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

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Message-ID: <e9724525-21e2-4c38-a5da-0e45a84a4b58@arm.com>
Date: Fri, 2 May 2025 11:34:03 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Kees Cook <kees@...nel.org>
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 02/05/2025 00:49, Kees Cook wrote:
> 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?

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.
>