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Subject: Re: [PATCH] ext4: do not use extent after put_bh
To:     Ritesh Harjani <riteshh@linux.ibm.com>, <adilger@dilger.ca>
CC:     <tytso@mit.edu>, <jack@suse.com>, <linux-ext4@vger.kernel.org>
References: <20201028055617.2569255-1-yangerkun@huawei.com>
 <93d5b1bf-0cf9-a483-ff5d-40a6a9c4b92b@linux.ibm.com>
From:   yangerkun <yangerkun@huawei.com>
Message-ID: <a77ce068-aa0f-77f3-abc4-58c0224757b0@huawei.com>
Date:   Fri, 30 Oct 2020 09:23:38 +0800
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在 2020/10/30 0:08, Ritesh Harjani 写道:
> 
> 
> On 10/28/20 11:26 AM, yangerkun wrote:
>> ext4_ext_search_right will read more extent block and call put_bh after
>> we get the information we need. However ret_ex will break this and may
>> cause use-after-free once pagecache has been freed. Fix it by dup the
>> extent we need.
> 
> 
> It would be good if we have a test case to reproduce it. Do you?
> Ideally it should go in fstests, if you have some way to forcefully
> reproduce it/simulate it. Let me know, if needed, I can as well help to
> get those into fstests.

Sorry for that. I found this bug while reading source code. Not with a 
testcase.

And time leave for drop pagecache is so small(time between 
get_implied_cluster_alloc and ext4_ext_search_right in 
ext4_ext_map_blocks, other caller for ext4_ext_search_right won't use 
@ret_ex). It may difficult to reproduce it expect a delay injection.

Thanks,
Kun.

> 
> -ritesh
> .