[0/2] Adjust brk randomness

Message

Kees Cook Feb. 17, 2024, 6:25 a.m. UTC

Hi,

It was recently pointed out[1] that x86_64 brk entropy was not great,
and that on all architectures the brk can (when the random offset is 0)
be immediately adjacent to .bss, leaving no gap that could stop linear
overflows from the .bss. Address both issues.

-Kees

Link: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=[e-mail] [1]

Kees Cook (2):
  x86: Increase brk randomness entropy on x86_64
  binfmt_elf: Leave a gap between .bss and brk

 arch/x86/kernel/process.c | 5 ++++-
 fs/binfmt_elf.c           | 3 +++
 2 files changed, 7 insertions(+), 1 deletion(-)

Comments

H. Peter Anvin Feb. 18, 2024, 12:25 a.m. UTC | #1

On February 16, 2024 10:25:42 PM PST, Kees Cook <[e-mail]> wrote:
>Hi,
>
>It was recently pointed out[1] that x86_64 brk entropy was not great,
>and that on all architectures the brk can (when the random offset is 0)
>be immediately adjacent to .bss, leaving no gap that could stop linear
>overflows from the .bss. Address both issues.
>
>-Kees
>
>Link: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=[e-mail] [1]
>
>Kees Cook (2):
>  x86: Increase brk randomness entropy on x86_64
>  binfmt_elf: Leave a gap between .bss and brk
>
> arch/x86/kernel/process.c | 5 ++++-
> fs/binfmt_elf.c           | 3 +++
> 2 files changed, 7 insertions(+), 1 deletion(-)
>

Why do we even have the brk, or perhaps more importantly, why do we use it? Is there any reason whatsoever why glibc uses brk instead of mmap to her heap memory?

I thought the base of the brk wasn't even known to userspace other than in the form of the image end...

Kees Cook Feb. 18, 2024, 1:25 a.m. UTC | #2

On Sat, Feb 17, 2024 at 04:25:33PM -0800, H. Peter Anvin wrote:
> On February 16, 2024 10:25:42 PM PST, Kees Cook <[e-mail]> wrote:
> >Hi,
> >
> >It was recently pointed out[1] that x86_64 brk entropy was not great,
> >and that on all architectures the brk can (when the random offset is 0)
> >be immediately adjacent to .bss, leaving no gap that could stop linear
> >overflows from the .bss. Address both issues.
> >
> >-Kees
> >
> >Link: https://lore.kernel.org/linux-hardening/CA+2EKTVLvc8hDZc+2Yhwmus=[e-mail] [1]
> >
> >Kees Cook (2):
> >  x86: Increase brk randomness entropy on x86_64
> >  binfmt_elf: Leave a gap between .bss and brk
> >
> > arch/x86/kernel/process.c | 5 ++++-
> > fs/binfmt_elf.c           | 3 +++
> > 2 files changed, 7 insertions(+), 1 deletion(-)
> >
> 
> Why do we even have the brk, or perhaps more importantly, why do we use it? Is there any reason whatsoever why glibc uses brk instead of mmap to her heap memory?
> 
> I thought the base of the brk wasn't even known to userspace other than in the form of the image end...

AFAIK, it's part of ELF ABI, and the loader uses it only for very early
allocations. e.g. it's what shows up as "[heap]" in /proc/$pid/maps.
It's also available to any program that wants it still (see "man brk").
I don't think glibc has plans to redirect it.