linux-kernel - Re: [RFC PATCH] Take mmap_min_addr into account while choosing unmapped address for x86-64.

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Date:   Tue, 27 Feb 2018 09:57:27 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Ilya Smith <blackzert@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, kirill.shutemov@...ux.intel.com,
        dsafonov@...tuozzo.com, hughd@...gle.com,
        gregkh@...uxfoundation.org, craigb@...gle.com, oleg@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] Take mmap_min_addr into account while choosing
 unmapped address for x86-64.

On Tue 27-02-18 00:12:57, Ilya Smith wrote:
[...]
> diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c
> index 676774b9bb8d..1752fe5cb735 100644
> --- a/arch/x86/kernel/sys_x86_64.c
> +++ b/arch/x86/kernel/sys_x86_64.c
> @@ -17,6 +17,7 @@
>  #include <linux/random.h>
>  #include <linux/uaccess.h>
>  #include <linux/elf.h>
> +#include <linux/security.h>
>  
>  #include <asm/elf.h>
>  #include <asm/compat.h>
> @@ -185,7 +186,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>  		return addr;
>  
>  	/* requested length too big for entire address space */
> -	if (len > TASK_SIZE)
> +	if (len > TASK_SIZE - mmap_min_addr)
>  		return -ENOMEM;
>  
>  	/* No address checking. See comment at mmap_address_hint_valid() */
> @@ -210,7 +211,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
>  
>  	info.flags = VM_UNMAPPED_AREA_TOPDOWN;
>  	info.length = len;
> -	info.low_limit = PAGE_SIZE;
> +	info.low_limit = max(PAGE_SIZE, mmap_min_addr);
>  	info.high_limit = get_mmap_base(0);
>  
>  	/*

mmap_min_addr handling is a bit mess... As you say, we would return
EPERM rather than ENOMEM which can be confusing but depleting the
address space like that is quite unlikely on 64b unless I am missing.
It is good to be in sync here with the generic implementation though,
IMO.

> diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
> index 155ecbac9e28..b6d0c317639e 100644
> --- a/arch/x86/mm/mmap.c
> +++ b/arch/x86/mm/mmap.c
> @@ -31,6 +31,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/sched/mm.h>
>  #include <linux/compat.h>
> +#include <linux/security.h>
>  #include <asm/elf.h>
>  
>  #include "physaddr.h"
> @@ -220,6 +221,9 @@ bool mmap_address_hint_valid(unsigned long addr, unsigned long len)
>  	if (TASK_SIZE - len < addr)
>  		return false;
>  
> +	if (addr < mmap_min_addr)
> +		return false;
> +
>  	return (addr > DEFAULT_MAP_WINDOW) == (addr + len > DEFAULT_MAP_WINDOW);

But is this one necessary? We do sanitze hint address before going to
get_unmapped_address AFAIR.

-- 
Michal Hocko
SUSE Labs