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Date: Mon, 27 May 2024 09:32:13 -0700
From: Kees Cook <kees@kernel.org>
To: Sasha Levin <sashal@kernel.org>, linux-kernel@vger.kernel.org,
 stable@vger.kernel.org
CC: Kees Cook <keescook@chromium.org>, y0un9n132@gmail.com,
 viro@zeniv.linux.org.uk, brauner@kernel.org, linux-fsdevel@vger.kernel.org,
 linux-mm@kvack.org
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_AUTOSEL_6=2E9_12/15=5D_binfmt=5F?=
 =?US-ASCII?Q?elf=3A_Leave_a_gap_between_=2Ebss_and_brk?=
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Hi,

Please don't backport this change=2E While it has been tested, it's a proc=
ess memory layout change, and I'd like to be as conservative as possible ab=
out it=2E If there is fall-out, I'd prefer to keep it limited to 6=2E10+=2E=
 :)

-Kees


On May 26, 2024 2:41:44 AM PDT, Sasha Levin <sashal@kernel=2Eorg> wrote:
>From: Kees Cook <keescook@chromium=2Eorg>
>
>[ Upstream commit 2a5eb9995528441447d33838727f6ec1caf08139 ]
>
>Currently the brk starts its randomization immediately after =2Ebss,
>which means there is a chance that when the random offset is 0, linear
>overflows from =2Ebss can reach into the brk area=2E Leave at least a sin=
gle
>page gap between =2Ebss and brk (when it has not already been explicitly
>relocated into the mmap range)=2E
>
>Reported-by:  <y0un9n132@gmail=2Ecom>
>Closes: https://lore=2Ekernel=2Eorg/linux-hardening/CA+2EKTVLvc8hDZc+2Yhw=
mus=3DdzOUG5E4gV7ayCbu0MPJTZzWkw@mail=2Egmail=2Ecom/
>Link: https://lore=2Ekernel=2Eorg/r/20240217062545=2E1631668-2-keescook@c=
hromium=2Eorg
>Signed-off-by: Kees Cook <keescook@chromium=2Eorg>
>Signed-off-by: Sasha Levin <sashal@kernel=2Eorg>
>---
> fs/binfmt_elf=2Ec | 3 +++
> 1 file changed, 3 insertions(+)
>
>diff --git a/fs/binfmt_elf=2Ec b/fs/binfmt_elf=2Ec
>index 5397b552fbeb5=2E=2E7862962f7a859 100644
>--- a/fs/binfmt_elf=2Ec
>+++ b/fs/binfmt_elf=2Ec
>@@ -1262,6 +1262,9 @@ static int load_elf_binary(struct linux_binprm *bpr=
m)
> 		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
> 		    elf_ex->e_type =3D=3D ET_DYN && !interpreter) {
> 			mm->brk =3D mm->start_brk =3D ELF_ET_DYN_BASE;
>+		} else {
>+			/* Otherwise leave a gap between =2Ebss and brk=2E */
>+			mm->brk =3D mm->start_brk =3D mm->brk + PAGE_SIZE;
> 		}
>=20
> 		mm->brk =3D mm->start_brk =3D arch_randomize_brk(mm);

--=20
Kees Cook