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

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Date:	Thu, 12 Jan 2012 13:19:54 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Pekka Enberg <penberg@...nel.org>
cc:	Sasha Levin <levinsasha928@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>, 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, 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.  I think memdup_user() should 
definitely be taking gfp flags, though, so the caller can specify things 
like __GFP_NORETRY on its own to avoid infinitely looping in the page 
allocator trying reclaim and possibly calling the oom killer.
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