From 981bedbbe61364fcc3a3b87ebaf648a66cd07108 Mon Sep 17 00:00:00 2001 From: Matthew Auld Date: Fri, 8 May 2026 11:26:36 +0100 Subject: [PATCH] drm/xe/dma-buf: handle empty bo and UAF races MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit There look to be some nasty races here when triggering the invalidate_mappings hook: 1) We do xe_bo_alloc() followed by the attach, before the actual full bo init step in xe_dma_buf_init_obj(). However the bo is visible on the attachments list after the attach. This is bad since exporter driver, say amdgpu, can at any time call back into our invalidate_mappings hook, with an empty/bogus bo, leading to potential bugs/crashes. 2) Similar to 1) but here we get a UAF, when the invalidate_mappings hook is triggered. For example, we get as far as xe_bo_init_locked() but this fails in some way. But here the bo will be freed on error, but we still have it attached from dma-buf pov, so if the invalidate_mappings is now triggered then the bo we access is gone and we trigger UAF and more bugs/crashes. To fix this, move the attach step until after we actually have a fully set up buffer object. Note that the bo is not published to userspace until later, so not sure what the comment "Don't publish the bo until we have a valid attachment", is referring to. We have at least two different customers reporting hitting a NULL ptr deref in evict_flags when importing something from amdgpu, followed by triggering the evict flow. Hit rate is also pretty low, which would hint at some kind of race, so something like 1) or 2) might explain this. v2: - Shuffle the order of the ops slightly (no functional change) - Improve the comment to better explain the ordering (Matt B) Assisted-by: Gemini:gemini-3 #debug Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/7903 Link: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/4055 Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs") Signed-off-by: Matthew Auld Cc: Thomas Hellström Cc: Matthew Brost Cc: # v6.8+ Reviewed-by: Matthew Brost Acked-by: Thomas Hellström Link: https://patch.msgid.link/20260508102635.149172-3-matthew.auld@intel.com (cherry picked from commit af1f2ad0c59fe4e2f924c526f66e968289d77971) Signed-off-by: Rodrigo Vivi --- drivers/gpu/drm/xe/xe_dma_buf.c | 31 ++++++++++++++++--------------- 1 file changed, 16 insertions(+), 15 deletions(-) diff --git a/drivers/gpu/drm/xe/xe_dma_buf.c b/drivers/gpu/drm/xe/xe_dma_buf.c index 855d32ba314d..f7b47e9d1a4c 100644 --- a/drivers/gpu/drm/xe/xe_dma_buf.c +++ b/drivers/gpu/drm/xe/xe_dma_buf.c @@ -377,15 +377,25 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev, } } - /* - * Don't publish the bo until we have a valid attachment, and a - * valid attachment needs the bo address. So pre-create a bo before - * creating the attachment and publish. - */ bo = xe_bo_alloc(); if (IS_ERR(bo)) return ERR_CAST(bo); + /* + * xe_dma_buf_init_obj() takes ownership of the raw bo, so do not touch + * on fail, since it will already take care of cleanup. On success we + * still need to drop the ref, if something later fails. + * + * In addition this needs to happen before the attach, since + * it will create a new attachment for this, and add it to the list of + * attachments, at which point it is globally visible, and at any point + * the export side can call into on invalidate_mappings callback, which + * require a working object. + */ + obj = xe_dma_buf_init_obj(dev, bo, dma_buf); + if (IS_ERR(obj)) + return obj; + attach_ops = &xe_dma_buf_attach_ops; #if IS_ENABLED(CONFIG_DRM_XE_KUNIT_TEST) if (test) @@ -398,21 +408,12 @@ struct drm_gem_object *xe_gem_prime_import(struct drm_device *dev, goto out_err; } - /* - * xe_dma_buf_init_obj() takes ownership of bo on both success - * and failure, so we must not touch bo after this call. - */ - obj = xe_dma_buf_init_obj(dev, bo, dma_buf); - if (IS_ERR(obj)) { - dma_buf_detach(dma_buf, attach); - return obj; - } get_dma_buf(dma_buf); obj->import_attach = attach; return obj; out_err: - xe_bo_free(bo); + xe_bo_put(bo); return obj; }