Linaro-mm-sig October 2026

linaro-mm-sig@lists.linaro.org

23 participants
70 discussions

Re: [RFC PATCH 1/2] iommu: Add iommu_map_sgtable_dma()

by Jason Gunthorpe

On Fri, Oct 09, 2026 at 02:21:13AM +0200, Karl Mehltretter wrote: > DMA-BUF attachments provide DMA addresses, not CPU-side pages. > iommu_map_sgtable() needs those pages, so private-domain importers such > as Rockchip cannot use it with strict DMABUF_DEBUG. > > Add an iommu-dma helper to map a live attachment into another domain by > translating through the device's default DMA domain. Roll back target > mappings on error. This still recovers physical addresses > internally. Absolutely not. You cannot recover the phys_addr's from a DMABUF expoter by using iommu_iova_to_phys(). Christian is adamant that drivers must not use physical addresses, and I will not agree to any hacks like this to try to end-run around his prohibition by abusing iommu subsystem. NAKed-by: Jason Gunthorpe <jgg(a)nvidia.com> If you want to keep the domain in the rockchip driver you have to fix DMABUF to allow an importer to use an iommu domain safely. It *must not* be done with a scatterlist interface, sorry, I know that is alot of work but Alistair is going to take a serious stab at it. Many users want this, not just rockchip. So help with the work to allow iommufd to do this and you will get it for free. Jason

2 weeks, 3 days

Re: [RFC PATCH 2/3] dma-buf: add a warn-only mode to DMABUF_DEBUG

by Jason Gunthorpe

On Wed, Oct 07, 2026 at 02:02:31PM +0200, Diederik de Haas wrote: > [ 1091.981354] rc rc3: two consecutive events of type space > [ 1104.670513] input: EDIFIER e235 (AVRCP) as /devices/virtual/input/input12 > [ 1172.090231] devfreq fb000000.gpu: Couldn't update frequency transition information. > [ 1189.403278] devfreq fb000000.gpu: Couldn't update frequency transition information. > [ 1206.309295] input: EDIFIER e235 (AVRCP) as /devices/virtual/input/input13 > [ 1268.968696] DMA-BUF: importer used the CPU side of an exporter's sg_table This is a nice stack trace, is that the point of this series? > [ 1268.968710] CPU: 6 UID: 1000 PID: 29025 Comm: sway Not tainted 7.3-rc6+unreleased-arm64-cknow #1 PREEMPTLAZY Debian 7.3~rc6-3 > [ 1268.968715] Hardware name: FriendlyElec NanoPC-T6 Plus (DT) > [ 1268.968717] Call trace: > [ 1268.968719] show_stack+0x20/0x38 (C) > [ 1268.968726] dump_stack_lvl+0x60/0x80 > [ 1268.968730] sg_dmabuf_cpu_access_warn.part.0+0x24/0x30 > [ 1268.968734] sg_dmabuf_cpu_access_warn+0x34/0x38 > [ 1268.968739] iommu_map_sg+0xc8/0x1e0 > [ 1268.968745] rockchip_gem_iommu_map+0x8c/0x128 [rockchipdrm] > [ 1268.968757] rockchip_gem_prime_import_sg_table+0x58/0x160 [rockchipdrm] > [ 1268.968761] drm_gem_prime_import_dev+0xa8/0x1d0 [drm] > [ 1268.968776] drm_gem_prime_fd_to_handle+0x1a4/0x280 [drm] So.. This is the exact same thing I need for iommufd. rockchip is managing its own iommu domain and you cannot map to an iommu domain without using a physical address. Of course it is *completely* illegal to call iommu_map_sgtable() in the importer side of a dmabuf. static int rockchip_gem_iommu_map(struct rockchip_gem_object *rk_obj) { [..] ret = iommu_map_sgtable(private->domain, rk_obj->dma_addr, rk_obj->sgt, prot); Jason

