xattr: Fix error results for non-existent / invisible attributes

Return -ENODATA when trying to read a user.* attribute which cannot
exist: user space otherwise does not have a reasonable way to
distinguish between non-existent and inaccessible attributes.

Likewise, return -ENODATA when an unprivileged process tries to read a
trusted.* attribute: to unprivileged processes, those attributes are
invisible (listxattr() won't include them).

Related to this bug report: https://bugzilla.redhat.com/660613

Signed-off-by: Andreas Gruenbacher <agruen@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 10 insertions, 6 deletions

diff --git a/fs/xattr.c b/fs/xattr.c
index f1ef94974dea..4be2e7666d02 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -46,18 +46,22 @@ xattr_permission(struct inode *inode, const char *name, int mask)
                 return 0;
 
         /*
-         * The trusted.* namespace can only be accessed by a privileged user.
+         * The trusted.* namespace can only be accessed by privileged users.
          */
-        if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN))
-                return (capable(CAP_SYS_ADMIN) ? 0 : -EPERM);
+        if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN)) {
+                if (!capable(CAP_SYS_ADMIN))
+                        return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
+                return 0;
+        }
 
-        /* In user.* namespace, only regular files and directories can have
+        /*
+         * In the user.* namespace, only regular files and directories can have
          * extended attributes. For sticky directories, only the owner and
-         * privileged user can write attributes.
+         * privileged users can write attributes.
          */
         if (!strncmp(name, XATTR_USER_PREFIX, XATTR_USER_PREFIX_LEN)) {
                 if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode))
-                        return -EPERM;
+                        return (mask & MAY_WRITE) ? -EPERM : -ENODATA;
                 if (S_ISDIR(inode->i_mode) && (inode->i_mode & S_ISVTX) &&
                     (mask & MAY_WRITE) && !inode_owner_or_capable(inode))
                         return -EPERM;