Re: [PATCH] ksys_mount: check for permissions before resource allocation

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 allocation
Date: Tue, 5 Jun 2018 13:34:09 +0100	[thread overview]
Message-ID: <20180605123409.GP30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <0F38EDA5-DEC3-48A1-9375-47949C26DAE8@gmail.com>

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.