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Open Source and information security mailing list archives
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Date: Mon, 28 Mar 2022 01:52:11 +0200 From: Halil Pasic <pasic@...ux.ibm.com> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Toke Høiland-Jørgensen <toke@...e.dk>, Robin Murphy <robin.murphy@....com>, Christoph Hellwig <hch@....de>, Maxime Bizon <mbizon@...ebox.fr>, Oleksandr Natalenko <oleksandr@...alenko.name>, Marek Szyprowski <m.szyprowski@...sung.com>, Kalle Valo <kvalo@...nel.org>, "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Olha Cherevyk <olha.cherevyk@...il.com>, iommu <iommu@...ts.linux-foundation.org>, linux-wireless <linux-wireless@...r.kernel.org>, Netdev <netdev@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable <stable@...r.kernel.org>, Halil Pasic <pasic@...ux.ibm.com> Subject: Re: [REGRESSION] Recent swiotlb DMA_FROM_DEVICE fixes break ath9k-based AP On Sat, 26 Mar 2022 22:21:03 -0700 Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Sat, Mar 26, 2022 at 10:06 PM Linus Torvalds > <torvalds@...ux-foundation.org> wrote: > > > > On Sat, Mar 26, 2022 at 8:49 PM Halil Pasic <pasic@...ux.ibm.com> wrote: > > > > > > I agree it CPU modified buffers *concurrently* with DMA can never work, > > > and I believe the ownership model was conceived to prevent this > > > situation. > > > > But that just means that the "ownership" model is garbage, and cannot > > handle this REAL LIFE situation. > > Just to clarify: I obviously agree that the "both sides modify > concurrently" obviously cannot work with bounce buffers. > > People still do want to do that, but they'll limit themselves to > actual cache-coherent DMA when they do so (or do nasty uncached > accesses but at least no bounce buffering). Thanks for the explanation! > > But the "bounce ownership back and forth" model comes up empty when > the CPU wants to read while the DMA is still going on. And that not > only can work, but *has* worked. > > You could have a new "get me a non-ownership copy" operation of > course, Yes, https://www.spinics.net/lists/linux-wireless/msg222442.html was mostly about exploring that idea. > but that hits the problem of "which existing drivers need it?" > > We have no idea, outside of ath9k. > > This is why I believe we have to keep the existing semantics in a way > that keep ath9k - and any number of unknown other drivers - happy. I agree. > > And then for the cases where you want to introduce the zeroing because > you don't know how much data the DMA returned - those are the ones > you'll have to mark some way. I have no intention of pursuing this. When fixing the information leak, I happened to realize, that a somewhat similar situation can emerge when mappings are reused. It seemed like an easy fix, so I asked the swiotlb maintainers, and they agreed. It ain't my field of expertise, and the drivers I'm interested in don't need this functionality. Regards, Halil > > Linus
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