On Mon, Oct 19, 2026 at 04:48:13PM +0200, Christian König wrote: > On 10/19/26 14:44, Laurent Pinchart wrote: > > On Mon, Oct 19, 2026 at 09:14:07AM +0200, Karl Mehltretter wrote: > >> UVC and vivid access imported DMA-BUFs with the CPU without cache > >> synchronization. On a Pi 400, captured data remained in the CPU cache > >> while physical RAM was stale. > >> > >> Add an opt-in queue flag to bracket CPU access from buf_prepare() through > >> buf_finish(). The allocator cannot imply CPU use because some vb2-vmalloc > >> drivers pass buffers to hardware. Patch 1 balances finish operations in > >> VIDIOC_REMOVE_BUFS so removing prepared buffers closes this interval. > > > > This seems completely wrong. > > It looks a bit questionable from 1000 miles above, but I might be wrong. > > Let me iterate the rules here: > > Accessing imported DMA-buf with the CPU from userspace should only be > possible by using the dma_buf_mmap() interface and that includes the > DMA_BUF_IOCTL_SYNC IOCTL to flush buffers dirtied by the CPU. If that > is not done this way then that would be a *major* security issue, but > I guess that this isn't the issue here. > > What the drivers do more likely is that they use the vmap interface to > access the DMA-buf with the CPU inside the kernel. Correct?
This patch series introduces a big penalty in the common case where a userspace application consumes the frame with the CPU. There should be no cache invalidation in that case. > In this case dma_buf_begin_cpu_access() and dma_buf_end_cpu_access() > should be called to properly frame that access. > > That seems to be added in patch #2, but I absolutely can't judge if > that is correct or not since at the same time you speak about userptr > access and the code really looks a bit like spaghetti. > > >> This follows the per-queue flag proposed here: > >> https://lists.openwall.net/linux-kernel/2024/06/20/236 > >> > >> Based on Linux 7.3. Tests cover the same code on these earlier bases: > >> > >> - Pi 400, 7.3-rc4, all four patches: eight 640x480 YUYV frames per run, > >> imported CMA DMA-BUFs with a DRM device mapping: > >> > >> Path Runs Stale frames > >> UVC, vb2-vmalloc 12 0/96 > >> vivid, vb2-vmalloc 12 0/96 > >> vivid, vb2-dma-contig 12 0/96 > >> > >> All six lifecycle cases passed with both vivid allocators. Read-only > >> output USERPTR passed. The test added no warnings or failed CPU-access > >> callbacks. A control without the DRM mapping found 4/16 stale frames. > >> > >> - SAM9X75 (ARMv5), 6.18 backport of all four patches: the baseline > >> PREPARE_BUF/REMOVE_BUFS imbalance was fixed. USERPTR and six lifecycle > >> cases passed. All 128 vivid and 96 UVC frames matched RAM, with 100/100 > >> begin/end calls. The baseline was already coherent. > >> > >> - BeagleV Ahead (RISC-V), 7.3-rc7, patches 1, 2 and 4: read-only USERPTR, > >> six traced lifecycle cases and 60 repeats passed with balanced callbacks > >> and no kernel diagnostics. A vendor 5.10 backport of patches 2 and 3 > >> captured 96 UVC frames without stale data, with 100/100 begin/end calls. > >> Its baseline was already coherent. > >> > >> Karl Mehltretter (4): > >> media: videobuf2: finish prepared buffers in VIDIOC_REMOVE_BUFS > >> media: videobuf2: add opt-in DMA-BUF CPU access > >> media: uvcvideo: synchronize imported DMA-BUFs for CPU access > >> media: vivid: synchronize imported DMA-BUFs for CPU access > >> > >> .../media/common/videobuf2/videobuf2-core.c | 96 ++++++++++++++++++- > >> .../common/videobuf2/videobuf2-dma-contig.c | 5 +- > >> .../media/common/videobuf2/videobuf2-dma-sg.c | 5 +- > >> .../common/videobuf2/videobuf2-vmalloc.c | 5 +- > >> drivers/media/test-drivers/vivid/vivid-core.c | 1 + > >> drivers/media/usb/uvc/uvc_queue.c | 1 + > >> include/media/videobuf2-core.h | 12 ++- > >> 7 files changed, 116 insertions(+), 9 deletions(-) > >> > >> > >> base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222 -- Regards, Laurent Pinchart
