Re: Memory zeroed when made available to user process

[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@xxxxxxxxx> 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.

> > Well, except for the weird MAP_UNINITIALIZED. Anyway agreed that this is
> > a bad idea and the flag should have never been merged. I've just
> > mentioned it for completness.
>
> Oh, I forgot about the crazy nommu world. :-)
>
> Thanks,
> //richard