linux-kernel - Re: [PATCH] kernel/kmod: fix use-after-free of the sub

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Date:	Thu, 16 Oct 2014 19:37:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info
	structure

On 10/16, Martin Schwidefsky wrote:
>
> There is a use-after-free bug on the subprocess_info structure allocated
> by the user mode helper. In case do_execve() returns with an error
> ____call_usermodehelper() stores the error code to sub_info->retval,
> but sub_info can already have been freed.

Hmm, yes... do_execve() can fail after mm_release(). CLONE_VFORK doesn't
help in this case.

> @@ -242,13 +263,14 @@ static int ____call_usermodehelper(void *data)
>  	retval = do_execve(getname_kernel(sub_info->path),
>  			   (const char __user *const __user *)sub_info->argv,
>  			   (const char __user *const __user *)sub_info->envp);
> -	if (!retval)
> -		return 0;
> -
> -	/* Exec failed? */
> -fail:
> +out:
>  	sub_info->retval = retval;
> -	do_exit(0);
> +	if (wait != UMH_WAIT_PROC)
> +		/* For UMH_WAIT_PROC wait_for_helper calls umh_complete */
> +		umh_complete(sub_info);
> +	if (retval)
> +		do_exit(0);
> +	return 0;
>  }

OK... I am wondering if __call_usermodehelper() still needs CLONE_VFORK
with this patch.

> @@ -588,7 +580,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
>  		goto out;
>  	}
>  
> -	sub_info->complete = &done;
> +	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;

This probably needs a comment, and the comment in umh_complete() should
be updated,

	- we own sub_info, the UMH_KILLABLE caller has gone away.
	+ we own sub_info, the UMH_KILLABLE caller has gone away
	+ or the caller used UMH_NO_WAIT.

The patch looks correct at first glance. I'll try to re-read it later
once again.

Thanks!

Oleg.

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