*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! -Kees [1] https://lore.kernel.org/lkml/20170807201542.GA21271@beast/ > > The new 32-bit PIE base is fine for ASan (since it matches the ET_EXEC > base), so only the 64-bit PIE base needs to be reverted to let x86 and > arm64 ASan binaries run again. Future changes to the 64-bit PIE base on > these architectures can be made optional once a more dynamic method for > dealing with AddressSanitizer is found. (e.g. always loading PIE into > the mmap region for marked binaries.) > > Reported-by: Kostya Serebryany <k...@google.com> > Cc: sta...@vger.kernel.org > Signed-off-by: Kees Cook <keesc...@chromium.org> > --- > arch/arm64/include/asm/elf.h | 4 ++-- > arch/x86/include/asm/elf.h | 4 ++-- > 2 files changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h > index acae781f7359..3288c2b36731 100644 > --- a/arch/arm64/include/asm/elf.h > +++ b/arch/arm64/include/asm/elf.h > @@ -114,10 +114,10 @@ > > /* > * This is the base location for PIE (ET_DYN with INTERP) loads. On > - * 64-bit, this is raised to 4GB to leave the entire 32-bit address > + * 64-bit, this is above 4GB to leave the entire 32-bit address > * space open for things that want to use the area for 32-bit pointers. > */ > -#define ELF_ET_DYN_BASE 0x100000000UL > +#define ELF_ET_DYN_BASE (2 * TASK_SIZE_64 / 3) > > #ifndef __ASSEMBLY__ > > diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h > index 1c18d83d3f09..9aeb91935ce0 100644 > --- a/arch/x86/include/asm/elf.h > +++ b/arch/x86/include/asm/elf.h > @@ -247,11 +247,11 @@ extern int force_personality32; > > /* > * This is the base location for PIE (ET_DYN with INTERP) loads. On > - * 64-bit, this is raised to 4GB to leave the entire 32-bit address > + * 64-bit, this is above 4GB to leave the entire 32-bit address > * space open for things that want to use the area for 32-bit pointers. > */ > #define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x000400000UL : \ > - 0x100000000UL) > + (TASK_SIZE / 3 * 2)) > > /* This yields a mask that user programs can use to figure out what > instruction set this CPU supports. This could be done in user space, > -- > 2.7.4 > > > -- > Kees Cook > Pixel Security -- Kees Cook