Subject: Re: [PATCH v2] powerpc/mm: Add support for runtime configuration of ASLR limits

From: Balbir Singh <[email-masked]>
Date: Mon, 24 Apr 2017 11:02:30 +1000

On Fri, 2017-04-21 at 00:36 +1000, Michael Ellerman wrote:
> Add powerpc support for mmap_rnd_bits and mmap_rnd_compat_bits, which are two
> sysctls that allow a user to configure the number of bits of randomness used for
> ASLR.
> 
> Because of the way the Kconfig for ARCH_MMAP_RND_BITS is defined, we have to
> construct at least the MIN value in Kconfig, vs in a header which would be more
> natural. Given that we just go ahead and do it all in Kconfig.
> 
> At least according to the code (the documentation makes no mention of it), the
> value is defined as the number of bits of randomisation *of the page*, not the
> address. This makes some sense, with larger page sizes more of the low bits are
> forced to zero, which would reduce the randomisation if we didn't take the
> PAGE_SIZE into account. However it does mean the min/max values have to change
> depending on the PAGE_SIZE in order to actually limit the amount of address
> space consumed by the randomisation.
> 
> The result of that is that we have to define the default values based on both
> 32-bit vs 64-bit, but also the configured PAGE_SIZE. Furthermore now that we
> have 128TB address space support on Book3S, we also have to take that into
> account.

Could we please suggest that this should not be sent to stable or back-ported
as the 128T default is new?

> 
> Finally we can wire up the value in arch_mmap_rnd().
> 
> Signed-off-by: Michael Ellerman <[email-masked]>
> Signed-off-by: Bhupesh Sharma <[email-masked]>
> ---
>  arch/powerpc/Kconfig   | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  arch/powerpc/mm/mmap.c | 11 ++++++-----
>  2 files changed, 50 insertions(+), 5 deletions(-)
> 
> v2: Fix the 32-bit MAX value incorrectly using MIN as spotted by Kees.
> 
> Kees/Bhupesh, would love a Review/Ack/Tested-by from you, I'll plan to merge
> this later today (Friday) my time.
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 97a8bc8a095c..6f0503951e94 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -22,6 +22,48 @@ config MMU
>  	bool
>  	default y
>  
> +config ARCH_MMAP_RND_BITS_MAX
> +	# On Book3S 64, the default virtual address space for 64-bit processes
> +	# is 2^47 (128TB). As a maximum, allow randomisation to consume up to
> +	# 32T of address space (2^45), which should ensure a reasonable gap
> +	# between bottom-up and top-down allocations for applications that
> +	# consume "normal" amounts of address space. Book3S 64 only supports 64K
> +	# and 4K page sizes.
> +	default 29 if PPC_BOOK3S_64 && PPC_64K_PAGES # 29 = 45 (32T) - 16 (64K)
> +	default 33 if PPC_BOOK3S_64		     # 33 = 45 (32T) - 12 (4K)
> +	#
> +	# On all other 64-bit platforms (currently only Book3E), the virtual
> +	# address space is 2^46 (64TB). Allow randomisation to consume up to 16T
> +	# of address space (2^44). Only 4K page sizes are supported.
> +	default 32 if 64BIT	# 32 = 44 (16T) - 12 (4K)
> +	#
> +	# For 32-bit, use the compat values, as they're the same.
> +	default ARCH_MMAP_RND_COMPAT_BITS_MAX
> +
> +config ARCH_MMAP_RND_BITS_MIN
> +	# Allow randomisation to consume up to 1GB of address space (2^30).
> +	default 14 if 64BIT && PPC_64K_PAGES	# 14 = 30 (1GB) - 16 (64K)
> +	default 18 if 64BIT			# 18 = 30 (1GB) - 12 (4K)
> +	#
> +	# For 32-bit, use the compat values, as they're the same.
> +	default ARCH_MMAP_RND_COMPAT_BITS_MIN
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> +	# Total virtual address space for 32-bit processes is 2^31 (2GB).
> +	# Allow randomisation to consume up to 512MB of address space (2^29).
> +	default 11 if PPC_256K_PAGES	# 11 = 29 (512MB) - 18 (256K)
> +	default 13 if PPC_64K_PAGES	# 13 = 29 (512MB) - 16 (64K)
> +	default 15 if PPC_16K_PAGES 	# 15 = 29 (512MB) - 14 (16K)
> +	default 17			# 17 = 29 (512MB) - 12 (4K)
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> +	# Total virtual address space for 32-bit processes is 2^31 (2GB).
> +	# Allow randomisation to consume up to 8MB of address space (2^23).
> +	default 5 if PPC_256K_PAGES	#  5 = 23 (8MB) - 18 (256K)
> +	default 7 if PPC_64K_PAGES	#  7 = 23 (8MB) - 16 (64K)
> +	default 9 if PPC_16K_PAGES	#  9 = 23 (8MB) - 14 (16K)
> +	default 11			# 11 = 23 (8MB) - 12 (4K)
> +
>  config HAVE_SETUP_PER_CPU_AREA
>  	def_bool PPC64
>  
> @@ -120,6 +162,8 @@ config PPC
>  	select HAVE_ARCH_HARDENED_USERCOPY
>  	select HAVE_ARCH_JUMP_LABEL
>  	select HAVE_ARCH_KGDB
> +	select HAVE_ARCH_MMAP_RND_BITS
> +	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_CBPF_JIT			if !PPC64
> diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
> index a5d9ef59debe..6141cfff634e 100644
> --- a/arch/powerpc/mm/mmap.c
> +++ b/arch/powerpc/mm/mmap.c
> @@ -59,13 +59,14 @@ static inline int mmap_is_legacy(void)
>  
>  unsigned long arch_mmap_rnd(void)
>  {
> -	unsigned long rnd;
> +	unsigned long shift, rnd;
>  
> -	/* 8MB for 32bit, 1GB for 64bit */
> +	shift = mmap_rnd_bits;
> +#ifdef CONFIG_COMPAT
>  	if (is_32bit_task())
> -		rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
> -	else
> -		rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
> +		shift = mmap_rnd_compat_bits;
> +#endif
> +	rnd = get_random_long() % (1 << shift);
>

Nitpick, 1 should be 1UL?
  
>  	return rnd << PAGE_SHIFT;
>  }

Acked-by: Balbir Singh <[email-masked]>