Re: [PATCH] tmpfs: security xattr setting on inode creation

[Hugh Dickins <hughd@google.com>]

Thanks a lot for doing this, Jarkko: I knew nothing about it.
And thank you James for reviewing, I felt much happier seeing that.

I did fiddle around with it slightly, to reduce the added textsize,
and eliminate it when CONFIG_TMPFS_XATTR is off.  Please, Jarkko
and James, complain bitterly if I've messed up the good work.

[PATCH] tmpfs: security xattr setting on inode creation

From: Jarkko Sakkinen <jarkko.sakkinen@intel.com>

Adds to generic xattr support introduced in Linux 3.0 by
implementing initxattrs callback. This enables consulting
of security attributes from LSM and EVM when inode is created.

[hughd: moved under CONFIG_TMPFS_XATTR, with memcpy in shmem_xattr_alloc]

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c |   88 +++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 72 insertions(+), 16 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1178,6 +1178,12 @@ static struct inode *shmem_get_inode(str
 static const struct inode_operations shmem_symlink_inode_operations;
 static const struct inode_operations shmem_short_symlink_operations;
 
+#ifdef CONFIG_TMPFS_XATTR
+static int shmem_initxattrs(struct inode *, const struct xattr *, void *);
+#else
+#define shmem_initxattrs NULL
+#endif
+
 static int
 shmem_write_begin(struct file *file, struct address_space *mapping,
 			loff_t pos, unsigned len, unsigned flags,
@@ -1490,7 +1496,7 @@ shmem_mknod(struct inode *dir, struct de
 	if (inode) {
 		error = security_inode_init_security(inode, dir,
 						     &dentry->d_name,
-						     NULL, NULL);
+						     shmem_initxattrs, NULL);
 		if (error) {
 			if (error != -EOPNOTSUPP) {
 				iput(inode);
@@ -1630,7 +1636,7 @@ static int shmem_symlink(struct inode *d
 		return -ENOSPC;
 
 	error = security_inode_init_security(inode, dir, &dentry->d_name,
-					     NULL, NULL);
+					     shmem_initxattrs, NULL);
 	if (error) {
 		if (error != -EOPNOTSUPP) {
 			iput(inode);
@@ -1704,6 +1710,66 @@ static void shmem_put_link(struct dentry
  * filesystem level, though.
  */
 
+/*
+ * Allocate new xattr and copy in the value; but leave the name to callers.
+ */
+static struct shmem_xattr *shmem_xattr_alloc(const void *value, size_t size)
+{
+	struct shmem_xattr *new_xattr;
+	size_t len;
+
+	/* wrap around? */
+	len = sizeof(*new_xattr) + size;
+	if (len <= sizeof(*new_xattr))
+		return NULL;
+
+	new_xattr = kmalloc(len, GFP_KERNEL);
+	if (!new_xattr)
+		return NULL;
+
+	new_xattr->size = size;
+	memcpy(new_xattr->value, value, size);
+	return new_xattr;
+}
+
+/*
+ * Callback for security_inode_init_security() for acquiring xattrs.
+ */
+static int shmem_initxattrs(struct inode *inode,
+			    const struct xattr *xattr_array,
+			    void *fs_info)
+{
+	struct shmem_inode_info *info = SHMEM_I(inode);
+	const struct xattr *xattr;
+	struct shmem_xattr *new_xattr;
+	size_t len;
+
+	for (xattr = xattr_array; xattr->name != NULL; xattr++) {
+		new_xattr = shmem_xattr_alloc(xattr->value, xattr->value_len);
+		if (!new_xattr)
+			return -ENOMEM;
+
+		len = strlen(xattr->name) + 1;
+		new_xattr->name = kmalloc(XATTR_SECURITY_PREFIX_LEN + len,
+					  GFP_KERNEL);
+		if (!new_xattr->name) {
+			kfree(new_xattr);
+			return -ENOMEM;
+		}
+
+		memcpy(new_xattr->name, XATTR_SECURITY_PREFIX,
+		       XATTR_SECURITY_PREFIX_LEN);
+		memcpy(new_xattr->name + XATTR_SECURITY_PREFIX_LEN,
+		       xattr->name, len);
+
+		spin_lock(&info->lock);
+		list_add(&new_xattr->list, &info->xattr_list);
+		spin_unlock(&info->lock);
+	}
+
+	return 0;
+}
+
 static int shmem_xattr_get(struct dentry *dentry, const char *name,
 			   void *buffer, size_t size)
 {
@@ -1731,24 +1797,17 @@ static int shmem_xattr_get(struct dentry
 	return ret;
 }
 
-static int shmem_xattr_set(struct dentry *dentry, const char *name,
+static int shmem_xattr_set(struct inode *inode, const char *name,
 			   const void *value, size_t size, int flags)
 {
-	struct inode *inode = dentry->d_inode;
 	struct shmem_inode_info *info = SHMEM_I(inode);
 	struct shmem_xattr *xattr;
 	struct shmem_xattr *new_xattr = NULL;
-	size_t len;
 	int err = 0;
 
 	/* value == NULL means remove */
 	if (value) {
-		/* wrap around? */
-		len = sizeof(*new_xattr) + size;
-		if (len <= sizeof(*new_xattr))
-			return -ENOMEM;
-
-		new_xattr = kmalloc(len, GFP_KERNEL);
+		new_xattr = shmem_xattr_alloc(value, size);
 		if (!new_xattr)
 			return -ENOMEM;
 
@@ -1757,9 +1816,6 @@ static int shmem_xattr_set(struct dentry
 			kfree(new_xattr);
 			return -ENOMEM;
 		}
-
-		new_xattr->size = size;
-		memcpy(new_xattr->value, value, size);
 	}
 
 	spin_lock(&info->lock);
@@ -1858,7 +1914,7 @@ static int shmem_setxattr(struct dentry
 	if (size == 0)
 		value = "";  /* empty EA, do not remove */
 
-	return shmem_xattr_set(dentry, name, value, size, flags);
+	return shmem_xattr_set(dentry->d_inode, name, value, size, flags);
 
 }
 
@@ -1878,7 +1934,7 @@ static int shmem_removexattr(struct dent
 	if (err)
 		return err;
 
-	return shmem_xattr_set(dentry, name, NULL, 0, XATTR_REPLACE);
+	return shmem_xattr_set(dentry->d_inode, name, NULL, 0, XATTR_REPLACE);
 }
 
 static bool xattr_is_trusted(const char *name)