linux-kernel - Re: [RFC, PATCH] pagemap: do not leak physical addresses to non-privileged userspace

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Date:	Tue, 10 Mar 2015 00:20:32 +0300
From:	Pavel Emelyanov <xemul@...allels.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	"Konstantin Khlebnikov" <khlebnikov@...nvz.org>,
	Mark Seaborn <mseaborn@...omium.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: Re: [RFC, PATCH] pagemap: do not leak physical addresses to non-privileged
 userspace

On 03/10/2015 12:11 AM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> 
> As pointed by recent post[1] on exploiting DRAM physical imperfection,
> /proc/PID/pagemap exposes sensitive information which can be used to do
> attacks.
> 
> This is RFC patch which disallow anybody without CAP_SYS_ADMIN to read
> the pagemap.
> 
> Any comments?

If I'm not mistaken, the pagemap file is used by some userspace that does 
working-set size analysis. But this thing only needs the flags (referenced
bit) from the PTE-s. Maybe it would be better not to lock this file completely,
but instead report the PFN part as zero?

Other than this, I don't mind :) Although we use this heavily in CRIU we
anyway work only with the CAP_SYS_ADMIN, so adding the new one doesn't hurt.

Thanks,
Pavel

> [1] http://googleprojectzero.blogspot.com/2015/03/exploiting-dram-rowhammer-bug-to-gain.html
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Pavel Emelyanov <xemul@...allels.com>
> Cc: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Mark Seaborn <mseaborn@...omium.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> ---
>  fs/proc/task_mmu.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 246eae84b13b..b72b36e64286 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1322,6 +1322,9 @@ out:
>  
>  static int pagemap_open(struct inode *inode, struct file *file)
>  {
> +	/* do not disclose physical addresses: attack vector */
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
>  	pr_warn_once("Bits 55-60 of /proc/PID/pagemap entries are about "
>  			"to stop being page-shift some time soon. See the "
>  			"linux/Documentation/vm/pagemap.txt for details.\n");
> 

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