[PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

The ops->receive_buf() may be accessed concurrently from these two
functions.  If the driver flushes data to the line discipline
receive_buf() method while tiocsti() is using the ops->receive_buf(),
the data race will happen.

For example:
tty_ioctl                       |tty_ldisc_receive_buf
 ->tioctsi                      | ->tty_port_default_receive_buf
                                |  ->tty_ldisc_receive_buf
   ->hci_uart_tty_receive       |   ->hci_uart_tty_receive
    ->h4_recv                   |    ->h4_recv

In this case, the h4 receive buffer will be overwritten by the
latecomer, and it cause a memory leak.

Hence, change tioctsi() function to use the exclusive lock interface
from tty_buffer to avoid the data race.

Signed-off-by: Nguyen Dinh Phi <redacted>
Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
---
 drivers/tty/tty_io.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960..746fe13a2054 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c

@@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
 	ld = tty_ldisc_ref_wait(tty);
 	if (!ld)
 		return -EIO;
+	tty_buffer_lock_exclusive(tty->port);
 	if (ld->ops->receive_buf)
 		ld->ops->receive_buf(tty, &ch, &mbz, 1);
+	tty_buffer_unlock_exclusive(tty->port);
 	tty_ldisc_deref(ld);
 	return 0;
 }
--

2.25.1

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On Sun, Aug 08, 2021 at 03:05:13AM +0800, Nguyen Dinh Phi wrote:

The ops->receive_buf() may be accessed concurrently from these two
functions.  If the driver flushes data to the line discipline
receive_buf() method while tiocsti() is using the ops->receive_buf(),
the data race will happen.

For example:
tty_ioctl                       |tty_ldisc_receive_buf
 ->tioctsi                      | ->tty_port_default_receive_buf
                                |  ->tty_ldisc_receive_buf
   ->hci_uart_tty_receive       |   ->hci_uart_tty_receive
    ->h4_recv                   |    ->h4_recv

In this case, the h4 receive buffer will be overwritten by the
latecomer, and it cause a memory leak.

That looks to be a bug in the h4 code, if the receive_buf() call can not
be run at the same time, it should have a lock in it, right?

Hence, change tioctsi() function to use the exclusive lock interface
from tty_buffer to avoid the data race.

Where is the lock being grabbed from the other receive_buf() call path
to ensure that the lock is always needed here?

Did this fix the syzbot reported issue?

thanks,

greg k-h
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On 8/13/2021 3:33 PM, Greg KH wrote:

quoted

Signed-off-by: Nguyen Dinh Phi <redacted>
Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
---
  drivers/tty/tty_io.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960..746fe13a2054 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c

@@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
  	ld = tty_ldisc_ref_wait(tty);
  	if (!ld)
  		return -EIO;
+	tty_buffer_lock_exclusive(tty->port);
  	if (ld->ops->receive_buf)
  		ld->ops->receive_buf(tty, &ch, &mbz, 1);
+	tty_buffer_unlock_exclusive(tty->port);

Did this fix the syzbot reported issue?

thanks,

greg k-h
Yes, this fixed the syzbot reported issue.

The lock is grabbed in flush_to_ldisc() and paste_selection().
Actually, I follow the document in tty_buffer.c, where it say the 
callers to receive_buff() other than flush_to_ldisc() need to exclude 
the kworker from concurrent use of the line discipline.

And function tiocsti() has the following comment:
/* FIXME: may race normal receive processing */
that why I add lock in this function.

BR,
Phi.
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On Sat, Aug 14, 2021 at 02:35:53AM +0800, Phi Nguyen wrote:

On 8/13/2021 3:33 PM, Greg KH wrote:

quoted

quoted

Signed-off-by: Nguyen Dinh Phi <redacted>
Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
---
  drivers/tty/tty_io.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960..746fe13a2054 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c

@@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
  	ld = tty_ldisc_ref_wait(tty);
  	if (!ld)
  		return -EIO;
+	tty_buffer_lock_exclusive(tty->port);
  	if (ld->ops->receive_buf)
  		ld->ops->receive_buf(tty, &ch, &mbz, 1);
+	tty_buffer_unlock_exclusive(tty->port);

Did this fix the syzbot reported issue?

thanks,

greg k-h
Yes, this fixed the syzbot reported issue.

The lock is grabbed in flush_to_ldisc() and paste_selection().
Actually, I follow the document in tty_buffer.c, where it say the callers to
receive_buff() other than flush_to_ldisc() need to exclude the kworker from
concurrent use of the line discipline.

And function tiocsti() has the following comment:
/* FIXME: may race normal receive processing */
that why I add lock in this function.

As you are fixing this FIXME, please remove it in this patch, otherwise
we will not know it is resolved :)

Can you add that to the change and submit a new version?

Also, this should go to stable kernels, right?  Any idea how far back?

thanks,

greg k-h
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On 8/18/2021 10:03 PM, Greg KH wrote:

On Sat, Aug 14, 2021 at 02:35:53AM +0800, Phi Nguyen wrote:

quoted

On 8/13/2021 3:33 PM, Greg KH wrote:

quoted

quoted

Signed-off-by: Nguyen Dinh Phi <redacted>
Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
---
   drivers/tty/tty_io.c | 2 ++
   1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960..746fe13a2054 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c

@@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
   	ld = tty_ldisc_ref_wait(tty);
   	if (!ld)
   		return -EIO;
+	tty_buffer_lock_exclusive(tty->port);
   	if (ld->ops->receive_buf)
   		ld->ops->receive_buf(tty, &ch, &mbz, 1);
+	tty_buffer_unlock_exclusive(tty->port);

Did this fix the syzbot reported issue?

thanks,

greg k-h
Yes, this fixed the syzbot reported issue.

The lock is grabbed in flush_to_ldisc() and paste_selection().
Actually, I follow the document in tty_buffer.c, where it say the callers to
receive_buff() other than flush_to_ldisc() need to exclude the kworker from
concurrent use of the line discipline.

And function tiocsti() has the following comment:
/* FIXME: may race normal receive processing */
that why I add lock in this function.

As you are fixing this FIXME, please remove it in this patch, otherwise
we will not know it is resolved :)

Can you add that to the change and submit a new version?

Yes, I will submit a new version.

Also, this should go to stable kernels, right?  Any idea how far back?

I have no idea about this question, but I see it meets specified 
requirements in stable-kernel-rules.rst

BR,
Phi.
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