2 weeks, 3 days

Re: [RFC PATCH 0/2] iommu, drm/rockchip: Map private-domain imports from DMA addresses

by Robin Murphy

On 09/10/2026 1:21 am, Karl Mehltretter wrote: > Diederik reported failed video-buffer imports on RK3588 with > DMABUF_DEBUG. The proposed warn-only mode [1] confirmed CPU-side > attachment access in rockchip_gem_iommu_map() during Sway video resizing. > > This RFC keeps Rockchip's private scanout domain. Patch 1 copies live > iommu-dma mappings into another domain. Patch 2 uses it for PRIME > imports. Unsupported inputs keep the old page-based path, which remains > unfixed under strict DMABUF_DEBUG. > > Rob's MSM approach assumes direct DMA [2]. Rockchip's attachment instead > provides IOVAs from the VOP's default domain, which cannot be mapped > unchanged into the private domain. Each import retains both mappings > and adds one reverse lookup per 4 KiB. > > Christian rejected reverse translation in the earlier MSM proposal [3]. > Robin NAKed exporting iommu_get_dma_domain() [4]. Keeping the helper in > dma-iommu.c does not resolve the physical-address objection. And if I'd had the context of the whole series, I would have said what I can see now, that the entire premise is fundamentally wrong, irrespective of abusing the internal helper or not. > Sharing the first VOP's default DMA domain, as Exynos does, would avoid > the translation but also change native-buffer mapping. > > Is retaining the private domain and reverse-translating the DMA mappings > an acceptable direction? If so, I'll address the remaining limitations > before posting a non-RFC version. No. If a driver has attached the device to its own unmanaged IOMMU domain then it is using that domain, not the default domain, and thus has even less reason to go poking at the default domain than usual (where the "usual" is tenuous enough in itself). In this situation DMA mapping only needs to take care of non-coherent cache maintenance, and 32-bit ARM is actually the better example here. The fact that iommu-dma does a load of unnecessary work and returns a bogus DMA address just to still get the cache maintenance as a side-effect is a hideous inefficiency (which folks have complained about before...) and absolutely should not be relied upon. It needs to go away. There were reasons why in the original iommu_dma_ops design it was rather impractical to do better (in fact iommu_get_dma_domain() itself is largely just a hack around some of those limitations), but since b67483b3c44e ("iommu/dma: Centralise iommu_setup_dma_ops()") and particularly b5c58b2fdc42 ("dma-mapping: direct calls for dma-iommu"), it now really could and should be cleaned up - it just needs something slightly different from the standard dma-direct behaviour, as for this case we need to ignore the DMA mask and any bouncing conditions. Thanks, Robin. > Base: mainline 602042bf29f6. The warn-only RFC is not a prerequisite. > No stable backport requested. Strict-by-default DMABUF_DEBUG took effect > in v7.3-rc4. > > Testing (builds and QEMU only): > > - W=1 object and stub builds passed on arm64, ARM32 with/without LPAE, > x86-64 GCC/Clang, i386, s390, RISC-V and UML, including dynamic SWIOTLB. > - Rockchip strict/warn A/B reproduced the control failure and warning. > Treatments checked every page and byte in 83 imports each. Primary > buffers had 1/127/507 DMA segments under the default 64 KiB limit. > - SMMUv3 strict/lazy tests, missing-source, bounds and rollback tests > passed. Real bounced attachments exercised alignment and pool > rejection with no target mapping or DMA. > - Invalid forced-SWIOTLB Rockchip provider runs are excluded. Expected > segment-limit and unsupported-fallback diagnostics remain. > > The rig uses a custom Rockchip IOMMU model and the real GEM callback in a > test module, not VOP2, Sway or a real exporter. RK3588 hardware, two-VOP > and 32-bit ARM runtime testing are outstanding. > > Hardware testing is welcome, particularly with Diederik's Sway resize > workload. Compare control and both patches on the same base/config, > first with strict DMABUF_DEBUG, then with the warn-only RFC on both. > Please report full dmesg, config, exporter and visible display problems. > Keep a known-good boot kernel. DMABUF_DEBUG=n testing is welcome too. > > Developed and tested with LLM assistance. > > [1] https://lore.kernel.org/r/20261005064133.7305-1-kmehltretter@gmail.com/ > [2] https://lore.kernel.org/r/20261006131000.81501-1-robin.clark@oss.qualcomm.c… > [3] https://lore.kernel.org/r/bd4e5ece-1358-4e0b-bb04-ba9de62d26f6@amd.com/ > [4] https://lore.kernel.org/r/47bf9a4c-2a47-4e93-bcdf-8d953c9a5ab8@arm.com/ > > Reports: > https://lore.kernel.org/r/DLR74W1U9YPC.375IK0HOYHDIG@cknow-tech.com/ > https://lore.kernel.org/r/DLYLA3WQNN3X.3FB9MO8ZWBQ5@cknow-tech.com/ > > Karl Mehltretter (2): > iommu: Add iommu_map_sgtable_dma() > drm/rockchip: Map imported buffers from DMA addresses > > drivers/gpu/drm/rockchip/rockchip_drm_gem.c | 19 ++- > drivers/iommu/dma-iommu.c | 131 ++++++++++++++++++++ > include/linux/iommu.h | 12 ++ > 3 files changed, 157 insertions(+), 5 deletions(-) > > > base-commit: 602042bf29f6efde39cfb5fdd9289bf4854bc0c5

