[PATCH] SELinux: canonicalize getxattr()

This patch allows SELinux to canonicalize the value returned from
getxattr() via the security_inode_getsecurity() hook, which is called after
the fs level getxattr() function.

The purpose of this is to allow the in-core security context for an inode
to override the on-disk value.  This could happen in cases such as
upgrading a system to a different labeling form (e.g.  standard SELinux to
MLS) without needing to do a full relabel of the filesystem.

In such cases, we want getxattr() to return the canonical security context
that the kernel is using rather than what is stored on disk.

The implementation hooks into the inode_getsecurity(), adding another
parameter to indicate the result of the preceding fs-level getxattr() call,
so that SELinux knows whether to compare a value obtained from disk with
the kernel value.

We also now allow getxattr() to work for mountpoint labeled filesystems
(i.e.  mount with option context=foo_t), as we are able to return the
kernel value to the user.

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

2 files changed, 33 insertions, 15 deletions

diff --git a/security/dummy.c b/security/dummy.c
index 3d34f3de7e82..2a0337a52d32 100644
--- a/security/dummy.c
+++ b/security/dummy.c
@@ -377,7 +377,7 @@ static int dummy_inode_removexattr (struct dentry *dentry, char *name)
 	return 0;
 }
 
-static int dummy_inode_getsecurity(struct inode *inode, const char *name, void *buffer, size_t size)
+static int dummy_inode_getsecurity(struct inode *inode, const char *name, void *buffer, size_t size, int err)
 {
 	return -EOPNOTSUPP;
 }
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 8cd33b2cd865..d9ec85292e1c 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2208,9 +2208,6 @@ static int selinux_inode_getxattr (struct dentry *dentry, char *name)
 	struct inode *inode = dentry->d_inode;
 	struct superblock_security_struct *sbsec = inode->i_sb->s_security;
 
-	if (sbsec->behavior == SECURITY_FS_USE_MNTPOINT)
-		return -EOPNOTSUPP;
-
 	return dentry_has_perm(current, NULL, dentry, FILE__GETATTR);
 }
 
@@ -2241,33 +2238,54 @@ static int selinux_inode_removexattr (struct dentry *dentry, char *name)
 	return -EACCES;
 }
 
-static int selinux_inode_getsecurity(struct inode *inode, const char *name, void *buffer, size_t size)
+/*
+ * Copy the in-core inode security context value to the user.  If the
+ * getxattr() prior to this succeeded, check to see if we need to
+ * canonicalize the value to be finally returned to the user.
+ *
+ * Permission check is handled by selinux_inode_getxattr hook.
+ */
+static int selinux_inode_getsecurity(struct inode *inode, const char *name, void *buffer, size_t size, int err)
 {
 	struct inode_security_struct *isec = inode->i_security;
 	char *context;
 	unsigned len;
 	int rc;
 
-	/* Permission check handled by selinux_inode_getxattr hook.*/
-
-	if (strcmp(name, XATTR_SELINUX_SUFFIX))
-		return -EOPNOTSUPP;
+	if (strcmp(name, XATTR_SELINUX_SUFFIX)) {
+		rc = -EOPNOTSUPP;
+		goto out;
+	}
 
 	rc = security_sid_to_context(isec->sid, &context, &len);
 	if (rc)
-		return rc;
+		goto out;
 
+	/* Probe for required buffer size */
 	if (!buffer || !size) {
-		kfree(context);
-		return len;
+		rc = len;
+		goto out_free;
 	}
+
 	if (size < len) {
-		kfree(context);
-		return -ERANGE;
+		rc = -ERANGE;
+		goto out_free;
+	}
+
+	if (err > 0) {
+		if ((len == err) && !(memcmp(context, buffer, len))) {
+			/* Don't need to canonicalize value */
+			rc = err;
+			goto out_free;
+		}
+		memset(buffer, 0, size);
 	}
 	memcpy(buffer, context, len);
+	rc = len;
+out_free:
 	kfree(context);
-	return len;
+out:
+	return rc;
 }
 
 static int selinux_inode_setsecurity(struct inode *inode, const char *name,