netdev - Re: Maybe a race condition in net/rds/rdma.c?

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Message-ID: <94b20d30-1d7d-7a66-b943-d75a05bcb46e@oracle.com>
Date:   Thu, 27 Feb 2020 10:10:53 -0800
From:   santosh.shilimkar@...cle.com
To:     zerons <sironhide0null@...il.com>
Cc:     netdev <netdev@...r.kernel.org>,
        OFED mailing list <linux-rdma@...r.kernel.org>
Subject: Re: Maybe a race condition in net/rds/rdma.c?


>> On 18 Feb 2020, at 14:13, zerons <sironhide0null@...il.com> wrote:
>>
>> Hi, all
>>
>> In net/rds/rdma.c
>> (https://urldefense.com/v3/__https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/net/rds/rdma.c?h=v5.5.3*n419__;Iw!!GqivPVa7Brio!OwwQCLtjDsKmhaIz0sfaOVSuC4ai5t5_FgB7yqNExGOCBtACtIGLF61NNJyqSDtIAcGoPg$ ),
>> there may be a race condition between rds_rdma_unuse() and rds_free_mr().
>>
Hmmm.. I didn't see email before in my inbox. Please post 
questions/patches on netdev in future which is the correct mailing list.

>> It seems that this one need some specific devices to run test,
>> unfortunately, I don't have any of these.
>> I've already sent two emails to the maintainer for help, no response yet,
>> (the email address may not be in use).
>>
>> 0) in rds_recv_incoming_exthdrs(), it calls rds_rdma_unuse() when receive an
>> extension header with force=0, if the victim mr does not have RDS_RDMA_USE_ONCE
>> flag set, then the mr would stay in the rbtree. Without any lock, it tries to
>> call mr->r_trans->sync_mr().
>>
>> 1) in rds_free_mr(), the same mr is found, and then freed. The mr->r_refcount
>> doesn't change while rds_mr_tree_walk().
>>
>> 0) back in rds_rdma_unuse(), the victim mr get used again, call
>> mr->r_trans->sync_mr().
>>
>> Could this race condition actually happen?
>>
force=0 is an interesting scenario. Let me think about it and get back.
Thanks for report.

Regards,
Santosh