2 weeks, 3 days

Re: [RFC PATCH 1/3] dma-buf: keep the DMA flags in the DMABUF_DEBUG copy

by Christian König

On 10/5/26 08:41, Karl Mehltretter wrote: > With DMABUF_DEBUG, dma_buf_map_attachment() hands the importer a copy > of the sg_table that keeps only sg_dma_address() and sg_dma_len(). The > dma_flags (SG_DMA_BUS_ADDRESS, SG_DMA_SWIOTLB) are dropped. > > The flags describe the DMA side of an entry, which is the side > importers may use. Copy them as well. > > This is not a bug fix. No importer reads the flags today. Their only > readers are dma-iommu, dma-direct and iommu_map_sg(), which are not > supposed to see an importer's copy at all. The warn mode added in the > next patch keeps its marker in dma_flags and lets importers that do > get there continue. They should then see the same flags as without > DMABUF_DEBUG. > > Assisted-by: LLM > Signed-off-by: Karl Mehltretter <kmehltretter(a)gmail.com> > --- > > Notes: > Built alone on x86_64 with NEED_SG_DMA_FLAGS=y (dma-buf.o, W=1). The > KUnit test in patch 3 checks the flags in the copy. It passes on x86_64 > under QEMU in strict and in warn mode. > > drivers/dma-buf/dma-buf.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > index 4c9add51f9ef..b3d311acb883 100644 > --- a/drivers/dma-buf/dma-buf.c > +++ b/drivers/dma-buf/dma-buf.c > @@ -904,6 +904,10 @@ static int dma_buf_wrap_sg_table(struct sg_table **sg_table) > sg_assign_page(to_sg, NULL); > sg_dma_address(to_sg) = sg_dma_address(from_sg); > sg_dma_len(to_sg) = sg_dma_len(from_sg); > +#ifdef CONFIG_NEED_SG_DMA_FLAGS > + /* the flags describe the DMA side, e.g. SG_DMA_BUS_ADDRESS */ > + to_sg->dma_flags = from_sg->dma_flags; We should probably have a WARN_ON_ONCE(sg_dma_is_swiotlb(from_sg)) here as well since some importers/exporters doesn't realize that this combination will usually not work. Apart from that looks good to me. Regards, Christian. > +#endif > to_sg = sg_next(to_sg); > } >

