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Message-ID: <b8c0c2e4-4d58-1d6e-5458-f0af3eb86d7c@ghiti.fr>
Date: Fri, 31 May 2019 00:49:29 -0400
From: Alex Ghiti <alex@...ti.fr>
To: Kees Cook <keescook@...omium.org>
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 5/29/19 3:26 PM, Kees Cook wrote:
> 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.
Yes, thanks for pointing that.
Thanks,
Alex
>
> -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
>>
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