linux-kernel - Re: [PATCH] x86: use WRITE

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]

Message-Id: <10030BE1-FE29-4C60-9963-4BE932EF09BA@gmail.com>
Date:   Thu, 6 Sep 2018 13:12:14 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Nadav Amit <namit@...are.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dave Hansen <dave.hansen@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86: use WRITE_ONCE() when setting PTEs

at 12:57 PM, Peter Zijlstra <peterz@...radead.org> wrote:

> On Sun, Sep 02, 2018 at 11:14:50AM -0700, Nadav Amit wrote:
>> When page-table entries are set, the compiler might optimize their
>> assignment by using multiple instructions to set the PTE. This might
>> turn into a security hazard if the user somehow manages to use the
>> interim PTE. L1TF does not make our lives easier, making even an interim
>> non-present PTE a security hazard.
>> 
>> Using WRITE_ONCE() to set PTEs and friends should prevent this potential
>> security hazard.
>> 
>> I skimmed the differences in the binary with and without this patch. The
>> differences are (obviously) greater when CONFIG_PARAVIRT=n as more
>> code optimizations are possible. For better and worse, the impact on the
>> binary with this patch is pretty small. Skimming the code did not cause
>> anything to jump out as a security hazard, but it seems that at least
>> move_soft_dirty_pte() caused set_pte_at() to use multiple writes.
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> 
> Also, its corollary would also make sense/be required, use READ_ONCE()
> when reading these.

I don’t know. This would obviously be much more intrusive. I can add a
get_pte() and write a Coccinelle script to use it instead of reading the
PTE, but in most cases, I presume, it would be an overkill.

The reason for that is that the PTEs are supposed to be accessed while
holding the page-table lock, and the hardware can only change dirty & access
bits. I think that any code that assumes that these bits do not change while
holding the lock is already broken in more ways.