Re: [PATCH] fs, elf: Make sure to page align bss in load_elf_library

[Kees Cook]

On Thu, Jul 5, 2018 at 7:55 AM,  <osalvador@xxxxxxxxxxxxxxxxxx> wrote:
> From: Oscar Salvador <osalvador@xxxxxxx>
>
> The current code does not make sure to page align bss before calling
> vm_brk(), and this can lead to a VM_BUG_ON() in __mm_populate()
> due to the requested lenght not being correctly aligned.
>
> Let us make sure to align it properly.
>
> Signed-off-by: Oscar Salvador <osalvador@xxxxxxx>
> Tested-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Reported-by: syzbot+5dcb560fe12aa5091c06@xxxxxxxxxxxxxxxxxxxxxxxxx

Wow. CONFIG_USELIB? I'm surprised distros are still using this. 32-bit
only, and libc5 and earlier only.

Regardless, this appears to match the current bss alignment logic in
the main elf loader, so:

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  fs/binfmt_elf.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 0ac456b52bdd..816cc921cf36 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1259,9 +1259,8 @@ static int load_elf_library(struct file *file)
>                 goto out_free_ph;
>         }
>
> -       len = ELF_PAGESTART(eppnt->p_filesz + eppnt->p_vaddr +
> -                           ELF_MIN_ALIGN - 1);
> -       bss = eppnt->p_memsz + eppnt->p_vaddr;
> +       len = ELF_PAGEALIGN(eppnt->p_filesz + eppnt->p_vaddr);
> +       bss = ELF_PAGEALIGN(eppnt->p_memsz + eppnt->p_vaddr);
>         if (bss > len) {
>                 error = vm_brk(len, bss - len);
>                 if (error)
> --
> 2.13.6
>



-- 
Kees Cook
Pixel Security