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

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Date:	Thu, 12 Jan 2012 14:29:10 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	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, 12 Jan 2012, Andrew Morton wrote:

> > 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.  So perhaps we should add a warning to
> kmalloc itself if the size_t is out of bounds, and !__GFP_NOWARN.
> 

That's already done.  For slub, for example, the largest object size 
handled by the allocator itself is an order-1 page; everything else gets 
passed through to the page allocator and its limitation is MAX_ORDER, 
which is the warning that we're seeing in Sasha's changelog when 
!__GFP_NOWARN.

> But none of this will be very effective.  If someone is passing an
> unchecked size_t into kmalloc then normal testing will not reveal the
> problem because the testers won't pass stupid numbers into their
> syscalls.
> 

They'll get the same warning that Sasha got, which is because the page 
allocator can't handle larger than MAX_ORDER orders.  The intention in my 
WARN_ON() above specifically for memdup_user() is to avoid the infinite 
loop in the page allocator for GFP_KERNEL allocations where the order 
is less than PAGE_ALLOC_COSTLY_ORDER and avoid the oom killer.  It returns 
immediately rather than passing __GFP_NORETRY since we don't want to incur 
the side-effects of direct reclaim or compaction as well.

The real fix would be to convert all callers to pass gfp flags into 
memdup_user() to determine the behavior they want, though.
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