ACK: [Xenial SRU][PATCH 1/1] mm: revert x86_64 and arm64 ELF_ET_DYN_BASE base changes

[Colin Ian King]

On 07/09/17 14:51, Kleber Sacilotto de Souza wrote:
> From: Kees Cook <keescook at chromium.org>
> 
> BugLink: http://bugs.launchpad.net/bugs/1715636
> 
> Moving the x86_64 and arm64 PIE base from 0x555555554000 to 0x000100000000
> broke AddressSanitizer.  This is a partial revert of:
> 
>   eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
>   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).
> 
> 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.)
> 
> Link: http://lkml.kernel.org/r/20170807201542.GA21271@beast
> Fixes: eab09532d400 ("binfmt_elf: use ELF_ET_DYN_BASE only for PIE")
> Fixes: 02445990a96e ("arm64: move ELF_ET_DYN_BASE to 4GB / 4MB")
> Signed-off-by: Kees Cook <keescook at chromium.org>
> Reported-by: Kostya Serebryany <kcc at google.com>
> Acked-by: Will Deacon <will.deacon at arm.com>
> Cc: Ingo Molnar <mingo at elte.hu>
> Cc: "H. Peter Anvin" <hpa at zytor.com>
> Cc: Thomas Gleixner <tglx at linutronix.de>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (cherry picked from commit c715b72c1ba406f133217b509044c38d8e714a37)
> Signed-off-by: Kleber Sacilotto de Souza <kleber.souza at canonical.com>
> ---
>  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 9e11dbe1cec3..329c127e13dc 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -121,10 +121,10 @@ typedef struct user_fpsimd_state elf_fpregset_t;
>  
>  /*
>   * 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)
>  
>  /*
>   * When the program starts, a1 contains a pointer to a function to be
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 07cf288b692e..bcd3d6199464 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,
> 

Clean cherry pick. Seems to do the right thing.

Acked-by: Colin Ian King <colin.king at canonical.com>