linux-kernel - Re: [PATCH v3 3/4] x86, mpx: add prctl commands PR_MPX_INIT, PR_MPX

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Date:	Sun, 26 Jan 2014 16:14:22 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	"Ren, Qiaowei" <qiaowei.ren@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 3/4] x86, mpx: add prctl commands PR_MPX_INIT,
 PR_MPX_RELEASE


* Ren, Qiaowei <qiaowei.ren@...el.com> wrote:

> The size of one bound table is 4M bytes for 64bit, and 16K bytes for 
> 32bit. It can not be accessed by user-space, and it will be accessed 
> automatically by hardware.

So, here's the bound-table allocation AFAICS:

+static bool allocate_bt(unsigned long bd_entry)
+{
+       unsigned long bt_size = 1UL << (MPX_L2_BITS+MPX_L2_SHIFT);
+       unsigned long bt_addr, old_val = 0;
+
+       bt_addr = sys_mmap_pgoff(0, bt_size, PROT_READ | PROT_WRITE,
+                       MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, -1, 0);

What ensures that user-space cannot access (and in particular, modify) 
the pages at bt_addr? It's a read-write anonymous mapping AFAICS.

Thanks,

	Ingo
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