[RFC PATCH V2 1/4] capabilities: use macros to make the logic easier to follow and verify

[Serge E. Hallyn]

On Thu, May 11, 2017 at 04:42:40PM -0400, Richard Guy Briggs wrote:
> This change is intended to be logic-neutral and simply make the logic easier to
> read in natural language and verify without getting distracted by details.
> 
> Signed-off-by: Richard Guy Briggs <rgb at redhat.com>
> ---
>  security/commoncap.c |   53 ++++++++++++++++++++++++++++++++-----------------
>  1 files changed, 34 insertions(+), 19 deletions(-)
> 
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 78b3783..9520f0a 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -497,6 +497,16 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  	int ret;
>  	kuid_t root_uid;
>  

The #defines make me uncomfortable, especially the lack of parens around
them.  The way they are used seems fine, but they seem like potential
future maintenance issues.  I definately appreciate the way you broke
the functionality down, though.  And I'm not sure I can improve on it.

> +#define SROOT !issecure(SECURE_NOROOT) /* root is special */

maybe

static inline bool root_privileged() { return !issecure(SECURE_NOROOT); }

> +#define RROOT uid_eq(new->uid, root_uid) /* real root */
> +#define EROOT uid_eq(new->euid, root_uid) /* effective root */
> +#define SETUIDROOT !RROOT && EROOT /* set uid root */

Yeah every time I start typing an alternative it doesn't look as good.

> +#define SUID !uid_eq(new->euid, old->uid) /* set uid */
> +#define SGID !gid_eq(new->egid, old->gid) /* set gid */
> +#define pPADD !cap_issubset(new->cap_permitted, old->cap_permitted) /* process permitted capabilities have been added */
> +#define pESET !cap_issubset(new->cap_effective, new->cap_ambient) /* process effective capabilities have been set */
> +#define pEALL cap_issubset(CAP_FULL_SET, new->cap_effective) /* process effective capabilities are full set */
> +#define pAADD !cap_issubset(new->cap_ambient, old->cap_ambient) /* process ambient capabilities have been added */
>  	if (WARN_ON(!cap_ambient_invariant_ok(old)))
>  		return -EPERM;
>  
> @@ -507,13 +517,13 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  
>  	root_uid = make_kuid(new->user_ns, 0);
>  
> -	if (!issecure(SECURE_NOROOT)) {
> +	if (SROOT) {
>  		/*
>  		 * If the legacy file capability is set, then don't set privs
>  		 * for a setuid root binary run by a non-root user.  Do set it
>  		 * for a root user just to cause least surprise to an admin.
>  		 */
> -		if (has_cap && !uid_eq(new->uid, root_uid) && uid_eq(new->euid, root_uid)) {
> +		if (has_cap && SETUIDROOT) {
>  			warn_setuid_and_fcaps_mixed(bprm->filename);
>  			goto skip;
>  		}
> @@ -521,33 +531,32 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  		 * To support inheritance of root-permissions and suid-root
>  		 * executables under compatibility mode, we override the
>  		 * capability sets for the file.
> -		 *
> -		 * If only the real uid is 0, we do not set the effective bit.
>  		 */
> -		if (uid_eq(new->euid, root_uid) || uid_eq(new->uid, root_uid)) {
> +		if (EROOT || RROOT) {
>  			/* pP' = (cap_bset & ~0) | (pI & ~0) */
>  			new->cap_permitted = cap_combine(old->cap_bset,
>  							 old->cap_inheritable);
>  		}
> -		if (uid_eq(new->euid, root_uid))
> +		/*
> +		 * If only the real uid is root, we do not set the effective bit.
> +		 */
> +		if (EROOT)
>  			effective = true;
>  	}
>  skip:
>  
>  	/* if we have fs caps, clear dangerous personality flags */
> -	if (!cap_issubset(new->cap_permitted, old->cap_permitted))
> +	if (pPADD)
>  		bprm->per_clear |= PER_CLEAR_ON_SETID;
>  
> +	is_setid = SUID || SGID;
>  
>  	/* Don't let someone trace a set[ug]id/setpcap binary with the revised
>  	 * credentials unless they have the appropriate permit.
>  	 *
>  	 * In addition, if NO_NEW_PRIVS, then ensure we get no new privs.
>  	 */
> -	is_setid = !uid_eq(new->euid, old->uid) || !gid_eq(new->egid, old->gid);
> -
> -	if ((is_setid ||
> -	     !cap_issubset(new->cap_permitted, old->cap_permitted)) &&
> +	if ((is_setid || pPADD) &&
>  	    ((bprm->unsafe & ~LSM_UNSAFE_PTRACE) ||
>  	     !ptracer_capable(current, new->user_ns))) {
>  		/* downgrade; they get no more than they had, and maybe less */
> @@ -599,14 +608,10 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  	 * Number 1 above might fail if you don't have a full bset, but I think
>  	 * that is interesting information to audit.
>  	 */
> -	if (!cap_issubset(new->cap_effective, new->cap_ambient)) {
> -		if (!cap_issubset(CAP_FULL_SET, new->cap_effective) ||
> -		    !uid_eq(new->euid, root_uid) || !uid_eq(new->uid, root_uid) ||
> -		    issecure(SECURE_NOROOT)) {
> -			ret = audit_log_bprm_fcaps(bprm, new, old);
> -			if (ret < 0)
> -				return ret;
> -		}
> +	if (pESET && (!pEALL || !EROOT || !RROOT || !SROOT) ) {

This might be better served by a separate helper 

	if (nonroot_raised_e(new, root_uid)) {
		ret = audit_log_bprm_fcaps(bprm, new, old);
		if (ret < 0)
			return ret;
	}

> +		ret = audit_log_bprm_fcaps(bprm, new, old);
> +		if (ret < 0)
> +			return ret;
>  	}
>  
>  	new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
> @@ -615,6 +620,16 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
>  		return -EPERM;
>  
>  	return 0;
> +#undef SROOT
> +#undef RROOT
> +#undef EROOT
> +#undef SETUIDROOT
> +#undef SUID
> +#undef SGID
> +#undef pPADD
> +#undef pESET
> +#undef pEALL
> +#undef pAADD
>  }
>  
>  /**
> -- 
> 1.7.1
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