Re: [PATCH 1/5] exec: move allow_write_access/fput to exec_binprm()

Kees Cook
2013-08-03T19:27:24Z

On Fri, Aug 2, 2013 at 12:27 PM, Oleg Nesterov <oleg@redhat.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@redhat.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