linux-kernel - Re: [PATCH AUTOSEL 6.9 12/15] binfmt_elf: Leave a gap between .bss and brk

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Date: Mon, 27 May 2024 09:32:13 -0700
From: Kees Cook <kees@...nel.org>
To: Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
CC: Kees Cook <keescook@...omium.org>, y0un9n132@...il.com,
 viro@...iv.linux.org.uk, brauner@...nel.org, linux-fsdevel@...r.kernel.org,
 linux-mm@...ck.org
Subject: Re: [PATCH AUTOSEL 6.9 12/15] binfmt_elf: Leave a gap between .bss and brk

Hi,

Please don't backport this change. While it has been tested, it's a process memory layout change, and I'd like to be as conservative as possible about it. If there is fall-out, I'd prefer to keep it limited to 6.10+. :)

-Kees



On May 26, 2024 2:41:44 AM PDT, Sasha Levin <sashal@...nel.org> wrote:
>From: Kees Cook <keescook@...omium.org>
>
>[ Upstream commit 2a5eb9995528441447d33838727f6ec1caf08139 ]
>
>Currently the brk starts its randomization immediately after .bss,
>which means there is a chance that when the random offset is 0, linear
>overflows from .bss can reach into the brk area. Leave at least a single
>page gap between .bss and brk (when it has not already been explicitly
>relocated into the mmap range).
>
>Reported-by:  <y0un9n132@...il.com>
>Closes: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=dzOUG5E4gV7ayCbu0MPJTZzWkw@mail.gmail.com/
>Link: https://lore.kernel.org/r/20240217062545.1631668-2-keescook@chromium.org
>Signed-off-by: Kees Cook <keescook@...omium.org>
>Signed-off-by: Sasha Levin <sashal@...nel.org>
>---
> fs/binfmt_elf.c | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
>index 5397b552fbeb5..7862962f7a859 100644
>--- a/fs/binfmt_elf.c
>+++ b/fs/binfmt_elf.c
>@@ -1262,6 +1262,9 @@ static int load_elf_binary(struct linux_binprm *bprm)
> 		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
> 		    elf_ex->e_type == ET_DYN && !interpreter) {
> 			mm->brk = mm->start_brk = ELF_ET_DYN_BASE;
>+		} else {
>+			/* Otherwise leave a gap between .bss and brk. */
>+			mm->brk = mm->start_brk = mm->brk + PAGE_SIZE;
> 		}
> 
> 		mm->brk = mm->start_brk = arch_randomize_brk(mm);

-- 
Kees Cook