xfs: add capability check to free eofblocks ioctl

commit 8c567a7fab6e086a0284eee2db82348521e7120c upstream.

Check for CAP_SYS_ADMIN since the caller can truncate preallocated
blocks from files they do not own nor have write access to. A more
fine grained access check was considered: require the caller to
specify their own uid/gid and to use inode_permission to check for
write, but this would not catch the case of an inode not reachable
via path traversal from the callers mount namespace.

Add check for read-only filesystem to free eofblocks ioctl.

Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Ben Myers <bpm@sgi.com>
Cc: Kees Cook <keescook@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/xfs/xfs_ioctl.c

@@ -1612,6 +1612,12 @@ xfs_file_ioctl(
 	case XFS_IOC_FREE_EOFBLOCKS: {
 		struct xfs_eofblocks eofb;
 
+		if (!capable(CAP_SYS_ADMIN))
+			return -EPERM;
+
+		if (mp->m_flags & XFS_MOUNT_RDONLY)
+			return -XFS_ERROR(EROFS);
+
 		if (copy_from_user(&eofb, arg, sizeof(eofb)))
 			return -XFS_ERROR(EFAULT);