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Date:   Tue, 5 Jun 2018 13:34:09 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Ilya Matveychikov <matvejchikov@gmail.com>
Cc:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] ksys_mount: check for permissions before resource
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On Tue, Jun 05, 2018 at 04:07:15PM +0400, Ilya Matveychikov wrote:

> > 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.
> 
> I agree that it’s impossible to completely avoid this kind of allocations
> and examining data in user-land will be the bigger problem than copying
> arguments to the kernel. But aside of that what’s wrong with the idea of
> having the permission check before doing any kind of work?

Presenting that as mitigating a vulnerability.  It's neither better nor worse
in that respect than the original.