linux-kernel - Re: [PATCHv5, REBASED 9/9] x86/mm: Allow to have userspace mappings above 47-bits

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Date:   Thu, 18 May 2017 13:43:59 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCHv5, REBASED 9/9] x86/mm: Allow to have userspace mappings
 above 47-bits

On Mon 15-05-17 15:12:18, Kirill A. Shutemov wrote:
[...]
> @@ -195,6 +207,16 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>  	info.length = len;
>  	info.low_limit = PAGE_SIZE;
>  	info.high_limit = get_mmap_base(0);
> +
> +	/*
> +	 * If hint address is above DEFAULT_MAP_WINDOW, look for unmapped area
> +	 * in the full address space.
> +	 *
> +	 * !in_compat_syscall() check to avoid high addresses for x32.
> +	 */
> +	if (addr > DEFAULT_MAP_WINDOW && !in_compat_syscall())
> +		info.high_limit += TASK_SIZE_MAX - DEFAULT_MAP_WINDOW;
> +
>  	info.align_mask = 0;
>  	info.align_offset = pgoff << PAGE_SHIFT;
>  	if (filp) {

I have two questions/concerns here. The above assumes that any address above
1<<47 will use the _whole_ address space. Is this what we want? What
if somebody does mmap(1<<52, ...) because he wants to (ab)use 53+ bits
for some other purpose? Shouldn't we cap the high_limit by the given
address?

Another thing would be that 
	/* requesting a specific address */
	if (addr) {
		addr = PAGE_ALIGN(addr);
		vma = find_vma(mm, addr);
		if (TASK_SIZE - len >= addr &&
				(!vma || addr + len <= vma->vm_start))
			return addr;
	}

would fail for mmap(-1UL, ...) which is good because we do want to
fallback to vm_unmapped_area and have randomized address which is
ensured by your info.high_limit += ... but that wouldn't work for
mmap(1<<N, ...) where N>47. So the first such mapping won't be
randomized while others will be. This is quite unexpected I would say.
So it should be documented at least or maybe we want to skip the above
shortcut for addr > DEFAULT_MAP_WINDOW altogether.

The patch looks sensible other than that.
-- 
Michal Hocko
SUSE Labs