Re: [PATCH] tty: fix atomicity violation in n_tty_read

[Ilpo Järvinen]

On Fri, 12 Jan 2024, Gui-Dong Han wrote:

> In n_tty_read():
>     if (packet && tty->link->ctrl.pktstatus) {
>     ...
>     spin_lock_irq(&tty->link->ctrl.lock);
>     cs = tty->link->ctrl.pktstatus;
>     tty->link->ctrl.pktstatus = 0;
>     spin_unlock_irq(&tty->link->ctrl.lock);
>     *kb++ = cs;
>     ...
> 
> In n_tty_read() function, there is a potential atomicity violation issue.
> The tty->link->ctrl.pktstatus might be set to 0 after being checked, which
> could lead to incorrect values in the kernel space buffer
> pointer (kb/kbuf). The check if (packet && tty->link->ctrl.pktstatus)
> occurs outside the spin_lock_irq(&tty->link->ctrl.lock) block. This may
> lead to tty->link->ctrl.pktstatus being altered between the check and the
> lock, causing *kb++ = cs; to be assigned with a zero pktstatus value.
> 
> This possible bug is found by an experimental static analysis tool
> developed by our team, BassCheck[1]. This tool analyzes the locking APIs
> to extract function pairs that can be concurrently executed, and then
> analyzes the instructions in the paired functions to identify possible
> concurrency bugs including data races and atomicity violations. The above
> possible bug is reported when our tool analyzes the source code of
> Linux 5.17.
> 
> To resolve this atomicity issue, it is suggested to move the condition
> check if (packet && tty->link->ctrl.pktstatus) inside the spin_lock block.
> With this patch applied, our tool no longer reports the bug, with the
> kernel configuration allyesconfig for x86_64. Due to the absence of the
> requisite hardware, we are unable to conduct runtime testing of the patch.
> Therefore, our verification is solely based on code logic analysis.
> 
> [1] https://sites.google.com/view/basscheck/
> 
> Fixes: 64d608db38ff ("tty: cumulate and document tty_struct::ctrl* members")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Gui-Dong Han <2045gemini@xxxxxxxxx>
> ---
>  drivers/tty/n_tty.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index f252d0b5a434..df54ab0c4d8c 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -2222,19 +2222,23 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, u8 *kbuf,
>  	add_wait_queue(&tty->read_wait, &wait);
>  	while (nr) {
>  		/* First test for status change. */
> +		spin_lock_irq(&tty->link->ctrl.lock);
>  		if (packet && tty->link->ctrl.pktstatus) {
>  			u8 cs;
> -			if (kb != kbuf)
> +			if (kb != kbuf) {
> +				spin_unlock_irq(&tty->link->ctrl.lock);
>  				break;
> -			spin_lock_irq(&tty->link->ctrl.lock);
> +			}
>  			cs = tty->link->ctrl.pktstatus;
>  			tty->link->ctrl.pktstatus = 0;
>  			spin_unlock_irq(&tty->link->ctrl.lock);
>  			*kb++ = cs;
>  			nr--;
>  			break;
> +		} else {
> +			spin_unlock_irq(&tty->link->ctrl.lock);

This seems way over-engineered. Wouldn't it be much simpler to just test 
if cs is non-zero after the original critical section to detect if there 
were any changes into pktstatus before the lock was acquired? That would 
avoid all this lock dance and enlarging the critical section.

>  		}
> -
> +

Spurious changes like this should not be included into patches.

>  		if (!input_available_p(tty, 0)) {
>  			up_read(&tty->termios_rwsem);
>  			tty_buffer_flush_work(tty->port);
> 

-- 
 i.