[PATCH 1/1] drm/amdgpu: Take a reference to an exported BO

[Felix Kuehling]

[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:

[ 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

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

Am 2020-05-01 um 10:29 a.m. schrieb Christian König:
> Am 01.05.20 um 16:21 schrieb Felix Kuehling:
>> From: Felix Kuehling <felix.kuehling at gmail.com>
>>
>> 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 <felix.kuehling at gmail.com>
>> ---
>>   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);
>> +    }
>
> Of hand that doesn't sounds correct to me. Why should the exported bo
> be closed through amdgpu_gem_object_free()?
>
> Regards,
> Christian.
>
>>         return buf;
>>   }
>
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