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From: Jeff Xu <jeffxu@chromium.org>
Date: Fri, 2 Feb 2024 09:58:53 -0800
Message-ID: <CABi2SkXPNPKgqheuFuQ9iZApQkJm8o6bypNn0B-QDz_W9b0JBQ@mail.gmail.com>
Subject: Re: [PATCH v8 0/4] Introduce mseal
To: Theo de Raadt <deraadt@openbsd.org>
Cc: Jeff Xu <jeffxu@google.com>, Linus Torvalds <torvalds@linux-foundation.org>, 
	"Liam R. Howlett" <Liam.Howlett@oracle.com>, Jonathan Corbet <corbet@lwn.net>, akpm@linux-foundation.org, 
	keescook@chromium.org, jannh@google.com, sroettger@google.com, 
	willy@infradead.org, gregkh@linuxfoundation.org, usama.anjum@collabora.com, 
	rdunlap@infradead.org, jorgelo@chromium.org, groeck@chromium.org, 
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, 
	linux-mm@kvack.org, pedro.falcato@gmail.com, dave.hansen@intel.com, 
	linux-hardening@vger.kernel.org
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On Thu, Feb 1, 2024 at 9:00=E2=80=AFPM Theo de Raadt <deraadt@openbsd.org> =
wrote:
>
> Jeff Xu <jeffxu@chromium.org> wrote:
>
> > Even without free.
> > I personally do not like the heap getting sealed like that.
> >
> > Component A.
> > p=3Dmalloc(4096);
> > writing something to p.
> >
> > Component B:
> > mprotect(p,4096, RO)
> > mseal(p,4096)
> >
> > This will split the heap VMA, and prevent the heap from shrinking, if
> > this is in a frequent code path, then it might hurt the process's
> > memory usage.
> >
> > The existing code is more likely to use malloc than mmap(), so it is
> > easier for dev to seal a piece of data belonging to another component.
> > I hope this pattern is not wide-spreading.
> >
> > The ideal way will be just changing the library A to use mmap.
>
> I think you are lacking some test programs to see how it actually
> behaves; the effect is worse than you think, and the impact is immediatel=
y
> visible to the programmer, and the lesson is clear:
>
>         you can only seal objects which you gaurantee never get recycled.
>
>         Pushing a sealed object back into reuse is a disasterous bug.
>
>         Noone should call this interface, unless they understand that.
>
> I'll say again, you don't have a test program for various allocators to
> understand how it behaves.  The failure modes described in your docuemnts
> are not correct.
>
I understand what you mean: I will add that part to the document:
Try to recycle a sealed memory is disastrous, e.g.
p=3Dmalloc(4096);
mprotect(p,4096,RO)
mseal(p,4096)
free(p);

My point is:
I think sealing an object from the heap is a bad pattern in general,
even dev doesn't free it. That was one of the reasons for the sealable
flag, I hope saying this doesn't be perceived as looking for excuses.

>