2 weeks, 3 days

Re: [RFC PATCH 2/3] dma-buf: add a warn-only mode to DMABUF_DEBUG

by Jason Gunthorpe

On Mon, Oct 05, 2026 at 08:41:32AM +0200, Karl Mehltretter wrote: > static inline struct page *sg_page(struct scatterlist *sg) > { > #ifdef CONFIG_DEBUG_SG > BUG_ON(sg_is_chain(sg)); > #endif > + sg_dmabuf_cpu_access_check(sg); > return (struct page *)((sg)->page_link & ~SG_PAGE_LINK_MASK); > } I'm not sure I understand the overall intention here, I get what this patch does, but no distro could turn this on by default when it touches *everyone* using scatterlist in a performance sensitive spot, and anyone doing testing can use the existing option - so what is the point? I certainly don't like this patch, and I don't like the word "DMABUF" in the scatterlist at all. If we want to add something it should be a general mechanism under DEBUG_SG that allows anyone to 'hide' the CPU list from any future access. Jason

2 weeks, 3 days

[PATCH v2 0/2] dma-buf: dmabuf->file release-time cleanup

by Matt Evans

Hi all, A fix for DMABUF's file-release path, to set dmabuf->file = NULL when the struct file itself is being freed and avoid a stale pointer lingering. Previously it could be assumed that dmabuf->file was always valid until .release was called, but commit 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops") separated dentry release (calls .release) from file release (can happen before .release). Setting the pointer to NULL clearly delimits the period in which it points to a valid struct file. Raw dmabuf->file access is bad news unless the file is known to have a reference held (for example, during export). A single-threaded problem scenario is a dentry held open (e.g. using O_PATH) but the file closed; .release is not called, so the exporter was unaware that dmabuf->file became stale. Exporter activities using dmabuf->file then led to a UAF. Or, threads racing concurrent access with a file closing. VFIO uses get_file_active(&dmabuf->file), which is a UAF when the pointer goes stale. Alex Mastro has created a VFIO-centric reproducer [1], which shows up with KASAN. To fix, dmabuf->file is either valid or NULL. get_file_active() is then safe and a recommended way to access dmabuf->file without an existing reference. The only possible outcomes are for it to get a reference to the file, to observe a valid pointer but the file in the DEAD state (being closed, fail), or NULL (fail). Patch [2/2] sets the pointer NULL when the file closes, which get_file_active() handles correctly. vmwgfx was identified as doing a problematic dmabuf->file dereference. Although already a UAF, making dmabuf->file = NULL turns it into an oops. So, the paired patch [1/2] correspondingly converts vmwgfx to use get_file_active() too. Added comments to highlight that dmabuf->file could go away before .release, and to use get_file_active() if a reference isn't otherwise already held. Thanks, Matt [1]: https://github.com/opsound/vfio-dmabuf-lab/blob/main/tests/vfio_dmabuf_opat… "v1" was the single "dma-buf: Annul dmabuf->file on file release" patch: https://lore.kernel.org/all/f8efaabd-c06e-4f04-8cfa-489c148e37ba@ozlabs.org/ Matt Evans (2): drm/vmwgfx: Use get_file_active() for dmabuf->file dma-buf: Annul dmabuf->file on file release drivers/dma-buf/dma-buf.c | 21 ++++++++++++++++++--- drivers/gpu/drm/vmwgfx/ttm_object.c | 2 +- include/linux/dma-buf.h | 4 +++- 3 files changed, 22 insertions(+), 5 deletions(-) -- 2.47.3

