linux-kernel - Re: [PATCH v2] mm: mmap: allow for the maximum number of bits for randomizing mmap

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Date: Mon, 10 Jun 2024 14:45:28 -0400
From: Rafael Aquini <aquini@...hat.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Heiko Carstens <hca@...ux.ibm.com>, Petr Mladek <pmladek@...e.com>,
	Mike Rapoport <rppt@...nel.org>, Paul McKenney <paulmck@...nel.org>,
	Samuel Holland <samuel.holland@...ive.com>
Subject: Re: [PATCH v2] mm: mmap: allow for the maximum number of bits for
 randomizing mmap_base by default

On Mon, Jun 10, 2024 at 11:11:39AM -0700, Andrew Morton wrote:
> On Thu,  6 Jun 2024 14:06:22 -0400 Rafael Aquini <aquini@...hat.com> wrote:
> 
> > An ASLR regression was noticed [1] and tracked down to file-mapped areas
> > being backed by THP in recent kernels. The 21-bit alignment constraint
> > for such mappings reduces the entropy for randomizing the placement of
> > 64-bit library mappings and breaks ASLR completely for 32-bit libraries.
> > 
> > The reported issue is easily addressed by increasing vm.mmap_rnd_bits
> > and vm.mmap_rnd_compat_bits. This patch just provides a simple way to
> > set ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS to their maximum
> > values allowed by the architecture at build time.
> > 
> > [1] https://zolutal.github.io/aslrnt/
> 
> Are we able to identify a Fixes: target for this?
>

Sure, it would be:

 Fixes: 1854bc6e2420 ("mm/readahead: Align file mappings for non-DAX")
 
> I assume a cc:stable is appropriate?
> 

Andrew, I admit I was somewhat hesitant on adding the Fixes: and the stable CC
to this patch because I didn't really think of it as a "fix" for the given 
commit, but just as a simple way to toggle ARCH_MMAP_RND{,_COMPAT}_BITS 
to maximum allowed at build time.

I don't disagree with doing it, though, if you think it might be appropriate.

Lemme know if you want me refreshing the patch to amend these bits.

-- Rafael