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Date: Wed, 18 Mar 2015 07:11:57 -0700
From: Dave Hansen <dave.hansen@intel.com>
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To: Konstantin Khlebnikov <koct9i@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
CC: Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH RFC] mm: protect suid binaries against rowhammer with
 copy-on-read mappings
References: <20150318083040.7838.76933.stgit@zurg>
In-Reply-To: <20150318083040.7838.76933.stgit@zurg>
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On 03/18/2015 01:30 AM, Konstantin Khlebnikov wrote:
> +		/*
> +		 * Read-only SUID/SGID binares are mapped as copy-on-read
> +		 * this protects them against exploiting with Rowhammer.
> +		 */
> +		if (!(file->f_mode & FMODE_WRITE) &&
> +		    ((inode->i_mode & S_ISUID) || ((inode->i_mode & S_ISGID) &&
> +			    (inode->i_mode & S_IXGRP)))) {
> +			vm_flags &= ~(VM_SHARED | VM_MAYSHARE);
> +			vm_flags |= VM_COR;
> +		}

I think we probably need to come to _some_ sort of understanding in the
kernel of how much we are willing to do to thwart these kinds of
attacks.  I suspect it's a very deep rabbit hole.

For this particular case, I don't see how this would be effective.  The
existing exploit which you reference attacks PTE pages which are
unmapped in to the user address space.  I'm confused how avoiding
mapping a page in to an attacker's process can keep it from being exploited.

Right now, there's a relatively small number of pages that will get
COW'd for a SUID binary.  This greatly increases the number which could
allow spraying of these (valuable) copy-on-read pages.