file caps: always start with clear bprm->caps_*

While Linux doesn't honor setuid on scripts.  However, it mistakenly
behaves differently for file capabilities.

This patch fixes that behavior by making sure that get_file_caps()
begins with empty bprm->caps_*.  That way when a script is loaded,
its bprm->caps_* may be filled when binfmt_misc calls prepare_binprm(),
but they will be cleared again when binfmt_elf calls prepare_binprm()
next to read the interpreter's file capabilities.
Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

security/commoncap.c

@@ -279,10 +279,10 @@ static int get_file_caps(struct linux_binprm *bprm)
 	struct vfs_cap_data vcaps;
 	struct inode *inode;
 
-	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID) {
 	bprm_clear_caps(bprm);
+
+	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
 		return 0;
-	}
 
 	dentry = dget(bprm->file->f_dentry);
 	inode = dentry->d_inode;