Re: [PATCH] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from granting privs

[John Johansen <john.johansen@canonical.com>]

On 01/13/2012 03:12 AM, Andy Lutomirski wrote:
> With this set, a lot of dangerous operations (chroot, unshare, etc)
> become a lot less dangerous because there is no possibility of
> subverting privileged binaries.
> 
> This patch completely breaks apparmor.  Someone who understands (and
> uses) apparmor should fix it or at least give me a hint.
> 

This should fix apparmor

---

>From b4d142f1a81d252ac2a72587841e7b430190a889 Mon Sep 17 00:00:00 2001
From: John Johansen <john.johansen@canonical.com>
Date: Fri, 13 Jan 2012 14:20:47 +0100
Subject: [PATCH] Fix apparmor for PR_{GET,SET}_NO_NEW_PRIV

Signed-off-by: John Johansen <john.johansen@canonical.com>
---
 security/apparmor/domain.c |   38 +++++++++++++++++++++++++++++++++++---
 1 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/security/apparmor/domain.c b/security/apparmor/domain.c
index 7f480b7..7316d77 100644
--- a/security/apparmor/domain.c
+++ b/security/apparmor/domain.c
@@ -360,9 +360,6 @@ int apparmor_bprm_set_creds(struct linux_binprm *bprm)
 	if (bprm->cred_prepared)
 		return 0;
 
-	/* XXX: someone who understands apparmor needs to fix this. */
-	BUG_ON(bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS);
-
 	cxt = bprm->cred->security;
 	BUG_ON(!cxt);
 
@@ -398,6 +395,11 @@ int apparmor_bprm_set_creds(struct linux_binprm *bprm)
 			new_profile = find_attach(ns, &ns->base.profiles, name);
 		if (!new_profile)
 			goto cleanup;
+		/*
+		 * NOTE: Domain transitions from unconfined are allowed
+		 * even when no_new_privs is set because this aways results
+		 * in a further reduction of permissions.
+		 */
 		goto apply;
 	}
 
@@ -458,6 +460,16 @@ int apparmor_bprm_set_creds(struct linux_binprm *bprm)
 		/* fail exec */
 		error = -EACCES;
 
+	/*
+	 * Policy has specified a domain transition, if no_new_privs then
+	 * fail the exec.
+	 */
+	if (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS) {
+		aa_put_profile(new_profile);
+		error = -EPERM;
+		goto cleanup;
+	}
+
 	if (!new_profile)
 		goto audit;
 
@@ -612,6 +624,14 @@ int aa_change_hat(const char *hats[], int count, u64 token, bool permtest)
 	const char *target = NULL, *info = NULL;
 	int error = 0;
 
+	/*
+	 * Fail explicitly requested domain transitions if no_new_privs.
+	 * There is no exception for unconfined as change_hat is not
+	 * available.
+	 */
+	if (current->no_new_privs)
+		return -EPERM;
+
 	/* released below */
 	cred = get_current_cred();
 	cxt = cred->security;
@@ -753,6 +773,18 @@ int aa_change_profile(const char *ns_name, const char *hname, bool onexec,
 	cxt = cred->security;
 	profile = aa_cred_profile(cred);
 
+	/*
+	 * Fail explicitly requested domain transitions if no_new_privs
+	 * and not unconfined.
+	 * Domain transitions from unconfined are allowed even when
+	 * no_new_privs is set because this aways results in a reduction
+	 * of permissions.
+	 */
+	if (current->no_new_privs && !unconfined(profile)) {
+		put_cred(cred);
+		return -EPERM;
+	}
+
 	if (ns_name) {
 		/* released below */
 		ns = aa_find_namespace(profile->ns, ns_name);
-- 
1.7.7.3