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Date: Wed, 29 May 2019 12:26:11 -0700
From: Kees Cook <keescook@...omium.org>
To: Alexandre Ghiti <alex@...ti.fr>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Christoph Hellwig <hch@....de>,
Russell King <linux@...linux.org.uk>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Ralf Baechle <ralf@...ux-mips.org>,
Paul Burton <paul.burton@...s.com>,
James Hogan <jhogan@...nel.org>,
Palmer Dabbelt <palmer@...ive.com>,
Albert Ou <aou@...s.berkeley.edu>,
Alexander Viro <viro@...iv.linux.org.uk>,
Luis Chamberlain <mcgrof@...nel.org>,
linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
linux-mips@...r.kernel.org, linux-riscv@...ts.infradead.org,
linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4 08/14] arm: Use generic mmap top-down layout and brk
randomization
On Sun, May 26, 2019 at 09:47:40AM -0400, Alexandre Ghiti wrote:
> arm uses a top-down mmap layout by default that exactly fits the generic
> functions, so get rid of arch specific code and use the generic version
> by selecting ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT.
> As ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT selects ARCH_HAS_ELF_RANDOMIZE,
> use the generic version of arch_randomize_brk since it also fits.
> Note that this commit also removes the possibility for arm to have elf
> randomization and no MMU: without MMU, the security added by randomization
> is worth nothing.
>
> Signed-off-by: Alexandre Ghiti <alex@...ti.fr>
Acked-by: Kees Cook <keescook@...omium.org>
It may be worth noting that STACK_RND_MASK is safe to remove here
because it matches the default that now exists in mm/util.c.
-Kees
> ---
> arch/arm/Kconfig | 2 +-
> arch/arm/include/asm/processor.h | 2 --
> arch/arm/kernel/process.c | 5 ---
> arch/arm/mm/mmap.c | 62 --------------------------------
> 4 files changed, 1 insertion(+), 70 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 8869742a85df..27687a8c9fb5 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -6,7 +6,6 @@ config ARM
> select ARCH_CLOCKSOURCE_DATA
> select ARCH_HAS_DEBUG_VIRTUAL if MMU
> select ARCH_HAS_DEVMEM_IS_ALLOWED
> - select ARCH_HAS_ELF_RANDOMIZE
> select ARCH_HAS_FORTIFY_SOURCE
> select ARCH_HAS_KEEPINITRD
> select ARCH_HAS_KCOV
> @@ -29,6 +28,7 @@ config ARM
> select ARCH_SUPPORTS_ATOMIC_RMW
> select ARCH_USE_BUILTIN_BSWAP
> select ARCH_USE_CMPXCHG_LOCKREF
> + select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
> select ARCH_WANT_IPC_PARSE_VERSION
> select BUILDTIME_EXTABLE_SORT if MMU
> select CLONE_BACKWARDS
> diff --git a/arch/arm/include/asm/processor.h b/arch/arm/include/asm/processor.h
> index 5d06f75ffad4..95b7688341c5 100644
> --- a/arch/arm/include/asm/processor.h
> +++ b/arch/arm/include/asm/processor.h
> @@ -143,8 +143,6 @@ static inline void prefetchw(const void *ptr)
> #endif
> #endif
>
> -#define HAVE_ARCH_PICK_MMAP_LAYOUT
> -
> #endif
>
> #endif /* __ASM_ARM_PROCESSOR_H */
> diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
> index 72cc0862a30e..19a765db5f7f 100644
> --- a/arch/arm/kernel/process.c
> +++ b/arch/arm/kernel/process.c
> @@ -322,11 +322,6 @@ unsigned long get_wchan(struct task_struct *p)
> return 0;
> }
>
> -unsigned long arch_randomize_brk(struct mm_struct *mm)
> -{
> - return randomize_page(mm->brk, 0x02000000);
> -}
> -
> #ifdef CONFIG_MMU
> #ifdef CONFIG_KUSER_HELPERS
> /*
> diff --git a/arch/arm/mm/mmap.c b/arch/arm/mm/mmap.c
> index 0b94b674aa91..b8d912ac9e61 100644
> --- a/arch/arm/mm/mmap.c
> +++ b/arch/arm/mm/mmap.c
> @@ -17,43 +17,6 @@
> ((((addr)+SHMLBA-1)&~(SHMLBA-1)) + \
> (((pgoff)<<PAGE_SHIFT) & (SHMLBA-1)))
>
> -/* gap between mmap and stack */
> -#define MIN_GAP (128*1024*1024UL)
> -#define MAX_GAP ((STACK_TOP)/6*5)
> -#define STACK_RND_MASK (0x7ff >> (PAGE_SHIFT - 12))
> -
> -static int mmap_is_legacy(struct rlimit *rlim_stack)
> -{
> - if (current->personality & ADDR_COMPAT_LAYOUT)
> - return 1;
> -
> - if (rlim_stack->rlim_cur == RLIM_INFINITY)
> - return 1;
> -
> - return sysctl_legacy_va_layout;
> -}
> -
> -static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
> -{
> - unsigned long gap = rlim_stack->rlim_cur;
> - unsigned long pad = stack_guard_gap;
> -
> - /* Account for stack randomization if necessary */
> - if (current->flags & PF_RANDOMIZE)
> - pad += (STACK_RND_MASK << PAGE_SHIFT);
> -
> - /* Values close to RLIM_INFINITY can overflow. */
> - if (gap + pad > gap)
> - gap += pad;
> -
> - if (gap < MIN_GAP)
> - gap = MIN_GAP;
> - else if (gap > MAX_GAP)
> - gap = MAX_GAP;
> -
> - return PAGE_ALIGN(STACK_TOP - gap - rnd);
> -}
> -
> /*
> * We need to ensure that shared mappings are correctly aligned to
> * avoid aliasing issues with VIPT caches. We need to ensure that
> @@ -181,31 +144,6 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
> return addr;
> }
>
> -unsigned long arch_mmap_rnd(void)
> -{
> - unsigned long rnd;
> -
> - rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
> -
> - return rnd << PAGE_SHIFT;
> -}
> -
> -void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
> -{
> - unsigned long random_factor = 0UL;
> -
> - if (current->flags & PF_RANDOMIZE)
> - random_factor = arch_mmap_rnd();
> -
> - if (mmap_is_legacy(rlim_stack)) {
> - mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
> - mm->get_unmapped_area = arch_get_unmapped_area;
> - } else {
> - mm->mmap_base = mmap_base(random_factor, rlim_stack);
> - mm->get_unmapped_area = arch_get_unmapped_area_topdown;
> - }
> -}
> -
> /*
> * You really shouldn't be using read() or write() on /dev/mem. This
> * might go away in the future.
> --
> 2.20.1
>
--
Kees Cook
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