<html><head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> </head> <body> [dropping my gmail address] We saw this backtrace showing the call chain while investigating a kernel oops caused by this issue on the DKMS branch with the KFD IPC API. It happens after a dma-buf file is released with fput: <pre>[ 1255.049330] BUG: kernel NULL pointer dereference, address: 000000000000051e [ 1255.049727] #PF: supervisor read access in kernel mode [ 1255.050092] #PF: error_code(0x0000) - not-present page [ 1255.050416] PGD 0 P4D 0 [ 1255.050736] Oops: 0000 [#1] SMP PTI [ 1255.051060] CPU: 27 PID: 2292 Comm: kworker/27:2 Tainted: G OE 5.3.0-46-generic #38~18.04.1-Ubuntu [ 1255.051400] Hardware name: Supermicro SYS-4029GP-TRT2/X11DPG-OT-CPU, BIOS 3.0a 02/26/2019 [ 1255.051752] Workqueue: events delayed_fput [ 1255.052111] RIP: 0010:drm_gem_object_put_unlocked+0x1c/0x70 [drm] [ 1255.052465] Code: 4d 80 c8 ee 0f 0b eb d8 66 0f 1f 44 00 00 0f 1f 44 00 00 48 85 ff 74 34 55 48 89 e5 41 54 53 48 89 fb 48 8b 7f 08 48 8b 47 20 <48> 83 b8 a0 00 00 00 00 74 1a 4c 8d 67 68 48 89 df 4c 89 e6 e8 9b [ 1255.053224] RSP: 0018:ffffb4b62035fdc8 EFLAGS: 00010286 [ 1255.053613] RAX: 000000000000047e RBX: ffff9f2add197850 RCX: 0000000000000000 [ 1255.054032] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9f2aa2548aa0 [ 1255.054440] RBP: ffffb4b62035fdd8 R08: 0000000000000000 R09: 0000000000000000 [ 1255.054860] R10: 0000000000000010 R11: ffff9f2a4b1cc310 R12: 0000000000080005 [ 1255.055268] R13: ffff9f2a4b1cc310 R14: ffff9f4e369161e0 R15: ffff9f2a1b2f9080 [ 1255.055674] FS: 0000000000000000(0000) GS:ffff9f4e3f740000(0000) knlGS:0000000000000000 [ 1255.056087] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 1255.056501] CR2: 000000000000051e CR3: 00000002df00a004 CR4: 00000000007606e0 [ 1255.056923] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 1255.057345] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 1255.057763] PKRU: 55555554 [ 1255.058179] Call Trace: [ 1255.058603] drm_gem_dmabuf_release+0x1a/0x30 [drm] [ 1255.059025] dma_buf_release+0x56/0x130 [ 1255.059443] __fput+0xc6/0x260 [ 1255.059856] delayed_fput+0x20/0x30 [ 1255.060272] process_one_work+0x1fd/0x3f0 [ 1255.060686] worker_thread+0x34/0x410 [ 1255.061099] kthread+0x121/0x140 [ 1255.061510] ? process_one_work+0x3f0/0x3f0 [ 1255.061923] ? kthread_park+0xb0/0xb0 [ 1255.062336] ret_from_fork+0x35/0x40 </pre> drm_gem_object_put_unlocked calls drm_gem_object_free when the obj->refcount reaches 0. From there it calls dev->driver->gem_free_object_unlocked, which is amdgpu_gem_object_free in amdgpu. Regards, Felix <div class="moz-cite-prefix">Am 2020-05-01 um 10:29 a.m. schrieb Christian König: </div> <blockquote type="cite" cite="mid:70e5d202-34ed-532f-e6b6-c195a3effad3@gmail.com">Am 01.05.20 um 16:21 schrieb Felix Kuehling: <blockquote type="cite">From: Felix Kuehling <a class="moz-txt-link-rfc2396E" href="mailto:felix.kuehling@gmail.com"><felix.kuehling@gmail.com></a> That reference gets dropped when the the dma-buf is freed. Not incrementing the refcount can lead to use-after-free errors. Signed-off-by: Felix Kuehling <a class="moz-txt-link-rfc2396E" href="mailto:felix.kuehling@gmail.com"><felix.kuehling@gmail.com></a> --- drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c index ffeb20f11c07..a0f9b3ef4aad 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_dma_buf.c @@ -398,8 +398,15 @@ struct dma_buf *amdgpu_gem_prime_export(struct drm_gem_object *gobj, return ERR_PTR(-EPERM); buf = drm_gem_prime_export(gobj, flags); - if (!IS_ERR(buf)) + if (!IS_ERR(buf)) { buf->ops = &amdgpu_dmabuf_ops; + /* GEM needs a reference to the underlying object + * that gets dropped when the dma-buf is released, + * through the amdgpu_gem_object_free callback + * from drm_gem_object_put_unlocked. + */ + amdgpu_bo_ref(bo); + } </blockquote> Of hand that doesn't sounds correct to me. Why should the exported bo be closed through amdgpu_gem_object_free()? Regards, Christian. <blockquote type="cite"> return buf; } </blockquote> </blockquote> </body> </html>