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Message-ID: <CAGXu5jKx_R3c7qzCbm6HdbZ69V9dz=kEHVs1y-rCi-sjCX4UQg@mail.gmail.com>
Date: Thu, 20 Apr 2017 13:08:29 -0700
From: Kees Cook <keescook@...omium.org>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: "linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>, Bhupesh Sharma <bhsharma@...hat.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2] powerpc/mm: Add support for runtime configuration of
ASLR limits
On Thu, Apr 20, 2017 at 7:36 AM, Michael Ellerman <mpe@...erman.id.au> 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.
>
> Finally we can wire up the value in arch_mmap_rnd().
>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
> Signed-off-by: Bhupesh Sharma <bhsharma@...hat.com>
> ---
> 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.
Reviewed-by: Kees Cook <keescook@...omium.org>
Looks good!
-Kees
>
> 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);
>
> return rnd << PAGE_SHIFT;
> }
> --
> 2.7.4
>
--
Kees Cook
Pixel Security
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