2 weeks, 3 days

Re: [RFT PATCH] Input: sur40 - fix DMA handling in video capture

by kernel test robot

Hi Karl, kernel test robot noticed the following build errors: [auto build test ERROR on 2c3418fffa9d037b2038a6db48be63f9e2291806] url: https://github.com/intel-lab-lkp/linux/commits/Karl-Mehltretter/Input-sur40… base: 2c3418fffa9d037b2038a6db48be63f9e2291806 patch link: https://lore.kernel.org/r/20261007054430.40155-1-kmehltretter%40gmail.com patch subject: [RFT PATCH] Input: sur40 - fix DMA handling in video capture config: arm-randconfig-1003-20261009 (https://download.01.org/0day-ci/archive/20261009/202610091847.RMZt5Cy1-lkp@…) compiler: arm-linux-gnueabi-gcc (GCC) 15.2.0 reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20261009/202610091847.RMZt5Cy1-lkp@…) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp(a)intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202610091847.RMZt5Cy1-lkp@intel.com/ All errors (new ones prefixed by >>, old ones prefixed by <<): >> ERROR: modpost: drivers/input/touchscreen/sur40.ko: module uses symbol 'dma_buf_begin_cpu_access' from namespace 'DMA_BUF', but does not import it. >> ERROR: modpost: drivers/input/touchscreen/sur40.ko: module uses symbol 'dma_buf_end_cpu_access' from namespace 'DMA_BUF', but does not import it. -- 0-DAY CI Kernel Test Service https://github.com/intel/lkp-tests/wiki

2 weeks, 3 days

Re: [RFT PATCH] Input: sur40 - fix DMA handling in video capture

by kernel test robot

Hi Karl, kernel test robot noticed the following build warnings: [auto build test WARNING on 2c3418fffa9d037b2038a6db48be63f9e2291806] url: https://github.com/intel-lab-lkp/linux/commits/Karl-Mehltretter/Input-sur40… base: 2c3418fffa9d037b2038a6db48be63f9e2291806 patch link: https://lore.kernel.org/r/20261007054430.40155-1-kmehltretter%40gmail.com patch subject: [RFT PATCH] Input: sur40 - fix DMA handling in video capture config: arm64-randconfig-1003-20261007 (https://download.01.org/0day-ci/archive/20261008/202610081252.0UqYKs2X-lkp@…) compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261) reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20261008/202610081252.0UqYKs2X-lkp@…) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp(a)intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202610081252.0UqYKs2X-lkp@intel.com/ All warnings (new ones prefixed by >>, old ones prefixed by <<): >> WARNING: modpost: drivers/input/touchscreen/sur40.ko: module uses symbol 'dma_buf_begin_cpu_access' from namespace 'DMA_BUF', but does not import it. >> WARNING: modpost: drivers/input/touchscreen/sur40.ko: module uses symbol 'dma_buf_end_cpu_access' from namespace 'DMA_BUF', but does not import it. -- 0-DAY CI Kernel Test Service https://github.com/intel/lkp-tests/wiki

