linux-kernel - Re: Memory zeroed when made available to user process

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Date:   Fri, 29 Jun 2018 08:10:45 +0200
From:   Richard Weinberger <richard@....at>
To:     Jefferson Carpenter <jeffersoncarpenter2@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Michal Hocko <mhocko@...nel.org>
Subject: Re: Memory zeroed when made available to user process

Am Freitag, 29. Juni 2018, 02:52:16 CEST schrieb Jefferson Carpenter:
> On 6/27/2018 1:18 PM, Richard Weinberger wrote:
> > Am Mittwoch, 27. Juni 2018, 15:12:48 CEST schrieb Michal Hocko:
> >> On Wed 27-06-18 13:29:05, Richard Weinberger wrote:
> >>> On Wed, Jun 27, 2018 at 11:34 AM, Jefferson Carpenter
> >>> <jeffersoncarpenter2@...il.com> wrote:
> >>>> Is there a way for a user process to mark memory as 'sensitive' or
> >>>> 'non-sensitive' when it is allocated?  That could allow it not to have to be
> >>>> zeroed before being allocated to another process.
> >>>
> >>> Isn't this what we have Meltdown and Spectre for? ;-)
> >>>
> >>> No, memory from the kernel is always zeroed.
> >>> libc offers malloc() and calloc() for this purpose.
> 
> Interesting.  Let's say
> 
> Process 1:
> free(use_memory(malloc(1024)));
> 
> Then Process 2:
> malloc(1024);
> 
> The physical RAM used to service Process 2's malloc call has to be 
> zeroed to prevent it from leaking data from Process 1.  However, if 
> Process 1 could mark that memory as non-sensitive, then it would not 
> have to be zeroed, saving the time it takes to do that.  However, this 
> would require at least a bit per memory page, so maybe it's not worth it.

Is this really a measurable overhead on your system?
Do you have numbers?

Thanks,
//richard