Re: [PATCH] drm: fix double free for gbo in drm_gem_vram_init and drm_gem_vram_create

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Thanks for the patch and apologies for being late with the review.

The fix is good, but I'd like to see different approach. I'd rather have
drm_gem_vram_init() being integrated into drm_gem_vram_create().

Do you prefer to make the patch or shall I type up something? Would you
be able to test?

I have some comments on the resulting changes below.

Am 01.07.20 um 04:32 schrieb Jia Yang:
> Ping...
> 
> On 2020/6/20 14:21, Jia Yang wrote:
>> I got a use-after-free report when doing some fuzz test:
>>
>> If ttm_bo_init() fails, the "gbo" and "gbo->bo.base" will be
>> freed by ttm_buffer_object_destroy() in ttm_bo_init(). But
>> then drm_gem_vram_create() and drm_gem_vram_init() will free
>> "gbo" and "gbo->bo.base" again.
>>
>> BUG: KMSAN: use-after-free in drm_vma_offset_remove+0xb3/0x150
>> CPU: 0 PID: 24282 Comm: syz-executor.1 Tainted: G    B   W         5.7.0-rc4-msan #2
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
>> Call Trace:
>>  __dump_stack
>>  dump_stack+0x1c9/0x220
>>  kmsan_report+0xf7/0x1e0
>>  __msan_warning+0x58/0xa0
>>  drm_vma_offset_remove+0xb3/0x150
>>  drm_gem_free_mmap_offset
>>  drm_gem_object_release+0x159/0x180
>>  drm_gem_vram_init
>>  drm_gem_vram_create+0x7c5/0x990
>>  drm_gem_vram_fill_create_dumb
>>  drm_gem_vram_driver_dumb_create+0x238/0x590
>>  drm_mode_create_dumb
>>  drm_mode_create_dumb_ioctl+0x41d/0x450
>>  drm_ioctl_kernel+0x5a4/0x710
>>  drm_ioctl+0xc6f/0x1240
>>  vfs_ioctl
>>  ksys_ioctl
>>  __do_sys_ioctl
>>  __se_sys_ioctl+0x2e9/0x410
>>  __x64_sys_ioctl+0x4a/0x70
>>  do_syscall_64+0xb8/0x160
>>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>> RIP: 0033:0x4689b9
>> Code: fd e0 fa ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 cb e0 fa ff c3 66 2e 0f 1f 84 00 00 00 00
>> RSP: 002b:00007f368fa4dc98 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
>> RAX: ffffffffffffffda RBX: 000000000076bf00 RCX: 00000000004689b9
>> RDX: 0000000020000240 RSI: 00000000c02064b2 RDI: 0000000000000003
>> RBP: 0000000000000004 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
>> R13: 00000000004d17e0 R14: 00007f368fa4e6d4 R15: 000000000076bf0c
>>
>> Uninit was created at:
>>  kmsan_save_stack_with_flags
>>  kmsan_internal_poison_shadow+0x66/0xd0
>>  kmsan_slab_free+0x6e/0xb0
>>  slab_free_freelist_hook
>>  slab_free
>>  kfree+0x571/0x30a0
>>  drm_gem_vram_destroy
>>  ttm_buffer_object_destroy+0xc8/0x130
>>  ttm_bo_release
>>  kref_put
>>  ttm_bo_put+0x117d/0x23e0
>>  ttm_bo_init_reserved+0x11c0/0x11d0
>>  ttm_bo_init+0x289/0x3f0
>>  drm_gem_vram_init
>>  drm_gem_vram_create+0x775/0x990
>>  drm_gem_vram_fill_create_dumb
>>  drm_gem_vram_driver_dumb_create+0x238/0x590
>>  drm_mode_create_dumb
>>  drm_mode_create_dumb_ioctl+0x41d/0x450
>>  drm_ioctl_kernel+0x5a4/0x710
>>  drm_ioctl+0xc6f/0x1240
>>  vfs_ioctl
>>  ksys_ioctl
>>  __do_sys_ioctl
>>  __se_sys_ioctl+0x2e9/0x410
>>  __x64_sys_ioctl+0x4a/0x70
>>  do_syscall_64+0xb8/0x160
>>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> If ttm_bo_init() fails, the "gbo" will be freed by
>> ttm_buffer_object_destroy() in ttm_bo_init(). But then
>> drm_gem_vram_create() and drm_gem_vram_init() will free
>> "gbo" again.
>>
>> Reported-by: Hulk Robot <hulkci@huawei.com>
>> Signed-off-by: Jia Yang <jiayang5@huawei.com>
>> ---
>>  drivers/gpu/drm/drm_gem_vram_helper.c | 28 +++++++++++++++------------
>>  1 file changed, 16 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_vram_helper.c b/drivers/gpu/drm/drm_gem_vram_helper.c
>> index 8b2d5c945c95..1d85af9a481a 100644
>> --- a/drivers/gpu/drm/drm_gem_vram_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_vram_helper.c
>> @@ -175,6 +175,10 @@ static void drm_gem_vram_placement(struct drm_gem_vram_object *gbo,
>>  	}
>>  }
>>  
>> +/*
>> + * Note that on error, drm_gem_vram_init will free the buffer object.
>> + */
>> +

