Re: [RFCv1 0/6] Page Detective

[Jann Horn <jannh@google.com>]

On Tue, Nov 19, 2024 at 4:14 PM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
> On Tue, Nov 19, 2024 at 7:52 AM Jann Horn <jannh@google.com> wrote:
> > On Tue, Nov 19, 2024 at 2:30 AM Pasha Tatashin
> > <pasha.tatashin@soleen.com> wrote:
> > > > Can you point me to where a refcounted reference to the page comes
> > > > from when page_detective_metadata() calls dump_page_lvl()?
> > >
> > > I am sorry, I remembered incorrectly, we are getting reference right
> > > after dump_page_lvl() in page_detective_memcg() -> folio_try_get(); I
> > > will move the folio_try_get() to before dump_page_lvl().
> > >
> > > > > > So I think dump_page() in its current form is not something we should
> > > > > > expose to a userspace-reachable API.
> > > > >
> > > > > We use dump_page() all over WARN_ONs in MM code where pages might not
> > > > > be locked, but this is a good point, that while even the existing
> > > > > usage might be racy, providing a user-reachable API potentially makes
> > > > > it worse. I will see if I could add some locking before dump_page(),
> > > > > or make a dump_page variant that does not do dump_mapping().
> > > >
> > > > To be clear, I am not that strongly opposed to racily reading data
> > > > such that the data may not be internally consistent or such; but this
> > > > is a case of racy use-after-free reads that might end up dumping
> > > > entirely unrelated memory contents into dmesg. I think we should
> > > > properly protect against that in an API that userspace can invoke.
> > > > Otherwise, if we race, we might end up writing random memory contents
> > > > into dmesg; and if we are particularly unlucky, those random memory
> > > > contents could be PII or authentication tokens or such.
> > > >
> > > > I'm not entirely sure what the right approach is here; I guess it
> > > > makes sense that when the kernel internally detects corruption,
> > > > dump_page doesn't take references on pages it accesses to avoid
> > > > corrupting things further. If you are looking at a page based on a
> > > > userspace request, I guess you could access the page with the
> > > > necessary locking to access its properties under the normal locking
> > > > rules?
> > >
> > > I will take reference, as we already do that for memcg purpose, but
> > > have not included dump_page().
> >
> > Note that taking a reference on the page does not make all of
> > dump_page() fine; in particular, my understanding is that
> > folio_mapping() requires that the page is locked in order to return a
> > stable pointer, and some of the code in dump_mapping() would probably
> > also require some other locks - probably at least on the inode and
> > maybe also on the dentry, I think? Otherwise the inode's dentry list
> > can probably change concurrently, and the dentry's name pointer can
> > change too.
>
> Agreed, once reference is taken, the page identity cannot change (i.e.
> if it is a named page it will stay a named page), but dentry can be
> renamed. I will look into what can be done to guarantee consistency in
> the next version. There is also a fallback if locking cannot be
> reliably resolved (i.e. for performance reasons) where we can make
> dump_mapping() optionally disabled from dump_page_lvl() with a new
> argument flag.

Yeah, I think if you don't need the details that dump_mapping() shows,
skipping that for user-requested dumps might be a reasonable option.