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Date:   Fri, 12 Jan 2018 16:15:12 -0800
From:   Tony Luck <tony.luck@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        kernel-hardening@...ts.openwall.com,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Cox <alan.cox@...el.com>,
        Will Deacon <will.deacon@....com>,
        Alexei Starovoitov <ast@...nel.org>,
        Solomon Peachy <pizza@...ftnet.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        Linux SCSI List <linux-scsi@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Kees Cook <keescook@...omium.org>, Jan Kara <jack@...e.com>,
        Al Viro <viro@...iv.linux.org.uk>, qla2xxx-upstream@...gic.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kalle Valo <kvalo@...eaurora.org>,
        Alan Cox <alan@...ux.intel.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux Wireless List <linux-wireless@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Network Development <netdev@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v2 00/19] prevent bounds-check bypass via speculative execution

On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> Should the array access in entry_SYSCALL_64_fastpath be made to use
> the masking approach?

That one has a bounds check for an inline constant.

     cmpq    $__NR_syscall_max, %rax

so should be safe.

The classic Spectre variant #1 code sequence is:

int array_size;

       if (x < array_size) {
               something with array[x]
       }

which runs into problems because the array_size variable may not
be in cache, and while the CPU core is waiting for the value it
speculates inside the "if" body.

The syscall entry is more like:

#define ARRAY_SIZE 10

     if (x < ARRAY_SIZE) {
          something with array[x]
     }

Here there isn't any reason for speculation. The core has the
value of 'x' in a register and the upper bound encoded into the
"cmp" instruction.  Both are right there, no waiting, no speculation.

-Tony

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