2014-11-17 23:03:03

by Yann Droneaud

Subject: [PATCH] arm64/mm: Remove hack in mmap randomize layout

Since commit 8a0a9bd4db63 ('random: make get_random_int() more
random'), get_random_int() returns a random value for each call,
so comment and hack introduced in mmap_rnd() as part of commit
1d18c47c735e ('arm64: MMU fault handling and page table management')
are incorrects.

Commit 1d18c47c735e seems to use the same hack introduced by
commit a5adc91a4b44 ('powerpc: Ensure random space between stack
and mmaps'), latter copied in commit 5a0efea09f42 ('sparc64: Sharpen
address space randomization calculations.').

But both architectures were cleaned up as part of commit
fa8cbaaf5a68 ('powerpc+sparc64/mm: Remove hack in mmap randomize
layout') as hack is no more needed since commit 8a0a9bd4db63.

So the present patch removes the comment and the hack around
get_random_int() on AArch64's mmap_rnd().

Cc: Dan McGee <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Anton Blanchard <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Fixes: 1d18c47c735e ('arm64: MMU fault handling and page table management')
Signed-off-by: Yann Droneaud <[email protected]>
---
arch/arm64/mm/mmap.c | 12 ++----------
1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 1d73662f00ff..54922d1275b8 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -47,22 +47,14 @@ static int mmap_is_legacy(void)
return sysctl_legacy_va_layout;
}

-/*
- * Since get_random_int() returns the same value within a 1 jiffy window, we
- * will almost always get the same randomisation for the stack and mmap
- * region. This will mean the relative distance between stack and mmap will be
- * the same.
- *
- * To avoid this we can shift the randomness by 1 bit.
- */
static unsigned long mmap_rnd(void)
{
unsigned long rnd = 0;

if (current->flags & PF_RANDOMIZE)
- rnd = (long)get_random_int() & (STACK_RND_MASK >> 1);
+ rnd = (long)get_random_int() & STACK_RND_MASK;

- return rnd << (PAGE_SHIFT + 1);
+ return rnd << PAGE_SHIFT;
}

static unsigned long mmap_base(void)
--
1.9.3

2014-11-18 11:41:28

by Will Deacon

Subject: Re: [PATCH] arm64/mm: Remove hack in mmap randomize layout

On Mon, Nov 17, 2014 at 11:02:19PM +0000, Yann Droneaud wrote:
> Since commit 8a0a9bd4db63 ('random: make get_random_int() more
> random'), get_random_int() returns a random value for each call,
> so comment and hack introduced in mmap_rnd() as part of commit
> 1d18c47c735e ('arm64: MMU fault handling and page table management')
> are incorrects.
>
> Commit 1d18c47c735e seems to use the same hack introduced by
> commit a5adc91a4b44 ('powerpc: Ensure random space between stack
> and mmaps'), latter copied in commit 5a0efea09f42 ('sparc64: Sharpen
> address space randomization calculations.').
>
> But both architectures were cleaned up as part of commit
> fa8cbaaf5a68 ('powerpc+sparc64/mm: Remove hack in mmap randomize
> layout') as hack is no more needed since commit 8a0a9bd4db63.
>
> So the present patch removes the comment and the hack around
> get_random_int() on AArch64's mmap_rnd().
>
> Cc: Dan McGee <[email protected]>
> Cc: David S. Miller <[email protected]>
> Cc: Anton Blanchard <[email protected]>
> Cc: Benjamin Herrenschmidt <[email protected]>
> Fixes: 1d18c47c735e ('arm64: MMU fault handling and page table management')
> Signed-off-by: Yann Droneaud <[email protected]>
> ---
> arch/arm64/mm/mmap.c | 12 ++----------
> 1 file changed, 2 insertions(+), 10 deletions(-)

Thanks, this looks sensible:

Acked-by: Will Deacon <[email protected]>

Will