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

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Date:   Wed, 16 Nov 2022 16:44:00 +0100
From:   Stefan Metzmacher <metze@...ba.org>
To:     Tom Talpey <tom@...pey.com>, Namjae Jeon <linkinjeon@...nel.org>
Cc:     David Howells <dhowells@...hat.com>, smfrench@...il.com,
        Long Li <longli@...rosoft.com>, linux-cifs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cifs: Fix problem with encrypted RDMA data read

Am 16.11.22 um 16:41 schrieb Tom Talpey:
> On 11/16/2022 3:36 AM, Stefan Metzmacher wrote:
>> Am 16.11.22 um 06:19 schrieb Namjae Jeon:
>>> 2022-11-16 9:57 GMT+09:00, Stefan Metzmacher <metze@...ba.org>:
>>>> Hi David,
>>>>
>>>> see below...
>>>>
>>>>> 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
>>>>> ---
>>>>>
>>>>>    fs/cifs/smb2ops.c |    3 +++
>>>>>    1 file changed, 3 insertions(+)
>>>>>
>>>>> diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c
>>>>> index 880cd494afea..8d459f60f27b 100644
>>>>> --- a/fs/cifs/smb2ops.c
>>>>> +++ b/fs/cifs/smb2ops.c
>>>>> @@ -4726,6 +4726,9 @@ handle_read_data(struct TCP_Server_Info *server,
>>>>> struct mid_q_entry *mid,
>>>>>            iov.iov_base = buf + data_offset;
>>>>>            iov.iov_len = data_len;
>>>>>            iov_iter_kvec(&iter, WRITE, &iov, 1, data_len);
>>>>> +    } else if (use_rdma_mr) {
>>>>> +        /* The data was delivered directly by RDMA. */
>>>>> +        rdata->got_bytes = data_len;
>>>>>        } else {
>>>>>            /* read response payload cannot be in both buf and pages */
>>>>>            WARN_ONCE(1, "buf can not contain only a part of read data");
>>>>
>>>> I'm not sure I understand why this would fix anything when encryption is
>>>> enabled.
>>>>
>>>> Is the payload still be offloaded as plaintext? Otherwise we wouldn't have
>>>> use_rdma_mr...
>>>> So this rather looks like a fix for the non encrypted case.
>>> ksmbd doesn't encrypt RDMA payload on read/write operation, Currently
>>> only smb2 response is encrypted for this. And as you pointed out, We
>>> need to implement SMB2 RDMA Transform to encrypt it.
>>
>> I haven't tested against a windows server yet, but my hope would be that
>> and encrypted request with SMB2_CHANNEL_RDMA_V1* receive NT_STATUS_ACCESS_DENIED or something similar...
>>
>> Is someone able to check that against Windows?
> 
> It's not going to fail, because it's perfectly legal per the protocol.
> And the new SMB3 extension to perform pre-encryption of RDMA payload
> is not a solution, because it's only supported by one server (Windows
> 22H2) and in any case it does not alter the transfer model. The client
> will see the same two-part response (headers in the inline portion,
> data via RDMA), so this same code will be entered when processing it.
> 
> I think David's change is on the right track because it actually
> processes the response. I'm a little bit skeptical of the got_bytes
> override however, still digging into that.
> 
>> But the core of it is a client security problem, shown in David's capture in frame 100.
> 
> Sorry, what's the security problem? