linux-kernel - Re: [PATCH 3/4] add ksm kernel shared memory driver.

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Date:	Tue, 2 Dec 2008 18:13:33 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Chris Wright <chrisw@...hat.com>
Cc:	Izik Eidus <ieidus@...hat.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	kvm@...r.kernel.org, aarcange@...hat.com, chrisw@...hat.com,
	avi@...hat.com, dlaor@...hat.com, kamezawa.hiroyu@...fujitsu.com,
	cl@...ux-foundation.org, corbet@....net
Subject: Re: [PATCH 3/4] add ksm kernel shared memory driver.

On Tue, 2 Dec 2008 10:07:24 -0800
Chris Wright <chrisw@...hat.com> wrote:

> * Alan Cox (alan@...rguk.ukuu.org.uk) wrote:
> > > +	r = !memcmp(old_digest, sha1_item->sha1val, SHA1_DIGEST_SIZE);
> > > +	mutex_unlock(&sha1_lock);
> > > +	if (r) {
> > > +		char *old_addr, *new_addr;
> > > +		old_addr = kmap_atomic(oldpage, KM_USER0);
> > > +		new_addr = kmap_atomic(newpage, KM_USER1);
> > > +		r = !memcmp(old_addr+PAGEHASH_LEN, new_addr+PAGEHASH_LEN,
> > > +			    PAGE_SIZE-PAGEHASH_LEN);
> > 
> > NAK - this isn't guaranteed to be robust so you could end up merging
> > different pages one provided by a malicious attacker.
> 
> I presume you're referring to the digest comparison.  While there's
> theoretical concern of hash collision, it's mitigated by hmac(sha1)
> so the attacker can't brute force for known collisions.

Using current known techniques. A random collision is just as bad news.

This code simply isn't fit for the kernel.

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