Re: [PATCH v1 1/2] mseal: Two fixes for madvise(MADV_DONTNEED) when sealed

[Pedro Falcato <pedro.falcato@gmail.com>]

On Thu, Oct 17, 2024 at 01:34:53PM -0700, Jeff Xu wrote:
> Hi Pedro
> 
> On Thu, Oct 17, 2024 at 12:37 PM Pedro Falcato <pedro.falcato@gmail.com> wrote:
> >
> > > For PROT_NONE mappings, the previous blocking of
> > > madvise(MADV_DONTNEED) is unnecessary. As PROT_NONE already prohibits
> > > memory access, madvise(MADV_DONTNEED) should be allowed to proceed in
> > > order to free the page.
> >
> > I don't get it. Is there an actual use case for this?
> >
> Sealing should not over-blocking API that it can allow to pass without
> security concern, this is a case in that principle.

Well, making the interface simple is also important. OpenBSD's mimmutable()
doesn't do any of this and it Just Works(tm)...

> 
> There is a user case for this as well: to seal NX stack on android,
> Android uses PROT_NONE/madvise to set up a guide page to prevent stack
> run over boundary. So we need to let madvise to pass.

And you need to MADV_DONTNEED this guard page?

> 
> > > For file-backed, private, read-only memory mappings, we previously did
> > > not block the madvise(MADV_DONTNEED). This was based on
> > > the assumption that the memory's content, being file-backed, could be
> > > retrieved from the file if accessed again. However, this assumption
> > > failed to consider scenarios where a mapping is initially created as
> > > read-write, modified, and subsequently changed to read-only. The newly
> > > introduced VM_WASWRITE flag addresses this oversight.
> >
> > We *do not* need this. It's sufficient to just block discard operations on read-only
> > private mappings.
> I think you meant blocking madvise(MADV_DONTNEED) on all read-only
> private file-backed mappings.
> 
> I considered that option, but there is a use case for madvise on those
> mappings that never get modified.
> 
> Apps can use that to free up RAM. e.g. Considering read-only .text
> section, which never gets modified, madvise( MADV_DONTNEED) can free
> up RAM when memory is in-stress, memory will be reclaimed from a
> backed-file on next read access. Therefore we can't just block all
> read-only private file-backed mapping, only those that really need to,
> such as mapping changed from rw=>r (what you described)

Does anyone actually do this? If so, why? WHYYYY?

The kernel's page reclaim logic should be perfectly cromulent. Please don't do this.
MADV_DONTNEED will also not free any pages if those are shared (rather they'll just be unmapped).

If we really need to do this, I'd maybe suggest walking through page tables, looking for
anon ptes or swap ptes (maybe inside the actual zap code?). But I would really prefer if we
didn't need to do this.

-- 
Pedro