2 weeks, 5 days

[PATCH] dma-buf: Annul dmabuf->file on file release

by Matt Evans

The dmabuf release path is split between file and dentry release: dma_buf_file_release() is called shortly before the file is freed, and dma_buf_release() calls an exporter's dmabuf->ops->release when the dentry is freed. However, the dentry can outlive the file (for example, if opened with O_PATH), meaning .release might be called some time after the file is freed. This presents a window, when closing a DMABUF file, in which dmabuf->file points to freed memory yet .release op has not yet been called. For VFIO, if a buffer's .release has not been called it's considered still active and subject to move/cleanup. If so, it attempts to get_file_active() on dmabuf->file: a file close with dentry held open will call this function with a stale pointer, a UAF. To make this pattern safe, set dmabuf->file to NULL in dma_buf_file_release() to reflect that the associated file is now dead even if the DMABUF is not yet gone. A get_file_active() ... fput() sequence concurrent with a file close will only execute as one of: - Gets the file before file_ref_put() (dmabuf->file valid) - Observes dmabuf->file pointing to a file, but it's DEAD (no file) - Observes dmabuf->file = NULL (no file) Originally, drivers could assume dmabuf->file was valid until .release was called from fops->release. This assumption was no longer valid after 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops"), which moved the callback to the dentry release (by which point the file might have been freed). With this commit, drivers must still consider that dmabuf->file could be NULL before .release. Fixes: 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops") Signed-off-by: Matt Evans <matt(a)ozlabs.org> --- Hi, This issue was found (by Claude Opus 5.5) in the context of VFIO's DMABUF export path. VFIO iterates live DMABUFs with a get_file_active()/fput() block, which now becomes safe if the file is closed (and memory freed!) yet DMABUF .release hasn't yet occurred. However, there are a couple of other places that directly use dmabuf->file and seem able to race a closing file (i.e. without holding the file reference)? If this is so, they'd be a UAF today; with this patch that goes away, but instead of a stale pointer dmabuf->file could be NULL: 1. drivers/gpu/drm/vmwgfx/ttm_object.c:get_dma_buf_unless_doomed() file_ref_get(&dmabuf->file->f_ref); on a non-refcounted DMABUF The commit message of 90ee6ed776c0 ("fs: port files to file_ref") hints this might be more subtle than replacing it with a get_file() variant (so as to accept a NULL file *). 2. drivers/gpu/drm/i915/gvt/dmabuf.c:intel_vgpu_get_dmabuf() gvt_dbg_dpy(... file_count(dmabuf->file) ...); Respective vmwgfx & i915 maintainers, what is your view? Or, indeed, if anyone sees any other questionable uses of dmabuf->file. Thanks, Matt drivers/dma-buf/dma-buf.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 4c9add51f9ef..726639130477 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -193,11 +193,15 @@ static void dma_buf_release(struct dentry *dentry) static int dma_buf_file_release(struct inode *inode, struct file *file) { + struct dma_buf *dmabuf = file->private_data; + if (!is_dma_buf_file(file)) return -EINVAL; - __dma_buf_list_del(file->private_data); + __dma_buf_list_del(dmabuf); + /* Must be observed by __get_file_rcu() before file_free() */ + smp_store_mb(dmabuf->file, NULL); return 0; } -- 2.47.3

2 weeks, 5 days

Re: [PATCH] dma-buf: Annul dmabuf->file on file release

by Matt Evans

Hi Alex, On 07/10/2026 10:19, Alex Mastro wrote: > On Tue, Oct 06, 2026 at 08:15:26PM +0100, Matt Evans wrote: >> static int dma_buf_file_release(struct inode *inode, struct file *file) >> { >> + struct dma_buf *dmabuf = file->private_data; > > I think dmabuf can be NULL here if the dmabuf allocation fails during > dma_buf_export(). > >> + >> if (!is_dma_buf_file(file)) >> return -EINVAL; >> - __dma_buf_list_del(file->private_data); >> + __dma_buf_list_del(dmabuf); >> + /* Must be observed by __get_file_rcu() before file_free() */ >> + smp_store_mb(dmabuf->file, NULL); > > Resulting in NULL deref here -- guard with null check? Oof, yes! Thanks for catching this. __dma_buf_list_del() protects itself against a NULL arg. Done. Thanks, Matt > >> return 0; >> } > > via this path > > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c > index d504c636dc29..557cd7a4c971 100644 > --- a/drivers/dma-buf/dma-buf.c > +++ b/drivers/dma-buf/dma-buf.c > @@ -725,6 +725,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info) > if (!try_module_get(exp_info->owner)) > return ERR_PTR(-ENOENT); > > + // succeeds > file = dma_buf_getfile(exp_info->size, exp_info->flags); > if (IS_ERR(file)) { > ret = PTR_ERR(file); > @@ -739,6 +740,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info) > dmabuf = kzalloc(alloc_size, GFP_KERNEL); > if (!dmabuf) { > ret = -ENOMEM; > + // go here > goto err_file; > } > > @@ -771,6 +773,7 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info) > return dmabuf; > > err_file: > + // ends up calling dma_buf_file_release() > fput(file); > err_module: > module_put(exp_info->owner); >

2 weeks, 5 days

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Linaro-mm-sig October 2026