Re: [PATCH 4/5] exec: Allow load_misc_binary to call prepare_binfmt unconditionally

[Kees Cook]

On Sat, May 09, 2020 at 02:42:23PM -0500, Eric W. Biederman wrote:
> 
> Add a flag preserve_creds that binfmt_misc can set to prevent
> credentials from being updated.  This allows binfmrt_misc to always
> call prepare_binfmt.  Allowing the credential computation logic to be
> consolidated.
> 
> Ref: c407c033de84 ("[PATCH] binfmt_misc: improve calculation of interpreter's credentials")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
>  fs/binfmt_misc.c        | 15 +++------------
>  fs/exec.c               | 14 +++++++++-----
>  include/linux/binfmts.h |  2 ++
>  3 files changed, 14 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
> index 127fae9c21ab..16bfafd2671d 100644
> --- a/fs/binfmt_misc.c
> +++ b/fs/binfmt_misc.c
> @@ -218,19 +218,10 @@ static int load_misc_binary(struct linux_binprm *bprm)
>  		goto error;
>  
>  	bprm->file = interp_file;
> -	if (fmt->flags & MISC_FMT_CREDENTIALS) {
> -		loff_t pos = 0;
> -
> -		/*
> -		 * No need to call prepare_binprm(), it's already been
> -		 * done.  bprm->buf is stale, update from interp_file.
> -		 */
> -		memset(bprm->buf, 0, BINPRM_BUF_SIZE);
> -		retval = kernel_read(bprm->file, bprm->buf, BINPRM_BUF_SIZE,
> -				&pos);
> -	} else
> -		retval = prepare_binprm(bprm);
> +	if (fmt->flags & MISC_FMT_CREDENTIALS)
> +		bprm->preserve_creds = 1;
>  
> +	retval = prepare_binprm(bprm);
>  	if (retval < 0)
>  		goto error;
>  
> diff --git a/fs/exec.c b/fs/exec.c
> index 8bbf5fa785a6..01dbeb025c46 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1630,14 +1630,18 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
>   */
>  int prepare_binprm(struct linux_binprm *bprm)
>  {
> -	int retval;
>  	loff_t pos = 0;
>  
> -	bprm_fill_uid(bprm);
> +	if (!bprm->preserve_creds) {

nit: hint this to the common execution path:

	if (likely(!bprm->preserve_creds) {

> +		int retval;
>  
> -	retval = cap_bprm_set_creds(bprm);
> -	if (retval)
> -		return retval;
> +		bprm_fill_uid(bprm);
> +
> +		retval = cap_bprm_set_creds(bprm);
> +		if (retval)
> +			return retval;
> +	}
> +	bprm->preserve_creds = 0;
>  
>  	memset(bprm->buf, 0, BINPRM_BUF_SIZE);
>  	return kernel_read(bprm->file, bprm->buf, BINPRM_BUF_SIZE, &pos);
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index 89f1135dcb75..cb016f001e7a 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -26,6 +26,8 @@ struct linux_binprm {
>  	unsigned long p; /* current top of mem */
>  	unsigned long argmin; /* rlimit marker for copy_strings() */
>  	unsigned int
> +		/* Don't update the creds for an interpreter (see binfmt_misc) */

I'd like a much more verbose comment here. How about this:

		/*
		 * Skip setting new privileges for an interpreter (see
		 * binfmt_misc) on the next call to prepare_binprm().
		 */

> +		preserve_creds:1,

Nit pick: we've seen there is a logical difference here between "creds"
(which mean "the creds struct itself") and "privileges" (which are
stored in the cred struct). I think we should reinforce this distinction
here and name this:

		preserve_privileges:1,

>  		/*
>  		 * True if most recent call to the commoncaps bprm_set_creds
>  		 * hook (due to multiple prepare_binprm() calls from the
> -- 
> 2.25.0
> 

Otherwise, yeah, this seems okay to me.

-- 
Kees Cook