linux-kernel - Re: [PATCH 3/5] SLIP: Prevent recursion stack overflow and scheduler crash

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Date:	Wed, 24 Jul 2013 21:12:20 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Dean Jenkins <Dean_Jenkins@...tor.com>
CC:	Andre Naujoks <nautsch2@...il.com>, linux-kernel@...r.kernel.org,
	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 3/5] SLIP: Prevent recursion stack overflow and scheduler
 crash

On 07/02/2013 11:31 AM, Dean Jenkins wrote:
> This is an issue when SLIP is bound to a PTY/TTY. If
> TTY_DO_WRITE_WAKEUP is set, pty_write() calls tty_wakeup()
> then slip_write_wakeup() can be called. slip_write_wakeup() can
> be called by pty_write(). This is a recursion loop.
>
> pty_write() is called in sl_encaps().
> slip_write_wakeup() can be called by the TTY wakeup event.
>
> The pty_write() call in sl_encaps() will also call
> slip_write_wakeup() but xleft has not been updated and contains
> the value from the previous SLIP frame transmission. xleft is zero
> unless the previous SLIP frame failed to be fully transmitted in
> which case xleft has a positive value. A failed transmission causes
> the next SLIP frame pending transmission to cause a crash.
>
> In the failure case when xleft is positive in slip_write_wakeup(),
> recursion causes the stack to overflow and task structures located
> near the stack are corrupted by the stack overflow. The corrupted
> task structures crash the kernel's scheduler and the system
> crashes with exception handlers crashing and the emergency reboot
> fails.
>
> The recursion loop is:
> slip_write_wakeup()-->pty_write()-->tty_wakeup()-->slip_write_wakeup()
> etc.

pty_write() calling tty_wakeup() directly is a no-no.

This is fixed in tty-next.

Regards,
Peter Hurley

> Therefore ensure xleft is zero before writing the SLIP frame to the
> PTY/TTY layers. This prevents the xleft value of the previous SLIP
> frame from interfering with the slip_write_wakeup() execution when
> SLIP is bound to a PTY/TTY.
>
> Note the transmission segmentation mechanism is broken and only a
> single call to the write() function pointer will take place per
> SLIP frame. This could cause missing or truncated SLIP frames to
> be transmitted when the write() function fails to write the complete
> frame. In other words the TTY wakeup event does nothing because
> the TTY_DO_WRITE_WAKEUP flag has been cleared.
>
> Signed-off-by: Dean Jenkins <Dean_Jenkins@...tor.com>
> ---
>   drivers/net/slip/slip.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c
> index bed819f..f7303e0 100644
> --- a/drivers/net/slip/slip.c
> +++ b/drivers/net/slip/slip.c
> @@ -395,6 +395,11 @@ static void sl_encaps(struct slip *sl, unsigned char *icp, int len)
>   #endif
>   		count = slip_esc(p, sl->xbuff, len);
>
> +	/* ensure xleft set by the previous SLIP frame is zero for this frame
> +	 * otherwise slip_write_wakeup() can cause a recursive loop.
> +	 */
> +	sl->xleft = 0;
> +
>   	/* Order of next two lines is *very* important.
>   	 * When we are sending a little amount of data,
>   	 * the transfer may be completed inside the ops->write()
>

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