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Subject[PATCH 2/2] binfmt_elf: Leave a gap between .bss and brk
Date
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

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