linux-kernel - Re: [PATCH] tty: Fix data race in tty_insert_flip_string_fixed

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Date:   Wed, 25 Apr 2018 15:41:59 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     DaeRyong Jeong <threeearcat@...il.com>
Cc:     jslaby@...e.com, byoungyoung@...due.edu, kt0755@...il.com,
        bammanag@...due.edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: Fix data race in tty_insert_flip_string_fixed_flag

On Wed, Apr 25, 2018 at 10:20:50PM +0900, DaeRyong Jeong wrote:
> tty_insert_flip_string_fixed_flag() copies chars to the buffer indicated
> by th->used and updates tb->used.
> But tty_insert_flip_string_fixed_flag() can be executed concurrently and
> tb->used can be updated improperly.
> It leads slab-out-of-bound write in tty_insert_flip_string_fixed_flag or
> slab-out-of-bounds read in flush_to_ldisc
> 
> BUG: KASAN: slab-out-of-bounds in tty_insert_flip_string_fixed_flag+0xb5/
> 0x130 drivers/tty/tty_buffer.c:316 at addr ffff880114fcc121
> Write of size 1792 by task syz-executor0/30017
> CPU: 1 PID: 30017 Comm: syz-executor0 Not tainted 4.8.0 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
>  0000000000000000 ffff88011638f888 ffffffff81694cc3 ffff88007d802140
>  ffff880114fcb300 ffff880114fcc300 ffff880114fcb300 ffff88011638f8b0
>  ffffffff8130075c ffff88011638f940 ffff88007d802140 ffff880194fcc121
> Call Trace:
>  __dump_stack lib/dump_stack.c:15 [inline]
>  dump_stack+0xb3/0x110 lib/dump_stack.c:51
>  kasan_object_err+0x1c/0x70 mm/kasan/report.c:156
>  print_address_description mm/kasan/report.c:194 [inline]
>  kasan_report_error+0x1f7/0x4e0 mm/kasan/report.c:283
>  kasan_report+0x36/0x40 mm/kasan/report.c:303
>  check_memory_region_inline mm/kasan/kasan.c:292 [inline]
>  check_memory_region+0x13e/0x1a0 mm/kasan/kasan.c:299
>  memcpy+0x37/0x50 mm/kasan/kasan.c:335
>  tty_insert_flip_string_fixed_flag+0xb5/0x130 drivers/tty/tty_buffer.c:316
>  tty_insert_flip_string include/linux/tty_flip.h:35 [inline]
>  pty_write+0x7f/0xc0 drivers/tty/pty.c:115
>  n_hdlc_send_frames+0x1d4/0x3b0 drivers/tty/n_hdlc.c:419
>  n_hdlc_tty_wakeup+0x73/0xa0 drivers/tty/n_hdlc.c:496
>  tty_wakeup+0x92/0xb0 drivers/tty/tty_io.c:601
>  __start_tty.part.26+0x66/0x70 drivers/tty/tty_io.c:1018
>  __start_tty+0x34/0x40 drivers/tty/tty_io.c:1013
>  n_tty_ioctl_helper+0x146/0x1e0 drivers/tty/tty_ioctl.c:1138
>  n_hdlc_tty_ioctl+0xb3/0x2b0 drivers/tty/n_hdlc.c:794
>  tty_ioctl+0xa85/0x16d0 drivers/tty/tty_io.c:2992
>  vfs_ioctl fs/ioctl.c:43 [inline]
>  do_vfs_ioctl+0x13e/0xba0 fs/ioctl.c:679
>  SYSC_ioctl fs/ioctl.c:694 [inline]
>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
>  entry_SYSCALL_64_fastpath+0x1f/0xbd
> 
> Call sequences are as follows.
> CPU0                                    CPU1
> n_tty_ioctl_helper                      n_tty_ioctl_helper
> __start_tty                             tty_send_xchar
> tty_wakeup                              pty_write
> n_hdlc_tty_wakeup                       tty_insert_flip_string
> n_hdlc_send_frames                      tty_insert_flip_string_fixed_flag
> pty_write
> tty_insert_flip_string
> tty_insert_flip_string_fixed_flag
> 
> Acquire tty->atomic_write_lock by calling tty_write_lock() before
> __start_tty() since __start_tty() can sends frames.
> 
> Signed-off-by: DaeRyong Jeong <threeearcat@...il.com>
> ---
>  drivers/tty/tty_io.c    | 16 +++++++++-------
>  drivers/tty/tty_ioctl.c |  5 +++++
>  include/linux/tty.h     |  2 ++
>  3 files changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 63114ea35ec1..41f83bd4cc40 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -873,13 +873,15 @@ static ssize_t tty_read(struct file *file, char __user *buf, size_t count,
>  	return i;
>  }
> 
> -static void tty_write_unlock(struct tty_struct *tty)
> +void tty_write_unlock(struct tty_struct *tty, int wakeup)
>  {
>  	mutex_unlock(&tty->atomic_write_lock);
> -	wake_up_interruptible_poll(&tty->write_wait, EPOLLOUT);
> +	if (wakeup) {
> +		wake_up_interruptible_poll(&tty->write_wait, EPOLLOUT);
> +	}

Always run scripts/checkpatch.pl before sending patches out so you do
not get grumpy maintainers telling you to run scripts/checkpatch.pl :)

Anyway, these "bool" type options are horrid for trying to remember what
is happening here.  You have to go look up what 1 or 0 is, right?

How about two functions:
	tty_write_unlock()
	tty_write_unlock_wakup()

and the second one calls the first and then does the wakeup?

But are you sure this is the correct fix?  What is protecting the race
from happening with this change?  How do you know to call wakeup or not?
What determines which is better to do?

thanks,

greg k-h