From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from zeniv.linux.org.uk ([195.92.253.2]:60654 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751747AbeFELxm (ORCPT ); Tue, 5 Jun 2018 07:53:42 -0400 Date: Tue, 5 Jun 2018 12:53:40 +0100 From: Al Viro To: Ilya Matveychikov Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] ksys_mount: check for permissions before resource allocation Message-ID: <20180605115340.GO30522@ZenIV.linux.org.uk> References: <20180605112641.GN30522@ZenIV.linux.org.uk> <1E519BA2-4198-4255-BAE4-3125C59741A3@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1E519BA2-4198-4255-BAE4-3125C59741A3@gmail.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Tue, Jun 05, 2018 at 03:35:55PM +0400, Ilya Matveychikov wrote: > > > On Jun 5, 2018, at 3:26 PM, Al Viro wrote: > >> > >>> On Jun 5, 2018, at 6:00 AM, Ilya Matveychikov wrote: > >>> > >>> Early check for mount permissions prevents possible allocation of 3 > >>> pages from kmalloc() pool by unpriveledged user which can be used for > >>> spraying the kernel heap. > > > > I'm sorry, but there are arseloads of unpriveleged syscalls that do the same, > > starting with read() from procfs files. So what the hell does it buy? > > Means that if all do the same shit no reason to fix it? Sounds weird... Fix *what*? You do realize that there's no permission checks to stop e.g. stat(2) from copying the pathname in, right? With user-supplied contents, even... If you depend upon preventing kmalloc'ed temporary allocations filled with user-supplied data, you are screwed, plain and simple. It really can't be prevented, in a lot of ways that are much less exotic than mount(2). Most of syscall arguments are copied in, before we get any permission checks. It does happen and it will happen - examining them while they are still in userland is a nightmare in a lot of respects, starting with security.