Message-ID: <1421028743.19708.44.camel@decadent.org.uk> (sfid-20150112_031252_276806_10823D09)
Subject: Re: [PATCH V3 for 3.19]  rtlwifi: Fix error when accessing unmapped
 memory in skb
From: Ben Hutchings <ben@decadent.org.uk>
To: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Eric Biggers <ebiggers3@gmail.com>, kvalo@codeaurora.org,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	Stable <stable@vger.kernel.org>
Date: Mon, 12 Jan 2015 02:12:23 +0000
In-Reply-To: <54A4665A.8090906@lwfinger.net>
References: <1419996787-17395-1-git-send-email-Larry.Finger@lwfinger.net>
	 <20141231050735.GA20639@zzz> <54A4665A.8090906@lwfinger.net>
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On Wed, 2014-12-31 at 15:10 -0600, Larry Finger wrote:
> On 12/30/2014 11:07 PM, Eric Biggers wrote:
> > On Tue, Dec 30, 2014 at 09:33:07PM -0600, Larry Finger wrote:
> >> v3 - Unmap skb before trying to allocate a new one so as to not leak m=
apping.
> >
> > Looks good to me, although I'm not sure about the handling of DMA mappi=
ng errors
> > (perhaps that's something that drivers typically don't even try to hand=
le?).
> > Anyway, the skb allocation issue appears to be resolved now.  I am runn=
ing your
> > patch with an extra hack to inject some occasional skb allocation failu=
res, and
> > I haven't noticed any problems except dropped packets.
>=20
> The last time I saw any DMA mapping errors were for some early BCM43xx ca=
rds=20
> that only had 20 bits of DMA addressing space. These Realtek devices have=
 a full=20
> 32 bits of addressing, thus any physical address in the first 4GB of RAM =
will be=20
> OK. I suppose that it might be possible to get a physical address outside=
 this=20
> range for machines with a lot of RAM, but they are unlikely to have wifi =
interfaces.
[...]

Really, you don't think laptops and desktops come with 4GB RAM or more?
Besides that, DMA mapping may involve calling an IOMMU driver that may
fail for some reason.

Drivers need to check for DMA mapping errors, it is not optional.

Ben.

--=20
Ben Hutchings
The world is coming to an end.	Please log off.

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