linux-kernel - Re: [PATCH 2/3] exec: Make unlocking exec_update

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Message-ID: <202004070902.5D36F15E@keescook>
Date:   Tue, 7 Apr 2020 09:02:32 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [PATCH 2/3] exec: Make unlocking exec_update_mutex explict

On Mon, Apr 06, 2020 at 08:31:52PM -0500, Eric W. Biederman wrote:
> 
> With install_exec_creds updated to follow immediately after
> setup_new_exec, the failure of unshare_sighand is the only
> code path where exec_update_mutex is held but not explicitly
> unlocked.
> 
> Update that code path to explicitly unlock exec_update_mutex.
> 
> Remove the unlocking of exec_update_mutex from free_bprm.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  fs/exec.c               | 6 +++---
>  include/linux/binfmts.h | 3 +--
>  2 files changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index d55710a36056..28c87020da9b 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1318,7 +1318,7 @@ int flush_old_exec(struct linux_binprm * bprm)
>  	 */
>  	retval = unshare_sighand(me);
>  	if (retval)
> -		goto out;
> +		goto out_unlock;
>  
>  	set_fs(USER_DS);
>  	me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
> @@ -1335,6 +1335,8 @@ int flush_old_exec(struct linux_binprm * bprm)
>  	do_close_on_exec(me->files);
>  	return 0;
>  
> +out_unlock:
> +	mutex_unlock(&me->signal->exec_update_mutex);
>  out:
>  	return retval;
>  }
> @@ -1451,8 +1453,6 @@ static void free_bprm(struct linux_binprm *bprm)
>  {
>  	free_arg_pages(bprm);
>  	if (bprm->cred) {
> -		if (bprm->called_exec_mmap)
> -			mutex_unlock(&current->signal->exec_update_mutex);
>  		mutex_unlock(&current->signal->cred_guard_mutex);
>  		abort_creds(bprm->cred);
>  	}
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index a345d9fed3d8..6f564b9ad882 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -47,8 +47,7 @@ struct linux_binprm {
>  		secureexec:1,
>  		/*
>  		 * Set by flush_old_exec, when exec_mmap has been called.
> -		 * This is past the point of no return, when the
> -		 * exec_update_mutex has been taken.
> +		 * This is past the point of no return.
>  		 */
>  		called_exec_mmap:1;
>  #ifdef __alpha__
> -- 
> 2.25.0
> 

-- 
Kees Cook