netdev - Re: [PATCH] net: fix setsockopt() locking errors

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Date:	Tue, 27 Jan 2009 09:52:49 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jarek Poplawski <jarkao2@...il.com>
Cc:	Martin MOKREJŠ <mmokrejs@...osome.natur.cuni.cz>,
	Vegard Nossum <vegard.nossum@...il.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] net: fix setsockopt() locking errors

On Tue, 2009-01-27 at 08:45 +0000, Jarek Poplawski wrote:
> On Mon, Jan 26, 2009 at 10:30:30PM +0100, Martin MOKREJŠ wrote:
> > The patch really did not help:
> > http://bugzilla.kernel.org/show_bug.cgi?id=12515#c5
> > Martin
> 
> Actually, there is a little change: the warning triggerd in another
> place (sock_setsockopt() -> sk_attach_filter()). So we could go deeper
> with these changes, but I'm not sure this is the right way to fix.
> 
> It looks like the scenario is very old, but probably wasn't reported
> (maybe there is some lockdep improvement):

Yes, they likely are very old, and yes we added a lockdep annotation to
copy_to/from_user() to catch these.

> A) sys_mmap2() -> mm->mmap_sem -> packet_mmap() -> sk_lock
> B) sock_setsockopt() -> sk_lock -> copy_from_user() -> mm->mmap_sem
> 
> packet_mmap() (net/packet/af_packet.c) seems to be the only place in
> net to implement mmap method, and using this lock order btw. On the
> other hand copy_from_user() could be more popular under sk_lock, and
> I'm not sure these changes are necessary.
> 
> Since I don't know enough neither sock/packet nor sys_mmap, I guess
> some advice would be precious. It looks like Peter Zijlstra solved
> similar problems in nfs, so I CC him.

The NFS/sunrpc case was special in that it did copy_to/from_kernel, that
is, it never actually touched user memory -- we taught the might_fault()
annotation about that.

Can't you simply do the copy_from_user() before you take the sk_lock?

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