This comment could go away.

>>  static int drm_gem_vram_init(struct drm_device *dev,
>>  			     struct drm_gem_vram_object *gbo,
>>  			     size_t size, unsigned long pg_align)
>> @@ -184,15 +188,19 @@ static int drm_gem_vram_init(struct drm_device *dev,
>>  	int ret;
>>  	size_t acc_size;
>>  
>> -	if (WARN_ONCE(!vmm, "VRAM MM not initialized"))
>> +	if (WARN_ONCE(!vmm, "VRAM MM not initialized")) {
>> +		kfree(gbo);
>>  		return -EINVAL;
>> +	}

This test can go to the top of drm_gem_vram_create()

>>  	bdev = &vmm->bdev;
>>  
>>  	gbo->bo.base.funcs = &drm_gem_vram_object_funcs;
>>  
>>  	ret = drm_gem_object_init(dev, &gbo->bo.base, size);
>> -	if (ret)
>> +	if (ret) {
>> +		kfree(gbo);
>>  		return ret;
>> +	}
>>  
>>  	acc_size = ttm_bo_dma_acc_size(bdev, size, sizeof(*gbo));
>>  
>> @@ -203,13 +211,13 @@ static int drm_gem_vram_init(struct drm_device *dev,
>>  			  &gbo->placement, pg_align, false, acc_size,
>>  			  NULL, NULL, ttm_buffer_object_destroy);
>>  	if (ret)
>> -		goto err_drm_gem_object_release;
>> +		/*
>> +		 * A failing ttm_bo_init will call ttm_buffer_object_destroy
>> +		 * to release gbo->bo.base and kfree gbo.
>> +		 */
>> +		return ret;

The rest of this function would be inlined where the call to
drm_gem_vram_init() currently happens.

>>  
>>  	return 0;
>> -
>> -err_drm_gem_object_release:
>> -	drm_gem_object_release(&gbo->bo.base);
>> -	return ret;
>>  }
>>  
>>  /**
>> @@ -243,13 +251,9 @@ struct drm_gem_vram_object *drm_gem_vram_create(struct drm_device *dev,
>>  
>>  	ret = drm_gem_vram_init(dev, gbo, size, pg_align);
>>  	if (ret < 0)
>> -		goto err_kfree;
>> +		return ERR_PTR(ret);
>>  
>>  	return gbo;
>> -
>> -err_kfree:
>> -	kfree(gbo);
>> -	return ERR_PTR(ret);

All inlined code up to ttm_bo_init() would jump to error labels on
errors. ttm_bo_init() would return directly.

Best regards
Thomas


>>  }
>>  EXPORT_SYMBOL(drm_gem_vram_create);
>>  
>>

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer


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