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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 17 Apr 2019 23:37:00 -0500
Message-ID: <CAGXu5jKo26zXw=jfKSzr_pnfx5Zux+fVbY7V9bJwEMApDcFi8w@mail.gmail.com>
Subject: Re: [PATCH v3 03/11] arm64: Consider stack randomization for mmap
 base only when necessary
To:     Alexandre Ghiti <alex@ghiti.fr>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@lst.de>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Paul Burton <paul.burton@mips.com>,
        James Hogan <jhogan@kernel.org>,
        Palmer Dabbelt <palmer@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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On Wed, Apr 17, 2019 at 12:26 AM Alexandre Ghiti <alex@ghiti.fr> wrote:
>
> Do not offset mmap base address because of stack randomization if
> current task does not want randomization.

Maybe mention that this makes this logic match the existing x86 behavior too?

> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>

Acked-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  arch/arm64/mm/mmap.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
> index ed4f9915f2b8..ac89686c4af8 100644
> --- a/arch/arm64/mm/mmap.c
> +++ b/arch/arm64/mm/mmap.c
> @@ -65,7 +65,11 @@ unsigned long arch_mmap_rnd(void)
>  static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
>  {
>         unsigned long gap = rlim_stack->rlim_cur;
> -       unsigned long pad = (STACK_RND_MASK << PAGE_SHIFT) + stack_guard_gap;
> +       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)
> --
> 2.20.1
>


-- 
Kees Cook