linux-kernel - Re: [PATCH 1/5] exec: move allow_write_access/fput to exec

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Message-ID: <CAGXu5jKY1S7wxWCv6ZULU=rvp1fQ9KemcFZxY7ccoX06jdvP3w@mail.gmail.com>
Date:	Sat, 3 Aug 2013 12:27:22 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Zach Levis <zml@...ux.vnet.ibm.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Evgeniy Polyakov <zbr@...emap.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] exec: move allow_write_access/fput to exec_binprm()

On Fri, Aug 2, 2013 at 12:27 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> When search_binary_handler() succeeds it does allow_write_access()
> and fput(), then it clears bprm->file to ensure the caller will not
> do the same.
>
> We can simply move this code to exec_binprm() which is called only
> once. In fact we could move this to free_bprm() and remove the same
> code in do_execve_common's error path.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  fs/exec.c |    9 +++++----
>  1 files changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index ad7d624..ef70320 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1400,10 +1400,6 @@ int search_binary_handler(struct linux_binprm *bprm)
>                         bprm->recursion_depth--;
>                         if (retval >= 0) {
>                                 put_binfmt(fmt);
> -                               allow_write_access(bprm->file);
> -                               if (bprm->file)
> -                                       fput(bprm->file);
> -                               bprm->file = NULL;
>                                 return retval;
>                         }
>                         read_lock(&binfmt_lock);
> @@ -1455,6 +1451,11 @@ static int exec_binprm(struct linux_binprm *bprm)
>                 ptrace_event(PTRACE_EVENT_EXEC, old_vpid);
>                 current->did_exec = 1;
>                 proc_exec_connector(current);
> +
> +               if (bprm->file) {
> +                       allow_write_access(bprm->file);
> +                       fput(bprm->file);
> +               }

Why not keep the bprm->file = NULL assignment? Seems reasonable to
keep that just to be avoid use-after-free accidents.

-Kees

>         }
>
>         return ret;
> --
> 1.5.5.1
>



-- 
Kees Cook
Chrome OS Security
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