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SubjectRe: [PATCH 2/2] block: fix use after free for bd_holder_dir
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Hi,

在 2022/11/03 16:12, Christoph Hellwig 写道:
> On Thu, Nov 03, 2022 at 10:55:41AM +0800, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> Currently, the caller of bd_link_disk_holer() get 'bdev' by
>> blkdev_get_by_dev(), which will look up 'bdev' by inode number 'dev'.
>> Howerver, it's possible that del_gendisk() can be called currently, and
>> 'bd_holder_dir' can be freed before bd_link_disk_holer() access it, thus
>> use after free is triggered.
>>
>> t1: t2:
>> bdev = blkdev_get_by_dev
>> del_gendisk
>> kobject_put(bd_holder_dir)
>> kobject_free()
>> bd_link_disk_holder
>>
>> Fix the problem by checking disk is still live and grabbing a reference
>> to 'bd_holder_dir' first in bd_link_disk_holder().
>
> Looks good with some minor stilistic nipicks:
>
>> + if (!disk_live(bdev->bd_disk)) {
>> + mutex_unlock(&bdev->bd_disk->open_mutex);
>> + return -ENODEV;
>> + }
>
> This can use a goto out_unlock;

This lock is different from current 'out_unlock', add a new lable will
make the code more complex, I think.
>
>> holder->refcnt++;
>> + kobject_put(bdev->bd_holder_dir);
>> goto out_unlock;
>> }
>>
>> holder = kzalloc(sizeof(*holder), GFP_KERNEL);
>> if (!holder) {
>> ret = -ENOMEM;
>> + kobject_put(bdev->bd_holder_dir);
>> goto out_unlock;
>> }
>>
>> @@ -101,16 +114,12 @@ int bd_link_disk_holder(struct block_device *bdev, struct gendisk *disk)
>> ret = __link_disk_holder(bdev, disk);
>> if (ret) {
>> kfree(holder);
>> + kobject_put(bdev->bd_holder_dir);
>
> And I think a goto out_put_holder and out_free_holder would clean this up
> nicely.

Yes, you're right.
>
>> list_add(&holder->list, &disk->slave_bdevs);
>> - /*
>> - * del_gendisk drops the initial reference to bd_holder_dir, so we need
>> - * to keep our own here to allow for cleanup past that point.
>> - */
>> - kobject_get(bdev->bd_holder_dir);
>
> .. with this then jumping straight to out_unlock.

Ok, I'll do that in next version.
>
>
> We should repost a series with my first 7 patches and your two. I can do
> that, but it might take some time as I just got through (minor) knee
> surgery and am still at the hospital, so if you have spare cycles feel
> free to do it.

I'm glad to do that, and have a good rest 😄

Thanks,
Kuai
>
> .
>

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