binfmt_elf: Leave a gap between .bss and brk

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>

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 5397b552fbeb5422f46a0bbe7ef838977a718377..7862962f7a8597563c0f5c3aba7b7ddd778e03d5 100644 (file)
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1262,6 +1262,9 @@ out_free_interp:
                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);