[PATCH] SELinux: add rootcontext= option to label root inode when mounting

Introduce a new rootcontext= option to FS mounting.  This option will allow
you to explicitly label the root inode of an FS being mounted before that
FS or inode because visible to userspace.  This was found to be useful for
things like stateless linux, see
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=190001

Signed-off-by: Eric Paris <eparis@parisplace.org>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

security/selinux/hooks.c

@@ -320,12 +320,14 @@ enum {
 	Opt_context = 1,
 	Opt_fscontext = 2,
 	Opt_defcontext = 4,
+	Opt_rootcontext = 8,
 };
 
 static match_table_t tokens = {
 	{Opt_context, "context=%s"},
 	{Opt_fscontext, "fscontext=%s"},
 	{Opt_defcontext, "defcontext=%s"},
+	{Opt_rootcontext, "rootcontext=%s"},
 };
 
 #define SEL_MOUNT_FAIL_MSG "SELinux:  duplicate or incompatible mount options\n"
@@ -346,10 +348,25 @@ static int may_context_mount_sb_relabel(u32 sid,
 	return rc;
 }
 
+static int may_context_mount_inode_relabel(u32 sid,
+			struct superblock_security_struct *sbsec,
+			struct task_security_struct *tsec)
+{
+	int rc;
+	rc = avc_has_perm(tsec->sid, sbsec->sid, SECCLASS_FILESYSTEM,
+			  FILESYSTEM__RELABELFROM, NULL);
+	if (rc)
+		return rc;
+
+	rc = avc_has_perm(sid, sbsec->sid, SECCLASS_FILESYSTEM,
+			  FILESYSTEM__ASSOCIATE, NULL);
+	return rc;
+}
+
 static int try_context_mount(struct super_block *sb, void *data)
 {
 	char *context = NULL, *defcontext = NULL;
-	char *fscontext = NULL;
+	char *fscontext = NULL, *rootcontext = NULL;
 	const char *name;
 	u32 sid;
 	int alloc = 0, rc = 0, seen = 0;
@@ -423,6 +440,22 @@ static int try_context_mount(struct super_block *sb, void *data)
 				seen |= Opt_fscontext;
 				break;
 
+			case Opt_rootcontext:
+				if (seen & Opt_rootcontext) {
+					rc = -EINVAL;
+					printk(KERN_WARNING SEL_MOUNT_FAIL_MSG);
+					goto out_free;
+				}
+				rootcontext = match_strdup(&args[0]);
+				if (!rootcontext) {
+					rc = -ENOMEM;
+					goto out_free;
+				}
+				if (!alloc)
+					alloc = 1;
+				seen |= Opt_rootcontext;
+				break;
+
 			case Opt_defcontext:
 				if (sbsec->behavior != SECURITY_FS_USE_XATTR) {
 					rc = -EINVAL;
@@ -501,6 +534,25 @@ static int try_context_mount(struct super_block *sb, void *data)
 		sbsec->behavior = SECURITY_FS_USE_MNTPOINT;
 	}
 
+	if (rootcontext) {
+		struct inode *inode = sb->s_root->d_inode;
+		struct inode_security_struct *isec = inode->i_security;
+		rc = security_context_to_sid(rootcontext, strlen(rootcontext), &sid);
+		if (rc) {
+			printk(KERN_WARNING "SELinux: security_context_to_sid"
+			       "(%s) failed for (dev %s, type %s) errno=%d\n",
+			       rootcontext, sb->s_id, name, rc);
+			goto out_free;
+		}
+
+		rc = may_context_mount_inode_relabel(sid, sbsec, tsec);
+		if (rc)
+			goto out_free;
+
+		isec->sid = sid;
+		isec->initialized = 1;
+	}
+
 	if (defcontext) {
 		rc = security_context_to_sid(defcontext, strlen(defcontext), &sid);
 		if (rc) {
@@ -513,13 +565,7 @@ static int try_context_mount(struct super_block *sb, void *data)
 		if (sid == sbsec->def_sid)
 			goto out_free;
 
-		rc = avc_has_perm(tsec->sid, sbsec->sid, SECCLASS_FILESYSTEM,
-				  FILESYSTEM__RELABELFROM, NULL);
-		if (rc)
-			goto out_free;
-
-		rc = avc_has_perm(sid, sbsec->sid, SECCLASS_FILESYSTEM,
-				  FILESYSTEM__ASSOCIATE, NULL);
+		rc = may_context_mount_inode_relabel(sid, sbsec, tsec);
 		if (rc)
 			goto out_free;
 
@@ -531,6 +577,7 @@ out_free:
 		kfree(context);
 		kfree(defcontext);
 		kfree(fscontext);
+		kfree(rootcontext);
 	}
 out:
 	return rc;
@@ -1882,7 +1929,8 @@ static inline int selinux_option(char *option, int len)
 {
 	return (match_prefix("context=", sizeof("context=")-1, option, len) ||
 	        match_prefix("fscontext=", sizeof("fscontext=")-1, option, len) ||
-	        match_prefix("defcontext=", sizeof("defcontext=")-1, option, len));
+	        match_prefix("defcontext=", sizeof("defcontext=")-1, option, len) ||
+		match_prefix("rootcontext=", sizeof("rootcontext=")-1, option, len));
 }
 
 static inline void take_option(char **to, char *from, int *first, int len)