linux-kernel - Re: [RFCv1 0/6] Page Detective

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Message-ID: <CA+CK2bByXtm8sLyFzDDzm5xC6xb=DEutaRUeujGJdwf-kmK1gA@mail.gmail.com>
Date: Tue, 19 Nov 2024 10:14:12 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Jann Horn <jannh@...gle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, linux-kernel@...r.kernel.org, 
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	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFCv1 0/6] Page Detective

On Tue, Nov 19, 2024 at 7:52 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Tue, Nov 19, 2024 at 2:30 AM Pasha Tatashin
> <pasha.tatashin@...een.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.

Thank you,
Pasha