Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

Tycho Andersen
2019-10-29T15:13:49Z

Hi Elena, Mike,

On Tue, Oct 29, 2019 at 11:25:12AM +0000, Reshetova, Elena wrote:
> > The patch below aims to allow applications to create mappins that have
> > pages visible only to the owning process. Such mappings could be used to
> > store secrets so that these secrets are not visible neither to other
> > processes nor to the kernel.
>
> Hi Mike,
>
> I have actually been looking into the closely related problem for the past
> couple of weeks (on and off). What is common here is the need for userspace
> to indicate to kernel that some pages contain secrets. And then there are
> actually a number of things that kernel can do to try to protect these secrets
> better. Unmap from direct map is one of them. Another thing is to map such
> pages as non-cached, which can help us to prevent or considerably restrict
> speculation on such pages. The initial proof of concept for marking pages as
> "UNCACHED" that I got from Dave Hansen was actually based on mlock2()
> and a new flag for it for this purpose. Since then I have been thinking on what
> interface suits the use case better and actually selected going with new madvise()
> flag instead because of all possible implications for fragmentation and performance.
> My logic was that we better allocate the secret data explicitly (using mmap())
> to make sure that no other process data accidentally gets to suffer.
> Imagine I would allocate a buffer to hold a secret key, signal with mlock
> to protect it and suddenly my other high throughput non-secret buffer
> (which happened to live on the same page by chance) became very slow
> and I don't even have an easy way (apart from mmap()ing it!) to guarantee
> that it won't be affected.
>
> So, I ended up towards smth like:
>
> secret_buffer = mmap(NULL, PAGE_SIZE, ...)
> madvise(secret_buffer, size, MADV_SECRET)
>
> I have work in progress code here:
> https://github.com/ereshetova/linux/commits/madvise
>
> I haven't sent it for review, because it is not ready yet and I am now working
> on trying to add the page wiping functionality. Otherwise it would be useless
> to protect the page during the time it is used in userspace, but then allow it
> to get reused by a different process later after it has been released back and
> userspace was stupid enough not to wipe the contents (or was crashed on
> purpose before it was able to wipe anything out).

I was looking at this and thinking that wiping during do_exit() might
be a nice place, but I haven't tried anything yet.

> We have also had some discussions with Tycho that XPFO can be also
> applied selectively for such "SECRET" marked pages and I know that he has also
> did some initial prototyping on this, so I think it would be great to decide
> on userspace interface first and then see how we can assemble together all
> these features.

Yep! Here's my tree with the direct un-mapping bits ported from XPFO:
https://github.com/tych0/linux/commits/madvise

As noted in one of the commit messages I think the bit math for page
prot flags needs a bit of work, but the test passes, so :)

In any case, I'll try to look at Mike's patches later today.

Cheers,

Tycho