netdev - Re: 2.6.34 Crash in dmaengine

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Date:	Tue, 22 Jun 2010 10:36:28 -0600
From:	Jeffrey Merkey <jeffmerkey@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: 2.6.34 Crash in dmaengine_put()

>
>> OK.  This bug occurs if you OR in the IFF_UP flag while creating
>> virtual interfaces without going through the normal ifup/ifdown
>> scripts.  Looks like a hole.  I will post the trace shortly.  It's
>> easy to reproduce, take the dummy net driver, OR in the IFF_UP flag in
>> dummy_setup, and watch the kernel crash.
>>
>
> Then dont do that ?
>
> No need to send us a trace, unless you use a pristine kernel.
>
> IFF_UP changes rules are very strict, dont try to avoid them and claim
> there is a hole or something wrong.
>
> Check __dev_open() , __dev_close() and __dev_change_flags() were OR/AND
> IFF_UP is done by core network.
>
> Net drivers are not allowed to change IFF_UP themselves.
>
> (DE-600 & DE-620 being the exceptions to confirm this rule, of course)
>

Gee.  OK, its a bug.  I see the crash when unregister_netdev is
called.  Changing a flag in a driver should not cause the kernel to
crash.  You should check the code.  The dmaengine registration should
have nothing to do with registering a netdev -- period.  I have coded
arounnd it but its damn convenient to create virtual drivers on the
fly and mark them as UP without needing to configure a bunch of text
scripts to bring one up or down.

The code itself is busted because it has a check if the dmaengine ref
count goes negative.  If you are going to have a registration layer
the rest of the OS should not have to wonder about its disconnected
state.  The busted code is right at the top of dmaengine_put where it
decrements the ref count then immediately jumps to a bug.  Need a
better way here I think to prevent needless crashes.  Someone could
just alter this flag remotely and crash a server -- HOLE - :)

Jeff
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