Re: [PATCH v4 08/14] arm: Use generic mmap top-down layout and brk randomization

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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@xxxxxxxx>
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

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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