linux-hardening - Re: [PATCH v8 0/4] Introduce mseal

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Date: Thu, 01 Feb 2024 21:05:10 -0700
From: "Theo de Raadt" <deraadt@...nbsd.org>
To: Jeff Xu <jeffxu@...gle.com>
cc: Linus Torvalds <torvalds@...ux-foundation.org>,
    Jeff Xu <jeffxu@...omium.org>,
    "Liam R. Howlett" <Liam.Howlett@...cle.com>,
    Jonathan Corbet <corbet@....net>, akpm@...ux-foundation.org,
    keescook@...omium.org, jannh@...gle.com, sroettger@...gle.com,
    willy@...radead.org, gregkh@...uxfoundation.org,
    usama.anjum@...labora.com, rdunlap@...radead.org,
    jorgelo@...omium.org, groeck@...omium.org,
    linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
    linux-mm@...ck.org, pedro.falcato@...il.com, dave.hansen@...el.com,
    linux-hardening@...r.kernel.org
Subject: Re: [PATCH v8 0/4] Introduce mseal

Jeff Xu <jeffxu@...gle.com> wrote:

> To me, the most important thing is to deliver a feature that's easy to
> use and works well. I don't want users to mess things up, so if I'm
> the one giving them the tools, I'm going to make sure they have all
> the information they need and that there are safeguards in place.
> 
> e.g. considering the following user case:
> 1> a security sensitive data is allocated from heap, using malloc,
> from the software component A, and filled with information.
> 2> software component B then uses mprotect to change it to RO, and
> seal it using mseal().

  p = malloc(80);
  mprotect(p & ~4095, 4096, PROT_NONE);
  free(p);

Will you save such a developer also?  No.

Since the same problem you describe already exists with mprotect() what
does mseal() even have to do with your proposal?

What about this?

  p = malloc(80);
  munmap(p & ~4095, 4096);
  free(p);

And since it is not sealed, how about madvise operations on a proper
non-malloc memory allocation?  Well, the process smashes it's own
memory.  And why is it not sealed?  You make it harder to seal memory!

How about this?

  p = malloc(80);
  bzero(p, 100000;

Yes it is a buffer overflow.  But this is all the same class of software
problem:

Memory belongs to processes, which belongs to the program, which is coded
by the programmer, who has to learn to be careful and handle the memory correctly.

mseal() / mimmutable() add *no new expectation* to a careful programmer,
because they expected to only use it on memory that they *promise will never
be de-allocated or re-permissioned*.

What you are proposing is not a "mitigation", it entirely cripples the
proposed subsystem because you are afraid of it; because you have cloned a
memory subsystem primitive you don't fully understand; and this is because
you've not seen a complete operating system using it.

When was the last time you developed outside of Chrome?

This is systems programming.  The kernel supports all the programs, not
just the one holy program from god.