Memory zeroed when made available to user process

Memory zeroed when made available to user process

[Jefferson Carpenter]

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.

Re: Memory zeroed when made available to user process

[Richard Weinberger]

On Wed, Jun 27, 2018 at 11:34 AM, Jefferson Carpenter
<jeffersoncarpenter2@gmail.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.

-- 
Thanks,
//richard

Re: Memory zeroed when made available to user process

[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@gmail.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.

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.

-- 
Michal Hocko
SUSE Labs

Re: Memory zeroed when made available to user process

[Richard Weinberger]

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@gmail.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.
> 
> 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

-- 
sigma star gmbh - Eduard-Bodem-Gasse 6 - 6020 Innsbruck - Austria
ATU66964118 - FN 374287y

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@gmail.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.

>>
>> 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
> 

Re: Memory zeroed when made available to user process

[Richard Weinberger]

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@gmail.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