On Sat, Oct 10, 2020 at 11:36 PM Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> wrote: > > Hi Tomasz, > > On Sat, Oct 10, 2020 at 07:22:48PM +0200, Tomasz Figa wrote: > > On Fri, Oct 9, 2020 at 7:52 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > > On Fri, Oct 9, 2020 at 2:48 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote: > > > > On Fri, Oct 09, 2020 at 02:37:23PM +0200, Mauro Carvalho Chehab wrote: > > > > > > > > > I'm not a mm/ expert, but, from what I understood from Daniel's patch > > > > > description is that this is unsafe *only if* __GFP_MOVABLE is used. > > > > > > > > No, it is unconditionally unsafe. The CMA movable mappings are > > > > specific VMAs that will have bad issues here, but there are other > > > > types too. > > > > > > > > The only way to do something at a VMA level is to have a list of OK > > > > VMAs, eg because they were creatd via a special mmap helper from the > > > > media subsystem. > > > > > > > > > Well, no drivers inside the media subsystem uses such flag, although > > > > > they may rely on some infrastructure that could be using it behind > > > > > the bars. > > > > > > > > It doesn't matter, nothing prevents the user from calling media APIs > > > > on mmaps it gets from other subsystems. > > > > > > I think a good first step would be to disable userptr of non struct > > > page backed storage going forward for any new hw support. Even on > > > existing drivers. dma-buf sharing has been around for long enough now > > > that this shouldn't be a problem. Unfortunately right now this doesn't > > > seem to exist, so the entire problem keeps getting perpetuated. > > > > > > > > If this is the case, the proper fix seems to have a GFP_NOT_MOVABLE > > > > > flag that it would be denying the core mm code to set __GFP_MOVABLE. > > > > > > > > We can't tell from the VMA these kinds of details.. > > > > > > > > It has to go the other direction, evey mmap that might be used as a > > > > userptr here has to be found and the VMA specially created to allow > > > > its use. At least that is a kernel only change, but will need people > > > > with the HW to do this work. > > > > > > I think the only reasonable way to keep this working is: > > > - add a struct dma_buf *vma_tryget_dma_buf(struct vm_area_struct *vma); > > > - add dma-buf export support to fbdev and v4l > > > > I assume you mean V4L2 and not the obsolete V4L that is emulated in > > the userspace by libv4l. If so, every video device that uses videobuf2 > > gets DMA-buf export for free and there is nothing needed to enable it. Yeah. And I missed that v4l2 added dma-buf export too. > > We probably still have a few legacy drivers using videobuf (non-2), > > but IMHO those should be safe to put behind some disabled-by-default > > Kconfig symbol or even completely drop, as the legacy framework has > > been deprecated for many years already. > > There's 8 drivers left, and they support a very large number of devices. > I expect unhappy users distros stop shipping them. On the other hand, > videobuf has been deprecated for a loooooooong time, so there has been > plenty of time to convert the remaining drivers to videobuf2. If nobody > can do it, then we'll have to drop support for these devices given the > security issues. Again, the issue here is _only_ with follow_pfn. For videobuf1 this means videbuf-dma-contig.c userptr support is broken. Unlike videobuf2 it means it's broken for all usage (not just zero-copy userptr), because videbuf-dma-contig.c lacks the pin_user_pages path. But that would be easy to add if this poses a problem I think - we just need to carry over the pin_user_pages_fast logic from videbuf2, no driver changes required. But of course I don't think we should do that before someone reports the regression, since videobuf1 userptr is doubly deprecated :-) Everything else keeps working with videobuf1 with my patch series. So depending upon which videobuf1 implementations these 8 drivers use, you might not even have any real breakage there. > We have moved media drivers to staging in the past when there wasn't > enough maintenance effort, we could do so here too. I'm not breaking the world with this, it's really very minimal use-case. At least as far as I'm understanding the entire media subsystem here. -Daniel > > > - roll this out everywhere we still need it. > > > > > > Realistically this just isn't going to happen. And anything else just > > > reimplements half of dma-buf, which is kinda pointless (you need > > > minimally refcounting and some way to get at a promise of a permanent > > > sg list for dma. Plus probably the vmap for kernel cpu access. > > > > > > > > Please let address the issue on this way, instead of broken an > > > > > userspace API that it is there since 1991. > > > > > > > > It has happened before :( It took 4 years for RDMA to undo the uAPI > > > > breakage caused by a security fix for something that was a 15 years > > > > old bug. > > > > > > Yeah we have a bunch of these on the drm side too. Some of them are > > > really just "you have to upgrade userspace", and there's no real fix > > > for the security nightmare without that. > > > > I think we need to phase out such userspace indeed. The Kconfig symbol > > allows enabling the unsafe functionality for anyone who still needs > > it, so I think it's not entirely a breakage. > > -- > Regards, > > Laurent Pinchart -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
