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

Adrian Vovk
2024-02-16T01:14:49Z

On 2/15/24 18:17, Dave Chinner wrote:
> 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 see. So even dismounting a filesystem and removing the device
completely doesn't fully protect from a cold-boot attack. Good to know.
>
>> - 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...

I see. Thank you for all the information.

So ~5% performance penalty isn't trivial, especially to protect against
something rare/unlikely like a cold-boot attack, but it would be quite
nice if we could have some semblance of effort put into making sure the
data is actually out of memory if we claim that we've done our best to
harden the system against this scenario. Again, I'm all for best-effort
solutions here; doing 90% is better than doing 0%...

I've got an alternative idea. How feasible would a second API be that
just goes through free regions of memory and zeroes them out? This would
be something we call immediately after we tell the kernel to drop
everything it can relating to a given filesystem. So the flow would be
something like follows:

1, user puts systemd-homed into this "locked" mode, homed wipes the
dm-crypt key out of memory and suspends the block device (this already
exists)
2. homed asks the kernel to drop whatever caches it can relating to that
filesystem (the topic of this email thread)
3. homed asks the kernel to zero out all unallocated memory to make sure
that the data is really gone (the second call I'm proposing now).

Sure this operation can take a while, but for our use-cases it's
probably fine. We would do this only in response to a direct user action
(and we can show a nice little progress spinner on screen), or right
before suspend. A couple of extra seconds of work while entering suspend
isn't going to be noticed by the user. If the hardware supports
something faster/better to mitigate cold-boot attacks, like memory
encryption / SEV, then we'd prefer to use that instead of course, but
for unsupported hardware I think just zeroing out all the memory that
has been marked free should do the trick just fine.

By the way, something like cryptsetup might want to use this second API
too to ensure data is purged from memory after it closes a LUKS volume,
for instance. So for example if you have an encrypted USB stick you use
on your computer, the data really gets wiped after you unplug it.

> -Dave.