linux-kernel - Re: [PATCH] binfmt_elf: Move brk for static PIE even if ASLR disabled

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Message-ID: <202505011633.82A962A7@keescook>
Date: Thu, 1 May 2025 16:49:11 -0700
From: Kees Cook <kees@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ali Saidi <alisaidi@...zon.com>, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] binfmt_elf: Move brk for static PIE even if ASLR disabled

On Thu, May 01, 2025 at 12:03:32PM +0100, Ryan Roberts wrote:
> I agree, as long as COMPAT_BRK is not set (which is the common case IFAICT).
> When COMPAT_BRK is enabled, I think you are breaking the purpose of that
> Kconfig? Perhaps it's not a real-world problem though...

When you turned off ASLR, what mechanism did you use? Personality or
randomize_va_space=0?

> > It's possible it could break running the loader directly against some
> > libc5-based binaries. If this turns out to be a real-world issue, we can
> > find a better solution (perhaps pre-allocating a large brk).
> 
> But how large is large enough...

Right -- Chrome has a 500MB brk on my laptop. :P Or with randomization
off, it could allocate to the top of the mmap space just to keep
"future" mmap allocations from landing in any holes...

> Perhaps it is safer to only move the brk if !IS_ENABLED(CONFIG_COMPAT_BRK) ?
> Then wait to see if there are any real-world COMPAT_BRK users that hit the issue?

Yeah, that might be the best middle-ground.

-- 
Kees Cook