[PATCH AUTOSEL 5.15 6/7] binfmt_elf: Leave a gap between .bss and brk

[Sasha Levin <sashal@kernel.org>]

From: Kees Cook <keescook@chromium.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@gmail.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@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/binfmt_elf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 30379c33ad20c..932141c93f3e3 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1289,6 +1289,9 @@ 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);
-- 
2.43.0