linux-kernel - Re: [PATCH 11/12] cifs: Fix problem with encrypted RDMA data read

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Message-ID: <4a43598d-51f0-b7bb-575a-d93f7879741f@samba.org>
Date:   Wed, 1 Feb 2023 15:52:59 +0100
From:   Stefan Metzmacher <metze@...ba.org>
To:     David Howells <dhowells@...hat.com>,
        Steve French <smfrench@...il.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Rohith Surabattula <rohiths.msft@...il.com>,
        Tom Talpey <tom@...pey.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jeff Layton <jlayton@...nel.org>, linux-cifs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Long Li <longli@...rosoft.com>,
        Namjae Jeon <linkinjeon@...nel.org>
Subject: Re: [PATCH 11/12] cifs: Fix problem with encrypted RDMA data read

Am 01.02.23 um 15:05 schrieb Stefan Metzmacher:
> Am 31.01.23 um 19:28 schrieb David Howells:
>> When the cifs client is talking to the ksmbd server by RDMA and the ksmbd
>> server has "smb3 encryption = yes" in its config file, the normal PDU
>> stream is encrypted, but the directly-delivered data isn't in the stream
>> (and isn't encrypted), but is rather delivered by DDP/RDMA packets (at
>> least with IWarp).
>>
>> Currently, the direct delivery fails with:
>>
>>     buf can not contain only a part of read data
>>     WARNING: CPU: 0 PID: 4619 at fs/cifs/smb2ops.c:4731 handle_read_data+0x393/0x405
>>     ...
>>     RIP: 0010:handle_read_data+0x393/0x405
>>     ...
>>      smb3_handle_read_data+0x30/0x37
>>      receive_encrypted_standard+0x141/0x224
>>      cifs_demultiplex_thread+0x21a/0x63b
>>      kthread+0xe7/0xef
>>      ret_from_fork+0x22/0x30
>>
>> The problem apparently stemming from the fact that it's trying to manage
>> the decryption, but the data isn't in the smallbuf, the bigbuf or the page
>> array).
>>
>> This can be fixed simply by inserting an extra case into handle_read_data()
>> that checks to see if use_rdma_mr is true, and if it is, just setting
>> rdata->got_bytes to the length of data delivered and allowing normal
>> continuation.
>>
>> This can be seen in an IWarp packet trace.  With the upstream code, it does
>> a DDP/RDMA packet, which produces the warning above and then retries,
>> retrieving the data inline, spread across several SMBDirect messages that
>> get glued together into a single PDU.  With the patch applied, only the
>> DDP/RDMA packet is seen.
>>
>> Note that this doesn't happen if the server isn't told to encrypt stuff and
>> it does also happen with softRoCE.
>>
>> Signed-off-by: David Howells <dhowells@...hat.com>
>> cc: Steve French <smfrench@...il.com>
>> cc: Tom Talpey <tom@...pey.com>
>> cc: Long Li <longli@...rosoft.com>
>> cc: Namjae Jeon <linkinjeon@...nel.org>
>> cc: Stefan Metzmacher <metze@...ba.org>
>> cc: linux-cifs@...r.kernel.org
>>
>> Link: https://lore.kernel.org/r/166855224228.1998592.2212551359609792175.stgit@warthog.procyon.org.uk/ # v1
>> ---
>>   fs/cifs/smb2ops.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
>> index cea578a45ed8..73b66ac86abf 100644
>> --- a/fs/cifs/smb2ops.c
>> +++ b/fs/cifs/smb2ops.c
>> @@ -4733,6 +4733,9 @@ handle_read_data(struct TCP_Server_Info *server, struct mid_q_entry *mid,
>>           if (length < 0)
>>               return length;
>>           rdata->got_bytes = data_len;
>> +    } else if (use_rdma_mr) {
>> +        /* The data was delivered directly by RDMA. */
>> +        rdata->got_bytes = data_len;
> 
> I actually don't understand why this would only be a problem with encryption.
> 
> I guess there's much more needed and data_offset should most likely be
> ignored completely in the rdma offload case. So I'd guess its just luck
> that we don't trigger the below warning/error more often.

I guess it might be related to smb3_handle_read_data passing
server->pdu_size to handle_read_data(), while server->pdu_size is
the outer size of the transform and not the size of the decrypted pdu.

Maybe receive_encrypted_standard() needs to reset server->pdu_size
during this:

         if (mid_entry && mid_entry->handle)
                 ret = mid_entry->handle(server, mid_entry);
         else
                 ret = cifs_handle_standard(server, mid_entry);

But this code is so overly complex, that's way to hard to understand...