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From: "Theo de Raadt" <deraadt@openbsd.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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Subject: Re: [PATCH v8 0/4] Introduce mseal
In-reply-to: <CAHk-=wjqozic6JuRimXD=RamnJmD6FoaQki7RtNYrezzx_OfOg@mail.gmail.com>
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Comments: In-reply-to Linus Torvalds <torvalds@linux-foundation.org>
   message dated "Thu, 01 Feb 2024 15:15:27 -0800."
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Content-ID: <95708.1706893543.1@cvs.openbsd.org>
Date: Fri, 02 Feb 2024 10:05:43 -0700
Message-ID: <66496.1706893543@cvs.openbsd.org>

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.

This is our documented strategy:

     On OpenBSD some additional restrictions prevent dangerous address space
     modifications.  The proposed space at ss_sp is verified to be
     contiguously mapped for read-write permissions (no execute) and incapable
     of syscall entry (see msyscall(2)).  If those conditions are met, a page-
     aligned inner region will be freshly mapped (all zero) with MAP_STACK
     (see mmap(2)), destroying the pre-existing data in the region.  Once the
     sigaltstack is disabled, the MAP_STACK attribute remains on the memory,
     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_STACK
if #ifdef MAP_STACK finds a definition.