Re: [PATCH] mm: Revert x86_64 and arm64 ELF_ET_DYN_BASE base

[Kees Cook <keescook@chromium.org>]

On Tue, Feb 20, 2024 at 08:47:09AM +0100, Dmitry Vyukov wrote:
> On Sat, 17 Feb 2024 at 07:50, Kees Cook <keescook@chromium.org> wrote:
> >
> > *extreme thread[1] necromancy*
> >
> > On Mon, Aug 07, 2017 at 01:15:42PM -0700, Kees Cook wrote:
> > > Moving the x86_64 and arm64 PIE base from 0x555555554000 to 0x000100000000
> > > broke AddressSanitizer. This is a partial revert of:
> > >
> > >   commit eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
> > >   commit 02445990a96e ("arm64: move ELF_ET_DYN_BASE to 4GB / 4MB")
> > >
> > > The AddressSanitizer tool has hard-coded expectations about where
> > > executable mappings are loaded. The motivation for changing the PIE
> > > base in the above commits was to avoid the Stack-Clash CVEs that
> > > allowed executable mappings to get too close to heap and stack. This
> > > was mainly a problem on 32-bit, but the 64-bit bases were moved too,
> > > in an effort to proactively protect those systems (proofs of concept
> > > do exist that show 64-bit collisions, but other recent changes to fix
> > > stack accounting and setuid behaviors will minimize the impact).
> >
> > I happened to be looking at this again today, and wondered where things
> > stood. It seems like ASan's mappings are documented here:
> > https://github.com/google/sanitizers/wiki/AddressSanitizerAlgorithm#64-bit
> >
> > This implies that it would be safe to move the ELF_ET_DYN_BASE from
> > 0x555555554000 down to 0x200000000000, since the shadow map ends at
> > 0x10007fff7fff. (Well, anything above there would work, I was just
> > picking a "round" number above it. We could just as well use
> > 0x100080000000, I think.)
> >
> > Is this correct? I'd like to open up some more room between mmap and
> > stack...

Thanks for the details!

> Note that there is also TSAN and MSAN with their own mappings.
> These are also different per-arch, e.g. TSAN/Linux/x86_64:
> https://github.com/llvm/llvm-project/blob/d2a26a7bd5fc7cc5752337b7f4f999642feb37dc/compiler-rt/lib/tsan/rtl/tsan_platform.h#L48-L58
> Search "linux/" in that file for other arches, e.g.:
> https://github.com/llvm/llvm-project/blob/d2a26a7bd5fc7cc5752337b7f4f999642feb37dc/compiler-rt/lib/tsan/rtl/tsan_platform.h#L156-L165
> https://github.com/llvm/llvm-project/blob/d2a26a7bd5fc7cc5752337b7f4f999642feb37dc/compiler-rt/lib/tsan/rtl/tsan_platform.h#L187-L196
> https://github.com/llvm/llvm-project/blob/d2a26a7bd5fc7cc5752337b7f4f999642feb37dc/compiler-rt/lib/tsan/rtl/tsan_platform.h#L218-L227
> https://github.com/llvm/llvm-project/blob/d2a26a7bd5fc7cc5752337b7f4f999642feb37dc/compiler-rt/lib/tsan/rtl/tsan_platform.h#L252-L263

Depending on VA size, these are effectively all below 0x3800 0000 0000.

> 
> And MSAN mappings:
> https://github.com/llvm/llvm-project/blob/d2a26a7bd5fc7cc5752337b7f4f999642feb37dc/compiler-rt/lib/msan/msan.h#L44-L61

These are all below 0x1000 0000 0000.

So there probably isn't much benefit in reducing the PIE program
position below the current 0x55....

Okay, thanks!

-- 
Kees Cook