Re: [PATCH RFC] mm: protect suid binaries against rowhammer with copy-on-read mappings

From: Dave Hansen
Date: Wed Mar 18 2015 - 10:12:06 EST

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.
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