Re: [PATCH] /dev/mem: Bail out upon SIGKILL when reading memory.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Aug 21, 2019 at 07:06:51AM +0900, Tetsuo Handa wrote:
> syzbot found that a thread can stall for minutes inside read_mem()
> after that thread was killed by SIGKILL [1]. Reading 2GB at one read()
> is legal, but delaying termination of killed thread for minutes is bad.
> 
>   [ 1335.912419][T20577] read_mem: sz=4096 count=2134565632
>   [ 1335.943194][T20577] read_mem: sz=4096 count=2134561536
>   [ 1335.978280][T20577] read_mem: sz=4096 count=2134557440
>   [ 1336.011147][T20577] read_mem: sz=4096 count=2134553344
>   [ 1336.041897][T20577] read_mem: sz=4096 count=2134549248
> 
> [1] https://syzkaller.appspot.com/bug?id=a0e3436829698d5824231251fad9d8e998f94f5e
> 
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Reported-by: syzbot <syzbot+8ab2d0f39fb79fe6ca40@syzkaller.appspotmail.com>
> ---
>  drivers/char/mem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index b08dc50..0f7d4c4 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -135,7 +135,7 @@ static ssize_t read_mem(struct file *file, char __user *buf,
>  	if (!bounce)
>  		return -ENOMEM;
>  
> -	while (count > 0) {
> +	while (count > 0 && !fatal_signal_pending(current)) {
>  		unsigned long remaining;
>  		int allowed, probe;
>  
> -- 
> 1.8.3.1
> 

Oh, nice!  This shouldn't break anything that is assuming that the read
will complete before a signal is delivered, right?

I know userspace handling of "short" reads is almost always not there...

thanks,

greg k-h