linux-kernel - [RFC PATCH] selinux: split no transition execve check

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Message-Id: <20220125145931.56831-1-cgzones@googlemail.com>
Date:   Tue, 25 Jan 2022 15:59:27 +0100
From:   Christian Göttsche <cgzones@...glemail.com>
To:     selinux@...r.kernel.org
Cc:     Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        "David S. Miller" <davem@...emloft.net>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Jeremy Kerr <jk@...econstruct.com.au>,
        Xiong Zhenwu <xiong.zhenwu@....com.cn>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org
Subject: [RFC PATCH] selinux: split no transition execve check

In case a setuid or setgid binary is mislabeled with a generic context,
either via a policy mistake or a move by the distribution package,
executing it will be checked by the file permission execute_no_trans on
the generic file context (e.g. bin_t).  The setuid(2)/setgid(2) syscall
within will then be checked against the unchanged caller process
context, which might have been granted the capability permission setuid/
setgid to initially drop privileges.  To avoid that scenario split the
execute_no_trans permission in case of a setuid/setgid binary into a new
permission execute_sxid_no_trans.

For backward compatibility this behavior is contained in a new policy
capability.

Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
---
 security/selinux/hooks.c                   | 9 ++++++++-
 security/selinux/include/classmap.h        | 2 +-
 security/selinux/include/policycap.h       | 1 +
 security/selinux/include/policycap_names.h | 3 ++-
 security/selinux/include/security.h        | 8 ++++++++
 5 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 5b6895e4fc29..b825fee39a70 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2348,9 +2348,16 @@ static int selinux_bprm_creds_for_exec(struct linux_binprm *bprm)
 	ad.u.file = bprm->file;
 
 	if (new_tsec->sid == old_tsec->sid) {
+		u32 perm;
+
+		if (selinux_policycap_execute_sxid_no_trans() && is_sxid(inode->i_mode))
+			perm = FILE__EXECUTE_SXID_NO_TRANS;
+		else
+			perm = FILE__EXECUTE_NO_TRANS;
+
 		rc = avc_has_perm(&selinux_state,
 				  old_tsec->sid, isec->sid,
-				  SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
+				  SECCLASS_FILE, perm, &ad);
 		if (rc)
 			return rc;
 	} else {
diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
index 35aac62a662e..53a1eeeb86fb 100644
--- a/security/selinux/include/classmap.h
+++ b/security/selinux/include/classmap.h
@@ -65,7 +65,7 @@ struct security_class_mapping secclass_map[] = {
 	    "quotaget", "watch", NULL } },
 	{ "file",
 	  { COMMON_FILE_PERMS,
-	    "execute_no_trans", "entrypoint", NULL } },
+	    "execute_no_trans", "entrypoint", "execute_sxid_no_trans", NULL } },
 	{ "dir",
 	  { COMMON_FILE_PERMS, "add_name", "remove_name",
 	    "reparent", "search", "rmdir", NULL } },
diff --git a/security/selinux/include/policycap.h b/security/selinux/include/policycap.h
index 2ec038efbb03..23929dc3e1db 100644
--- a/security/selinux/include/policycap.h
+++ b/security/selinux/include/policycap.h
@@ -11,6 +11,7 @@ enum {
 	POLICYDB_CAPABILITY_CGROUPSECLABEL,
 	POLICYDB_CAPABILITY_NNP_NOSUID_TRANSITION,
 	POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS,
+	POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS,
 	__POLICYDB_CAPABILITY_MAX
 };
 #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - 1)
diff --git a/security/selinux/include/policycap_names.h b/security/selinux/include/policycap_names.h
index b89289f092c9..4c014c2cf352 100644
--- a/security/selinux/include/policycap_names.h
+++ b/security/selinux/include/policycap_names.h
@@ -12,7 +12,8 @@ const char *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = {
 	"always_check_network",
 	"cgroup_seclabel",
 	"nnp_nosuid_transition",
-	"genfs_seclabel_symlinks"
+	"genfs_seclabel_symlinks",
+	"execute_sxid_no_trans",
 };
 
 #endif /* _SELINUX_POLICYCAP_NAMES_H_ */
diff --git a/security/selinux/include/security.h b/security/selinux/include/security.h
index ac0ece01305a..ab95241b6b7b 100644
--- a/security/selinux/include/security.h
+++ b/security/selinux/include/security.h
@@ -219,6 +219,14 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
 	return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
 }
 
+static inline bool selinux_policycap_execute_sxid_no_trans(void)
+{
+	struct selinux_state *state = &selinux_state;
+
+	return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_EXECUTE_SXID_NO_TRANS]);
+}
+
+
 struct selinux_policy_convert_data;
 
 struct selinux_load_state {
-- 
2.34.1