[PATCH 2/8] move xattr permission checks into the VFS

[Christoph Hellwig <hch@lst.de>]

The xattr code has rather complex permission checks because the
rules are very different for different attribute namespaces.  This
patch moves as much as we can into the generic code.  Currenyly
all the major disk based filesystems duplicate these checks, while
many minor filesystems or network filesystems lack some or all of them.

To do this we need defines for the extended attribute names in common
code, I moved them up from JFS which had the nicest defintions.


Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: linux-2.6/fs/jfs/xattr.c
===================================================================
--- linux-2.6.orig/fs/jfs/xattr.c	2005-10-31 12:23:57.000000000 +0100
+++ linux-2.6/fs/jfs/xattr.c	2005-10-31 14:34:40.000000000 +0100
@@ -83,21 +83,6 @@
 #define EA_NEW		0x0004
 #define EA_MALLOC	0x0008
 
-/* Namespaces */
-#define XATTR_SYSTEM_PREFIX "system."
-#define XATTR_SYSTEM_PREFIX_LEN (sizeof (XATTR_SYSTEM_PREFIX) - 1)
-
-#define XATTR_USER_PREFIX "user."
-#define XATTR_USER_PREFIX_LEN (sizeof (XATTR_USER_PREFIX) - 1)
-
-#define XATTR_OS2_PREFIX "os2."
-#define XATTR_OS2_PREFIX_LEN (sizeof (XATTR_OS2_PREFIX) - 1)
-
-/* XATTR_SECURITY_PREFIX is defined in include/linux/xattr.h */
-#define XATTR_SECURITY_PREFIX_LEN (sizeof (XATTR_SECURITY_PREFIX) - 1)
-
-#define XATTR_TRUSTED_PREFIX "trusted."
-#define XATTR_TRUSTED_PREFIX_LEN (sizeof (XATTR_TRUSTED_PREFIX) - 1)
 
 /*
  * These three routines are used to recognize on-disk extended attributes
Index: linux-2.6/fs/xattr.c
===================================================================
--- linux-2.6.orig/fs/xattr.c	2005-10-31 14:34:33.000000000 +0100
+++ linux-2.6/fs/xattr.c	2005-10-31 14:35:23.000000000 +0100
@@ -20,6 +20,47 @@
 #include <asm/uaccess.h>
 
 
+/*
+ * Check permissions for extended attribute access.  This is a bit complicated
+ * because different namespaces have very different rules.
+ */
+static int
+xattr_permission(struct inode *inode, const char *name, int mask)
+{
+	/*
+	 * We can never set or remove an extended attribute on a read-only
+	 * filesystem  or on an immutable / append-only inode.
+	 */
+	if (mask & MAY_WRITE) {
+		if (IS_RDONLY(inode))
+			return -EROFS;
+		if (IS_IMMUTABLE(inode) || IS_APPEND(inode))
+			return -EPERM;
+	}
+
+	/*
+	 * No restriction for security.* and system.* from the VFS.  Decision
+	 * on these is left to the underlying filesystem / security module.
+	 */
+	if (!strncmp(name, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN) ||
+	    !strncmp(name, XATTR_SYSTEM_PREFIX, XATTR_SYSTEM_PREFIX_LEN))
+		return 0;
+
+	/*
+	 * The trusted.* namespace can only accessed by a privilegued user.
+	 */
+	if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN))
+		return (capable(CAP_SYS_ADMIN) ? 0 : -EPERM);
+
+	if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
+		if (!S_ISREG(inode->i_mode) &&
+		    (!S_ISDIR(inode->i_mode) || inode->i_mode & S_ISVTX))
+			return -EPERM;
+	}
+
+	return permission(inode, mask, NULL);
+}
+
 int
 vfs_setxattr(struct dentry *dentry, char *name, void *value,
 		size_t size, int flags)
@@ -27,6 +68,10 @@
 	struct inode *inode = dentry->d_inode;
 	int error;
 
+	error = xattr_permission(inode, name, MAY_WRITE);
+	if (error)
+		return error;
+
 	down(&inode->i_sem);
 	error = security_inode_setxattr(dentry, name, value, size, flags);
 	if (error)
@@ -40,8 +85,8 @@
 						     size, flags);
 		}
 	} else if (!strncmp(name, XATTR_SECURITY_PREFIX,
-				sizeof XATTR_SECURITY_PREFIX - 1)) {
-		const char *suffix = name + sizeof XATTR_SECURITY_PREFIX - 1;
+				XATTR_SECURITY_PREFIX_LEN)) {
+		const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
 		error = security_inode_setsecurity(inode, suffix, value,
 						   size, flags);
 		if (!error)
@@ -59,6 +104,10 @@
 	struct inode *inode = dentry->d_inode;
 	int error;
 
+	error = xattr_permission(inode, name, MAY_READ);
+	if (error)
+		return error;
+
 	error = security_inode_getxattr(dentry, name);
 	if (error)
 		return error;
@@ -69,8 +118,8 @@
 		error = -EOPNOTSUPP;
 
 	if (!strncmp(name, XATTR_SECURITY_PREFIX,
-				sizeof XATTR_SECURITY_PREFIX - 1)) {
-		const char *suffix = name + sizeof XATTR_SECURITY_PREFIX - 1;
+				XATTR_SECURITY_PREFIX_LEN)) {
+		const char *suffix = name + XATTR_SECURITY_PREFIX_LEN;
 		int ret = security_inode_getsecurity(inode, suffix, value,
 						     size, error);
 		/*
@@ -94,6 +143,10 @@
 	if (!inode->i_op->removexattr)
 		return -EOPNOTSUPP;
 
+	error = xattr_permission(inode, name, MAY_WRITE);
+	if (error)
+		return error;
+
 	error = security_inode_removexattr(dentry, name);
 	if (error)
 		return error;
Index: linux-2.6/include/linux/xattr.h
===================================================================
--- linux-2.6.orig/include/linux/xattr.h	2005-10-31 14:34:33.000000000 +0100
+++ linux-2.6/include/linux/xattr.h	2005-10-31 14:34:40.000000000 +0100
@@ -13,7 +13,22 @@
 #define XATTR_CREATE	0x1	/* set value, fail if attr already exists */
 #define XATTR_REPLACE	0x2	/* set value, fail if attr does not exist */
 
+/* Namespaces */
+#define XATTR_OS2_PREFIX "os2."
+#define XATTR_OS2_PREFIX_LEN (sizeof (XATTR_OS2_PREFIX) - 1)
+
 #define XATTR_SECURITY_PREFIX	"security."
+#define XATTR_SECURITY_PREFIX_LEN (sizeof (XATTR_SECURITY_PREFIX) - 1)
+
+#define XATTR_SYSTEM_PREFIX "system."
+#define XATTR_SYSTEM_PREFIX_LEN (sizeof (XATTR_SYSTEM_PREFIX) - 1)
+
+#define XATTR_TRUSTED_PREFIX "trusted."
+#define XATTR_TRUSTED_PREFIX_LEN (sizeof (XATTR_TRUSTED_PREFIX) - 1)
+
+#define XATTR_USER_PREFIX "user."
+#define XATTR_USER_PREFIX_LEN (sizeof (XATTR_USER_PREFIX) - 1)
+
 
 struct xattr_handler {
 	char *prefix;