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Subject: Re: [PATCH v6 09/14] mips: Properly account for stack randomization
 and stack guard gap
To:     Alexandre Ghiti <alex@ghiti.fr>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Luis Chamberlain <mcgrof@kernel.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>,
        Kees Cook <keescook@chromium.org>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org
References: <20190808061756.19712-1-alex@ghiti.fr>
 <20190808061756.19712-10-alex@ghiti.fr>
From:   Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Message-ID: <bd67507e-8a5b-34b5-1a33-5500bbb724b2@cogentembedded.com>
Date:   Thu, 8 Aug 2019 12:16:04 +0300
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Hello!

On 08.08.2019 9:17, Alexandre Ghiti wrote:

> This commit takes care of stack randomization and stack guard gap when
> computing mmap base address and checks if the task asked for randomization.
> 
> This fixes the problem uncovered and not fixed for arm here:
> https://lkml.kernel.org/r/20170622200033.25714-1-riel@redhat.com
> 
> Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
> Acked-by: Kees Cook <keescook@chromium.org>
> Acked-by: Paul Burton <paul.burton@mips.com>
> Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>   arch/mips/mm/mmap.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
> index d79f2b432318..f5c778113384 100644
> --- a/arch/mips/mm/mmap.c
> +++ b/arch/mips/mm/mmap.c
> @@ -21,8 +21,9 @@ unsigned long shm_align_mask = PAGE_SIZE - 1;	/* Sane caches */
>   EXPORT_SYMBOL(shm_align_mask);
>   
>   /* gap between mmap and stack */
> -#define MIN_GAP (128*1024*1024UL)
> -#define MAX_GAP ((TASK_SIZE)/6*5)
> +#define MIN_GAP		(128*1024*1024UL)
> +#define MAX_GAP		((TASK_SIZE)/6*5)

    Could add spaces around *, while touching this anyway? And parens
around TASK_SIZE shouldn't be needed...

> +#define STACK_RND_MASK	(0x7ff >> (PAGE_SHIFT - 12))
>   
>   static int mmap_is_legacy(struct rlimit *rlim_stack)
>   {
> @@ -38,6 +39,15 @@ static int mmap_is_legacy(struct rlimit *rlim_stack)
>   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);

    Parens not needed here.

> +
> +	/* Values close to RLIM_INFINITY can overflow. */
> +	if (gap + pad > gap)
> +		gap += pad;
>   
>   	if (gap < MIN_GAP)
>   		gap = MIN_GAP;
>