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From: Kees Cook <keescook@chromium.org>
To: Jiri Kosina <jikos@kernel.org>
Cc: Kees Cook <keescook@chromium.org>,
	y0un9n132@gmail.com,
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	Christophe Leroy <christophe.leroy@csgroup.eu>,
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	linux-mm@kvack.org,
	Thomas Gleixner <tglx@linutronix.de>,
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	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
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	Bjorn Helgaas <bhelgaas@google.com>,
	Tony Battersby <tonyb@cybernetics.com>,
	linux-kernel@vger.kernel.org,
	x86@kernel.org,
	linux-hardening@vger.kernel.org
Subject: [PATCH 2/2] binfmt_elf: Leave a gap between .bss and brk
Date: Fri, 16 Feb 2024 22:25:44 -0800
Message-Id: <20240217062545.1631668-2-keescook@chromium.org>
In-Reply-To: <20240217062035.work.493-kees@kernel.org>
References: <20240217062035.work.493-kees@kernel.org>
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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@gmail.com
Closes: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=dzOUG5E4gV7ayCbu0MPJTZzWkw@mail.gmail.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
---
 fs/binfmt_elf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 5397b552fbeb..7862962f7a85 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);
-- 
2.34.1