linux-kernel - Re: [PATCH] mm: Don't warn if memdup

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Message-ID: <20120113071752.GA3802@mwanda>
Date:	Fri, 13 Jan 2012 10:17:52 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Sasha Levin <levinsasha928@...il.com>, lizf@...fujitsu.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tyler Hicks <tyhicks@...onical.com>,
	Dustin Kirkland <kirkland@...onical.com>,
	ecryptfs@...r.kernel.org
Subject: Re: [PATCH] mm: Don't warn if memdup_user fails

On Thu, Jan 12, 2012 at 01:58:03PM -0800, Andrew Morton wrote:
> On Thu, 12 Jan 2012 13:19:54 -0800 (PST)
> David Rientjes <rientjes@...gle.com> wrote:
> 
> > On Thu, 12 Jan 2012, Pekka Enberg wrote:
> > 
> > > I think you missed Andrew's point. We absolutely want to issue a
> > > kernel warning here because ecryptfs is misusing the memdup_user()
> > > API. We must not let userspace processes allocate large amounts of
> > > memory arbitrarily.
> > > 
> > 
> > I think it's good to fix ecryptfs like Tyler is doing and, at the same 
> > time, ensure that the len passed to memdup_user() makes sense prior to 
> > kmallocing memory with GFP_KERNEL.  Perhaps something like
> > 
> > 	if (WARN_ON(len > PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
> > 		return ERR_PTR(-ENOMEM);
> > 
> > in which case __GFP_NOWARN is irrelevant.
> 
> If someone is passing huge size_t's into kmalloc() and getting failures
> then that's probably a bug.

It's pretty common to pass high values to kmalloc().  We've added
a bunch of integer overflow checks recently where we do:

	if (n > ULONG_MAX / size)
		return -EINVAL;

The problem is that we didn't set a maximum bound before and we
can't know which maximum will break compatibility.

Probably we shouldn't do that, I guess.

regards,
dan carpenter


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