linux-kernel - RE: [PATCH 12/14] x86: remove address space overrides using set

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 4 Sep 2020 07:59:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>
CC:     Christoph Hellwig <hch@....de>, Al Viro <viro@...iv.linux.org.uk>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "x86@...nel.org" <x86@...nel.org>,
        "Alexey Dobriyan" <adobriyan@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        "Kees Cook" <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH 12/14] x86: remove address space overrides using set_fs()

From: Linus Torvalds
> Sent: 04 September 2020 00:26
> 
> On Thu, Sep 3, 2020 at 2:30 PM David Laight <David.Laight@...lab.com> wrote:
> >
> > A non-canonical (is that the right term) address between the highest
> > valid user address and the lowest valid kernel address (7ffe to fffe?)
> > will fault anyway.
> 
> Yes.
> 
> But we actually warn against that fault, because it's been a good way
> to catch places that didn't use the proper "access_ok()" pattern.
> 
> See ex_handler_uaccess() and the
> 
>         WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in
> user access. Non-canonical address?");
> 
> warning. It's been good for randomized testing - a missing range check
> on a user address will often hit this.
> 
> Of course, you should never see it in real life (and hopefully not in
> testing either any more). But belt-and-suspenders..

That could still be effective, just pick an address limit that is
appropriate for the one access_ok() is using.

Even if access_ok() uses 1<<63 there are plenty of addresses above it that fault.
But the upper limit for 5-level page tables could be used all the time.

One option is to test '(address | length) < (3<<62)' in access_ok().
That is also moderately suitable for masking invalid addresses to 0.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)