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SubjectRe: race between flush_to_ldisc and pty_cleanup
Date
Hi,

02/01/2019 07:31 PM에 Maninder Singh 이(가) 쓴 글:
> Hi,
>
>> On Fri, Feb 01, 2019 at 07:03:26PM +0530, Maninder Singh wrote:
>>> Hi,
>>>
>>>
>>> There is some race condition between tty_port_put and flush_to_ldisc
>>> which lead to use after free case:
>>> (Kernel 4.1)
>>>
>>> [1403.5130] Unable to handle kernel paging request at virtual address 6b6b6b83
>>> ...
>>> ...
>>> ...
>>>
>>> [1403.5132] [<c036a65c>] (ldsem_down_read_trylock) from [<c0367db4>] (tty_ldisc_ref+0x24/0x60)
>>> [1403.5132] [<c0367d90>] (tty_ldisc_ref) from [<c03690cc>] (flush_to_ldisc+0x6c/0x21c)
>>> [1403.5132] r5:dbcd4a84 r4:00000000
>>> [1403.5132] [<c0369060>] (flush_to_ldisc) from [<c0049f18>] (process_one_work+0x214/0x570)
>>> [1403.5132] r10:00000000 r9:ddab0000 r8:e3d6e000 r7:00000000 r6:e453f740 r5:cb37b780
>>> [1403.5132] r4:dbcd4a84
>>> [1403.5132] [<c0049d04>] (process_one_work) from [<c004a2d4>] (worker_thread+0x60/0x580)
>>> [1403.5132] r10:e453f740 r9:ddab0000 r8:e453f764 r7:00000088 r6:e453f740 r5:cb37b798
>>> [1403.5132] r4:cb37b780
>>> [1403.5132] [<c004a274>] (worker_thread) from [<c005022c>] (kthread+0xec/0x104)
>>> [1403.5132] r10:00000000 r9:00000000 r8:00000000 r7:c004a274 r6:cb37b780 r5:d8a3fc80
>>> [1403.5132] r4:00000000
>>> [1403.5132] [<c0050140>] (kthread) from [<c00107d8>] (ret_from_fork+0x14/0x3c)
>>>
>>>
>>> for checking further we entered some debug prints and added delay in flush_to_ldisc to reproduce
>>> and seems there is some issue with workqueue implementation of TTY:
>>>
>>> bool tty_buffer_cancel_work(struct tty_port *port)
>>> {
>>> bool ret;
>>> ret = cancel_work_sync(&port->buf.work); // Check return value of cancel_work_sync
>>> pr_emerg("Work cancelled is 0x%x %pS %d\n", (unsigned int)&port->buf.work, (void *)_RET_IP_, ret);
>>> return ret;
>>> }
>>>
>>> static void flush_to_ldisc(struct work_struct *work)
>>> {
>>> ...
>>> mdelay(100); // Added Delay to reproduce race
>>>
>>> if (flag_work_cancel) {
>>> pr_emerg("scheduled work after stopping work %x\n", (unsigned int)work);
>>>
>>> ....
>>> }
>>>
>>> static void pty_cleanup(struct tty_struct *tty)
>>> {
>>> ...
>>> flag_work_cancel = 1;
>>> ...
>>> }
>>>
>>>
>>> [1403.4158]Work cancelled is dbcd4a84 tty_port_destroy+0x1c/0x6c 0 // Since return is 0 so no work is pending
>>>
>>> [1403.5129] scheduled work after stopping work dbcd4a84 // Still same work is scheduled after cancelled
>>> [1403.5130] Unable to handle kernel paging request at virtual address 6b6b6b83 // Kernel OOPs occured because of use after free
>> Ok, after my initial "use a newer kernel" comment, this really does look
>> strange. There has also been a lot of workqueue fixes and rework since
>> 4.1, and that might be the thing that fixes this issue here.
>>
>> However, are you sure you are not just calling flush_to_ldisc() directly
>> through some codepath somehow? If you look at the stack in the
> Yes, there is no call path for flush_to_disc directly. It is all aligned with kernel 4.1.
>
>
>> pr_emerg() message, where did it come from? From the same workqueue
>> that you already stopped?
> We added debug prints to check "work" in pty_cleanup() & flush_to_ldisc
>
>> Testing on a newer kernel would be great, if possible.
> We are facing it hard, but currently we have 4.1 and able to reproduce on that.
> Not really possible to have the latest kernel on the same target and may be reproduce the same race.
> Tried to track for changes in the other stable branches, but no change looks really relevant for this race.
> I might be wrong, please help if there is any commit related with this.
Please find & review the attached patch for this issue.

I think the tty which called pty_cleanup may be different from the tty
which called flush_to_ldisc.
Please check if those two are same or different. I don't think this is a
workqueue problem.
Thanks.

Best Regards,
Sahara

>
>> thanks,
>>
>> greg k-h
> Our initial debugging direction was with "tty" but looks some issue in workqueue.
> Also, the most surprising looks to be the CANCEL and FLUSH to be occuring on the same CORE in sequence.
> So, if the CANCEL really worked how can the flush_to_ldisc be scheduled.
>
> Thanks,
> Maninder Singh
>



Keun-O Park | Principal Software Engineer
Tel: +971 2 417 1417 | Mob: +971 55 260 7343
Keun-O.Park@darkmatter.ae

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From 3a65ec01f8849c3e11267794b9d1c70ef32d13e3 Mon Sep 17 00:00:00 2001
From: Sahara <keun-o.park@darkmatter.ae>
Date: Sun, 3 Feb 2019 09:27:28 +0400
Subject: [PATCH] tty: pty: Fix race condition between release_one_tty and
pty_write

Especially when a linked tty is used such as pty, the linked tty
port's buf works have not been cancelled while master tty port's
buf work has been cancelled. Since release_one_tty and flush_to_ldisc
run in workqueue threads separately, when pty_cleanup happens and
link tty port is freed, flush_to_ldisc tries to access freed port
and port->itty, eventually it causes a panic.
This patch utilizes the magic value with holding the tty_mutex to
check if the tty->link is valid.

Fixes: 2b022ab7542d ("pty: cancel pty slave port buf's work in tty_release")
Signed-off-by: Sahara <keun-o.park@darkmatter.ae>
---
drivers/tty/pty.c | 7 +++++++
drivers/tty/tty_io.c | 3 +++
2 files changed, 10 insertions(+)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 00099a84..ef72031 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -116,6 +116,12 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
if (tty->stopped)
return 0;

+ mutex_lock(&tty_mutex);
+ if (to->magic != TTY_MAGIC) {
+ mutex_unlock(&tty_mutex);
+ return -EIO;
+ }
+
if (c > 0) {
spin_lock_irqsave(&to->port->lock, flags);
/* Stuff the data into the input queue of the other end */
@@ -125,6 +131,7 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
tty_flip_buffer_push(to->port);
spin_unlock_irqrestore(&to->port->lock, flags);
}
+ mutex_unlock(&tty_mutex);
return c;
}

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 23c6fd2..6c4a206 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1446,10 +1446,13 @@ static void release_one_tty(struct work_struct *work)
struct tty_driver *driver = tty->driver;
struct module *owner = driver->owner;

+ mutex_lock(&tty_mutex);
if (tty->ops->cleanup)
tty->ops->cleanup(tty);

tty->magic = 0;
+ mutex_unlock(&tty_mutex);
+
tty_driver_kref_put(driver);
module_put(owner);

--
2.7.4
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