linux-kernel - Re: [PATCH AUTOSEL 4.19 06/14] ELF: fix kernel.randomize_va

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Message-ID: <Zs3EIrnulQ38qJ6o@duo.ucw.cz>
Date: Tue, 27 Aug 2024 14:18:42 +0200
From: Pavel Machek <pavel@...x.de>
To: Sasha Levin <sashal@...nel.org>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Alexey Dobriyan <adobriyan@...il.com>, Kees Cook <kees@...nel.org>,
	viro@...iv.linux.org.uk, brauner@...nel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH AUTOSEL 4.19 06/14] ELF: fix kernel.randomize_va_space
 double read

Hi!

> [ Upstream commit 2a97388a807b6ab5538aa8f8537b2463c6988bd2 ]
> 
> ELF loader uses "randomize_va_space" twice. It is sysctl and can change
> at any moment, so 2 loads could see 2 different values in theory with
> unpredictable consequences.
> 
> Issue exactly one load for consistent value across one exec.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> Link: https://lore.kernel.org/r/3329905c-7eb8-400a-8f0a-d87cff979b5b@p183
> Signed-off-by: Kees Cook <kees@...nel.org>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>  fs/binfmt_elf.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index c41c568ad1b8a..af8830878fa0b 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -876,7 +876,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	if (elf_read_implies_exec(loc->elf_ex, executable_stack))
>  		current->personality |= READ_IMPLIES_EXEC;
>  
> -	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
> +	const int snapshot_randomize_va_space = READ_ONCE(randomize_va_space);
> +	if (!(current->personality & ADDR_NO_RANDOMIZE) && snapshot_randomize_va_space)
>  		current->flags |= PF_RANDOMIZE;
>  
>  	setup_new_exec(bprm);

We normally put variable declaration at start of the function. I'd not
be surprised if this broke with older compilers.

Best regards,
								Pavel
-- 
DENX Software Engineering GmbH,        Managing Director: Erika Unter
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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