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

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Date:   Tue, 5 Jun 2018 14:28:29 +0200
From:   David Sterba <dsterba@...e.cz>
To:     Ilya Matveychikov <matvejchikov@...il.com>
Cc:     Al Viro <viro@...IV.linux.org.uk>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ksys_mount: check for permissions before resource
 allocation

On Tue, Jun 05, 2018 at 04:07:15PM +0400, Ilya Matveychikov wrote:
> > On Jun 5, 2018, at 3:53 PM, Al Viro <viro@...IV.linux.org.uk> wrote:
> > On Tue, Jun 05, 2018 at 03:35:55PM +0400, Ilya Matveychikov wrote:
> >>> On Jun 5, 2018, at 3:26 PM, Al Viro <viro@...IV.linux.org.uk> wrote:
> >>>>> On Jun 5, 2018, at 6:00 AM, Ilya Matveychikov <matvejchikov@...il.com> 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.
> 
> 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?

Isn't there some sysctl knob or config option to sanitize freed memory?
I doubt that using kzfree everywhere unconditionally would be welcome,
also would not scale as there are too many of them. This IMHO leaves
only the build-time option for those willing to pay the performance hit.

> BTW, sys_umount() has this check in the right place - before doing anything.
> So, why not to have the same logic for mount/umount?

What if the check is not equivalent to the one done later? may_mount
needs namespace, it will be available at umount time but not necessarily
during mount due to the security hooks.