[PATCH v2 bpf-next 1/2] fs/xattr: bpf: Introduce security.bpf xattr name prefix

[Song Liu <song@xxxxxxxxxx>]

Introduct new xattr name prefix security.bpf, and enable reading these
xattrs from bpf kfuncs bpf_get_[file|dentry]_xattr(). Note that
"security.bpf" could be the name of the xattr or the prefix of the
name. For example, both "security.bpf" and "security.bpf.xxx" are
valid xattr name; while "security.bpfxxx" is not valid.

As we are on it, correct the comments for return value of
bpf_get_[file|dentry]_xattr(), i.e. return length the xattr value on
successl.

Signed-off-by: Song Liu <song@xxxxxxxxxx>
Acked-by: Christian Brauner <brauner@xxxxxxxxxx>
---
 fs/bpf_fs_kfuncs.c         | 29 ++++++++++++++++++++++++-----
 include/uapi/linux/xattr.h |  4 ++++
 2 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 3fe9f59ef867..be78a13f1d82 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -93,6 +93,23 @@ __bpf_kfunc int bpf_path_d_path(struct path *path, char *buf, size_t buf__sz)
 	return len;
 }
 
+static bool bpf_xattr_name_allowed(const char *name__str)
+{
+	/* Allow xattr names with user. prefix */
+	if (!strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))
+		return true;
+
+	/* Allow security.bpf. prefix or just security.bpf */
+	if (!strncmp(name__str, XATTR_NAME_BPF_LSM, XATTR_NAME_BPF_LSM_LEN) &&
+	    (name__str[XATTR_NAME_BPF_LSM_LEN] == '\0' ||
+	     name__str[XATTR_NAME_BPF_LSM_LEN] == '.')) {
+		return true;
+	}
+
+	/* Disallow anything else */
+	return false;
+}
+
 /**
  * bpf_get_dentry_xattr - get xattr of a dentry
  * @dentry: dentry to get xattr from
@@ -101,9 +118,10 @@ __bpf_kfunc int bpf_path_d_path(struct path *path, char *buf, size_t buf__sz)
  *
  * Get xattr *name__str* of *dentry* and store the output in *value_ptr*.
  *
- * For security reasons, only *name__str* with prefix "user." is allowed.
+ * For security reasons, only *name__str* with prefix "user." or
+ * "security.bpf" is allowed.
  *
- * Return: 0 on success, a negative value on error.
+ * Return: length of the xattr value on success, a negative value on error.
  */
 __bpf_kfunc int bpf_get_dentry_xattr(struct dentry *dentry, const char *name__str,
 				     struct bpf_dynptr *value_p)
@@ -117,7 +135,7 @@ __bpf_kfunc int bpf_get_dentry_xattr(struct dentry *dentry, const char *name__st
 	if (WARN_ON(!inode))
 		return -EINVAL;
 
-	if (strncmp(name__str, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN))
+	if (!bpf_xattr_name_allowed(name__str))
 		return -EPERM;
 
 	value_len = __bpf_dynptr_size(value_ptr);
@@ -139,9 +157,10 @@ __bpf_kfunc int bpf_get_dentry_xattr(struct dentry *dentry, const char *name__st
  *
  * Get xattr *name__str* of *file* and store the output in *value_ptr*.
  *
- * For security reasons, only *name__str* with prefix "user." is allowed.
+ * For security reasons, only *name__str* with prefix "user." or
+ * "security.bpf" is allowed.
  *
- * Return: 0 on success, a negative value on error.
+ * Return: length of the xattr value on success, a negative value on error.
  */
 __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
 				   struct bpf_dynptr *value_p)
diff --git a/include/uapi/linux/xattr.h b/include/uapi/linux/xattr.h
index 9463db2dfa9d..166ef2f1f1b3 100644
--- a/include/uapi/linux/xattr.h
+++ b/include/uapi/linux/xattr.h
@@ -76,6 +76,10 @@
 #define XATTR_CAPS_SUFFIX "capability"
 #define XATTR_NAME_CAPS XATTR_SECURITY_PREFIX XATTR_CAPS_SUFFIX
 
+#define XATTR_BPF_LSM_SUFFIX "bpf"
+#define XATTR_NAME_BPF_LSM (XATTR_SECURITY_PREFIX XATTR_BPF_LSM_SUFFIX)
+#define XATTR_NAME_BPF_LSM_LEN (sizeof(XATTR_NAME_BPF_LSM) - 1)
+
 #define XATTR_POSIX_ACL_ACCESS  "posix_acl_access"
 #define XATTR_NAME_POSIX_ACL_ACCESS XATTR_SYSTEM_PREFIX XATTR_POSIX_ACL_ACCESS
 #define XATTR_POSIX_ACL_DEFAULT  "posix_acl_default"
-- 
2.43.5