot 
Cc: Michal Hocko 
Cc: Vegard Nossum 
Cc: Dmitry Vyukov 
Cc: Jiri Slaby 
Cc: Peter Hurley 
Cc: One Thousand Gnomes 
Cc: Linus Torvalds 
Cc: stable 
Signed-off-by: Greg Kroah-Hartman 
Signed-off-by: Greg Kroah-Hartman

tty: Don't call panic() at tty_ldisc_init()

2018-05-01T22:13:05+00:00

commit 903f9db10f18f735e62ba447147b6c434b6af003 upstream.

syzbot is reporting kernel panic [1] triggered by memory allocation failure
at tty_ldisc_get() from tty_ldisc_init(). But since both tty_ldisc_get()
and caller of tty_ldisc_init() can cleanly handle errors, tty_ldisc_init()
does not need to call panic() when tty_ldisc_get() failed.

[1] https://syzkaller.appspot.com/bug?id=883431818e036ae6a9981156a64b821110f39187

Signed-off-by: Tetsuo Handa 
Reported-by: syzbot 
Cc: Greg Kroah-Hartman 
Cc: Jiri Slaby 
Cc: stable 
Signed-off-by: Greg Kroah-Hartman

tty: fix data race between tty_init_dev and flush of buf

2018-02-03T16:05:41+00:00

commit b027e2298bd588d6fa36ed2eda97447fb3eac078 upstream.

There can be a race, if receive_buf call comes before
tty initialization completes in n_tty_open and tty->disc_data
may be NULL.

CPU0					CPU1
----					----
 000|n_tty_receive_buf_common()   	n_tty_open()
-001|n_tty_receive_buf2()		tty_ldisc_open.isra.3()
-002|tty_ldisc_receive_buf(inline)	tty_ldisc_setup()

Using ldisc semaphore lock in tty_init_dev till disc_data
initializes completely.

Signed-off-by: Gaurav Kohli 
Reviewed-by: Alan Cox 
Signed-off-by: Greg Kroah-Hartman

tty: fix data race in tty_ldisc_ref_wait()

2017-12-20T09:07:26+00:00

[ Upstream commit a4a3e061149f09c075f108b6f1cf04d9739a6bc2 ]

tty_ldisc_ref_wait() checks tty->ldisc under tty->ldisc_sem.
But if ldisc==NULL it releases them sem and reloads
tty->ldisc without holding the sem. This is wrong and
can lead to returning non-NULL ldisc without protection.

Don't reload tty->ldisc second time.

Signed-off-by: Dmitry Vyukov 
Cc: syzkaller@googlegroups.com
Cc: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman 
Cc: Jiri Slaby 
Cc: Peter Hurley 
Cc: One Thousand Gnomes 
Signed-off-by: Greg Kroah-Hartman 
Signed-off-by: Sasha Levin 
Signed-off-by: Greg Kroah-Hartman

tty: don't panic on OOM in tty_set_ldisc()

2017-12-20T09:07:25+00:00

[ Upstream commit 5362544bebe85071188dd9e479b5a5040841c895 ]

If tty_ldisc_open() fails in tty_set_ldisc(), it tries to go back
to the old discipline or N_TTY. But that can fail as well, in such
case it panics. This is not a graceful way to handle OOM.

Leave ldisc==NULL if all attempts fail instead.
Also use existing tty_ldisc_reinit() helper function instead of
tty_ldisc_restore(). Also don't WARN/BUG in tty_ldisc_reinit()
if N_TTY fails, which would have the same net effect of bringing
kernel down on OOM. Instead print a single line message about
what has happened.

Signed-off-by: Dmitry Vyukov 
Cc: syzkaller@googlegroups.com
Cc: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman 
Cc: Jiri Slaby 
Cc: Peter Hurley 
Cc: One Thousand Gnomes 
Signed-off-by: Greg Kroah-Hartman 
Signed-off-by: Sasha Levin 
Signed-off-by: Greg Kroah-Hartman

tty: Eliminate global symbol tty_ldisc_N_TTY

2016-01-27T23:13:28+00:00

Reduce global tty symbols; move and rename tty_ldisc_begin() as
n_tty_init() and redefine the N_TTY ldisc ops as file scope.

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman

tty: Avoid unnecessary temporaries for tty->ldisc

2016-01-27T23:01:44+00:00

tty_ldisc_setup() is race-free and can reference tty->ldisc without
snapshots.

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman

tty: Destroy ldisc instance on hangup

2016-01-27T23:01:44+00:00

Currently, when the tty is hungup, the ldisc is re-instanced; ie., the
current instance is destroyed and a new instance is created. The purpose
of this design was to guarantee a valid, open ldisc for the lifetime of
the tty.

However, now that tty buffers are owned by and have lifetime equivalent
to the tty_port (since v3.10), any data received immediately after the
ldisc is re-instanced may cause continued driver i/o operations
concurrently with the driver's hangup() operation. For drivers that
shutdown h/w on hangup, this is unexpected and usually bad. For example,
the serial core may free the xmit buffer page concurrently with an
in-progress write() operation (triggered by echo).

With the existing stable and robust ldisc reference handling, the
cleaned-up tty_reopen(), the straggling unsafe ldisc use cleaned up, and
the preparation to properly handle a NULL tty->ldisc, the ldisc instance
can be destroyed and only re-instanced when the tty is re-opened.

If the tty was opened as /dev/console or /dev/tty0, the original behavior
of re-instancing the ldisc is retained (the 'reinit' parameter to
tty_ldisc_hangup() is true). This is required since those file descriptors
are never hungup.

This patch has neglible impact on userspace; the tty file_operations ptr
is changed to point to the hungup file operations _before_ the ldisc
instance is destroyed, so only racing file operations might now retrieve
a NULL ldisc reference (which is simply handled as if the hungup file
operation had been called instead -- see "tty: Prepare for destroying
line discipline on hangup").

This resolves a long-standing FIXME and several crash reports.

Signed-off-by: Peter Hurley 
Signed-off-by: Greg Kroah-Hartman