Re: [LSF/MM/BPF TOPIC] Dropping page cache of individual fs

Dave Chinner
2024-02-15T23:17:33Z

On Thu, Feb 15, 2024 at 02:46:52PM -0500, Adrian Vovk wrote:
> On 2/15/24 08:57, Jan Kara wrote:
> > On Mon 29-01-24 19:13:17, Adrian Vovk wrote:
> > > Hello! I'm the "GNOME people" who Christian is referring to
> > Got back to thinking about this after a while...
> >
> > > On 1/17/24 09:52, Matthew Wilcox wrote:
> > > > I feel like we're in an XY trap [1]. What Christian actually wants is
> > > > to not be able to access the contents of a file while the device it's
> > > > on is suspended, and we've gone from there to "must drop the page cache".
> > > What we really want is for the plaintext contents of the files to be gone
> > > from memory while the dm-crypt device backing them is suspended.
> > >
> > > Ultimately my goal is to limit the chance that an attacker with access to a
> > > user's suspended laptop will be able to access the user's encrypted data. I
> > > need to achieve this without forcing the user to completely log out/power
> > > off/etc their system; it must be invisible to the user. The key word here is
> > > limit; if we can remove _most_ files from memory _most_ of the time Ithink
> > > luksSuspend would be a lot more useful against cold boot than it is today.
> > Well, but if your attack vector are cold-boot attacks, then how does
> > freeing pages from the page cache help you? I mean sure the page allocator
> > will start tracking those pages with potentially sensitive content as free
> > but unless you also zero all of them, this doesn't help anything against
> > cold-boot attacks? The sensitive memory content is still there...
> >
> > So you would also have to enable something like zero-on-page-free and
> > generally the cost of this is going to be pretty big?
>
> Yes you are right. Just marking pages as free isn't enough.
>
> I'm sure it's reasonable enough to zero out the pages that are getting
> free'd at our request. But the difficulty here is to try and clear pages
> that were freed previously for other reasons, unless we're zeroing out all
> pages on free. So I suppose that leaves me with a couple questions:
>
> - As far as I know, the kernel only naturally frees pages from the page
> cache when they're about to be given to some program for imminent use.

Memory pressure does cause cache reclaim. Not just page cache, but
also slab caches and anything else various subsystems can clean up
to free memory..

> But
> then in the case the page isn't only free'd, but also zero'd out before it's
> handed over to the program (because giving a program access to a page filled
> with potentially sensitive data is a bad idea!). Is this correct?

Memory exposed to userspace is zeroed before userspace can access
it. Kernel memory is not zeroed unless the caller specifically asks
for it to be zeroed.

> - Are there other situations (aside from drop_caches) where the kernel frees
> pages from the page cache? Especially without having to zero them anyway? In

truncate(), fallocate(), direct IO, fadvise(), madvise(), etc. IOWs,
there are lots of runtime vectors that cause page cache to be freed.

> other words, what situations would turning on some zero-pages-on-free
> setting actually hurt performance?

Lots. page contents are typically cold when the page is freed so
the zeroing is typically memory latency and bandwidth bound. And
doing it on free means there isn't any sort of "cache priming"
performance benefits that we get with zeroing at allocation because
the page contents are not going to be immediately accessed by the
kernel or userspace.

> - Does dismounting a filesystem completely zero out the removed fs's pages
> from the page cache?

No. It just frees them. No explicit zeroing.

> - I remember hearing somewhere of some Linux support for zeroing out all
> pages in memory if they're free'd from the page cache. However, I spent a
> while trying to find this (how to turn it on, benchmarks) and I couldn't
> find it. Do you know if such a thing exists, and if so how to turn it on?
> I'm curious of the actual performance impact of it.

You can test it for yourself: the init_on_free kernel command line
option controls whether the kernel zeroes on free.

Typical distro configuration is:

$ sudo dmesg |grep auto-init
[ 0.018882] mem auto-init: stack:all(zero), heap alloc:on, heap free:off
$

So this kernel zeroes all stack memory, page and heap memory on
allocation, and does nothing on free...

-Dave.
--
Dave Chinner
david@fromorbit.com