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From: Jeff Xu <jeffxu@chromium.org>
Date: Fri, 2 Feb 2024 13:02:38 -0800
Message-ID: <CABi2SkUkFLpLaK3nivWz-wNUC=eqiR-uq9PG0YSdmaT94MUAuQ@mail.gmail.com>
Subject: Re: [PATCH v8 0/4] Introduce mseal
To: Theo de Raadt <deraadt@openbsd.org>
Cc: 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, jeffxu@google.com, 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 Fri, Feb 2, 2024 at 9:05=E2=80=AFAM Theo de Raadt <deraadt@openbsd.org> =
wrote:
>
> Another interaction to consider is sigaltstack().
>
> In OpenBSD, sigaltstack() forces MAP_STACK onto the specified
> (pre-allocated) region, because on kernel-entry we require the "sp"
> register to point to a MAP_STACK region (this severely damages ROP pivot
> methods).  Linux does not have MAP_STACK enforcement (yet), but one day
> someone may try to do that work.
>
> This interacted poorly with mimmutable() because some applications
> allocate the memory being provided poorly.  I won't get into the details
> unless pushed, because what we found makes me upset.  Over the years,
> we've upstreamed diffs to applications to resolve all the nasty
> allocation patterns.  I think the software ecosystem is now mostly
> clean.
>
> I suggest someone in Linux look into whether sigaltstack() is a mseal()
> bypass, perhaps somewhat similar to madvise MADV_FREE, and consider the
> correct strategy.
>

Thanks for bringing this up. I will follow up on sigaltstack() in Linux.

> This is our documented strategy:
>
>      On OpenBSD some additional restrictions prevent dangerous address sp=
ace
>      modifications.  The proposed space at ss_sp is verified to be
>      contiguously mapped for read-write permissions (no execute) and inca=
pable
>      of syscall entry (see msyscall(2)).  If those conditions are met, a =
page-
>      aligned inner region will be freshly mapped (all zero) with MAP_STAC=
K
>      (see mmap(2)), destroying the pre-existing data in the region.  Once=
 the
>      sigaltstack is disabled, the MAP_STACK attribute remains on the memo=
ry,
>      so it is best to deallocate the memory via a method that results in
>      munmap(2).
>
> OK, I better provide the details of what people were doing.
> sigaltstacks() in .data, in .bss, using malloc(), on a buffer on the
> stack, we even found one creating a sigaltstack inside a buffer on a
> pthread stack.  We told everyone to use mmap() and munmap(), with MAP_STA=
CK
> if #ifdef MAP_STACK finds a definition.
>