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Date:   Thu, 7 Jan 2021 10:58:39 +1100
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hannes Reinecke <hare@...e.de>, Jens Axboe <axboe@...nel.dk>,
        Tejun Heo <tj@...nel.org>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH kernel] block: initialize block_device::bd_bdi for
 bdev_cache



On 06/01/2021 21:41, Jan Kara wrote:
> On Wed 06-01-21 20:29:00, Alexey Kardashevskiy wrote:
>> This is a workaround to fix a null derefence crash:
>>
>> [c00000000b01f840] c00000000b01f880 (unreliable)
>> [c00000000b01f880] c000000000769a3c bdev_evict_inode+0x21c/0x370
>> [c00000000b01f8c0] c00000000070bacc evict+0x11c/0x230
>> [c00000000b01f900] c00000000070c138 iput+0x2a8/0x4a0
>> [c00000000b01f970] c0000000006ff030 dentry_unlink_inode+0x220/0x250
>> [c00000000b01f9b0] c0000000007001c0 __dentry_kill+0x190/0x320
>> [c00000000b01fa00] c000000000701fb8 dput+0x5e8/0x860
>> [c00000000b01fa80] c000000000705848 shrink_dcache_for_umount+0x58/0x100
>> [c00000000b01fb00] c0000000006cf864 generic_shutdown_super+0x54/0x200
>> [c00000000b01fb80] c0000000006cfd48 kill_anon_super+0x38/0x60
>> [c00000000b01fbc0] c0000000006d12cc deactivate_locked_super+0xbc/0x110
>> [c00000000b01fbf0] c0000000006d13bc deactivate_super+0x9c/0xc0
>> [c00000000b01fc20] c00000000071a340 cleanup_mnt+0x1b0/0x250
>> [c00000000b01fc80] c000000000278fa8 task_work_run+0xf8/0x180
>> [c00000000b01fcd0] c00000000002b4ac do_notify_resume+0x4dc/0x5d0
>> [c00000000b01fda0] c00000000004ba0c syscall_exit_prepare+0x28c/0x370
>> [c00000000b01fe10] c00000000000e06c system_call_common+0xfc/0x27c
>> --- Exception: c00 (System Call) at 0000000010034890
>>
>> Is this fixed properly already somewhere? Thanks,
>>
>> Fixes: e6cb53827ed6 ("block: initialize struct block_device in bdev_alloc")
> 
> I don't think it's fixed anywhere and I've seen the syzbot report and I was
> wondering how this can happen when bdev_alloc() initializes bdev->bd_bdi
> and it also wasn't clear to me whether bd_bdi is really the only field that
> is problematic - if we can get to bdev_evict_inode() without going through
> bdev_alloc(), we are probably missing initialization of other fields in
> that place as well...
> 
> But now I've realized that probably the inode is a root inode for bdev
> superblock which is allocated by VFS through new_inode() and thus doesn't
> undergo the initialization in bdev_alloc(). 

yup, this is the case.

> And AFAICT the root inode on
> bdev superblock can get only to bdev_evict_inode() and bdev_free_inode().
> Looking at bdev_evict_inode() the only thing that's used there from struct
> block_device is really bd_bdi. bdev_free_inode() will also access
> bdev->bd_stats and bdev->bd_meta_info. So we need to at least initialize
> these to NULL as well.

These are all NULL.

> IMO the most logical place for all these
> initializations is in bdev_alloc_inode()...


This works. We can also check for NULL where it crashes. But I do not 
know the code to make an informed decision...

> 
> 								Honza
> 
>> ---
>>   fs/block_dev.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 3e5b02f6606c..86fdc28d565e 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -792,8 +792,10 @@ static void bdev_free_inode(struct inode *inode)
>>   static void init_once(void *data)
>>   {
>>   	struct bdev_inode *ei = data;
>> +	struct block_device *bdev = &ei->bdev;
>>   
>>   	inode_init_once(&ei->vfs_inode);
>> +	bdev->bd_bdi = &noop_backing_dev_info;
>>   }
>>   
>>   static void bdev_evict_inode(struct inode *inode)
>> -- 
>> 2.17.1
>>

-- 
Alexey

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