linux-kernel - Re: [RFC PATCH v3 11/11] mseal:add documentation

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Message-ID: <CAKbZUD09VKfxsz4VCp+0-Y-cWJ-n=dV+t45Aan10HEhFCjxWdQ@mail.gmail.com>
Date: Thu, 14 Dec 2023 20:11:32 +0000
From: Pedro Falcato <pedro.falcato@...il.com>
To: Stephen Röttger <sroettger@...gle.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Jeff Xu <jeffxu@...gle.com>, 
	jeffxu@...omium.org, akpm@...ux-foundation.org, keescook@...omium.org, 
	jannh@...gle.com, willy@...radead.org, gregkh@...uxfoundation.org, 
	jorgelo@...omium.org, groeck@...omium.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, linux-mm@...ck.org, dave.hansen@...el.com, 
	linux-hardening@...r.kernel.org, deraadt@...nbsd.org
Subject: Re: [RFC PATCH v3 11/11] mseal:add documentation

On Thu, Dec 14, 2023 at 6:07 PM Stephen Röttger <sroettger@...gle.com> wrote:
>
> On Thu, Dec 14, 2023 at 2:31 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Wed, 13 Dec 2023 at 16:36, Jeff Xu <jeffxu@...gle.com> wrote:
> > >
> > >
> > > > IOW, when would you *ever* say "seal this area, but MADV_DONTNEED is ok"?
> > > >
> > > The MADV_DONTNEED is OK for file-backed mapping.
> >
> > Right. It makes no semantic difference. So there's no point to it.
> >
> > My point was that you added this magic flag for "not ok for RO anon mapping".
> >
> > It's such a *completely* random flag, that I go "that's just crazy
> > random - make sealing _always_ disallow that case".
> >
> > So what I object to in this series is basically random small details
> > that should just eb part of the basic act of sealing.
> >
> > I think sealing should just mean "you can't do any operations that
> > have semantic meaning for the mapping, because it is SEALED".
> >
> > So I think sealing should automatically mean "can't do MADV_DONTNEED
> > on anon memory", because that's basically equivalent to a munmap/remap
> > operation.
>
> In Chrome, we have a use case to allow MADV_DONTNEED on sealed memory.

I don't want to be that guy (*believe me*), but what if there was a
way to attach BPF programs to mm's? Such that you could handle 'seal
failures' in BPF, and thus allow for this sort of weird semantics?
e.g: madvise(MADV_DONTNEED) on a sealed region fails, kernel invokes
the BPF program (that chrome loaded), BPF program sees it was a
MADV_DONTNEED and allows it to proceed.

It requires BPF but sounds like a good compromise in order to not get
an ugly API?

-- 
Pedro