On Mon, Jun 06, 2022 at 05:02:50PM +0100, Robin Murphy wrote: > On 2022-06-06 16:35, Russell King (Oracle) wrote: > > On Mon, Jun 06, 2022 at 04:21:50PM +0100, Will Deacon wrote: > > > (1) What if the DMA transfer doesn't write to every byte in the buffer? > > > > The data that is in RAM gets pulled into the cache and is visible to > > the CPU - but if DMA doesn't write to every byte in the buffer, isn't > > that a DMA failure? Should a buffer that suffers DMA failure be passed > > to the user? > > No, partial DMA writes can sometimes effectively be expected behaviour, see > the whole SWIOTLB CVE fiasco for the most recent discussion on that: > > https://lore.kernel.org/lkml/1812355.tdWV9SEqCh@xxxxxxxxxxxxxx/ So we have a CVE'd security hole that was never reported to maintainers... > > > (2) What if the buffer has a virtual alias in userspace (e.g. because > > > the kernel has GUP'd the buffer? > > > > Then userspace needs to avoid writing to cachelines that overlap the > > buffer to avoid destroying the action of the DMA. It shouldn't be doing > > this anyway (what happens if userspace writes to the same location that > > is being DMA'd to... who wins?) > > > > However, you're right that invalidating in this case could expose data > > that userspace shouldn't see, and I'd suggest in this case that DMA > > buffers should be cleaned in this circumstance before they're exposed > > to userspace - so userspace only ever gets to see the data that was > > there at the point they're mapped, or is subsequently written to > > afterwards by DMA. > > > > I don't think there's anything to be worried about if the invalidation > > reveals stale data provided the stale data is not older than the data > > that was there on first mapping. > > Indeed as above that may actually be required. I think cleaning the caches > on dma_map_* is the most correct thing to do. It's also the most expensive thing to do as it can push up the slow on-bus traffic to memory quite a bit, especially when big buffers are involved. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
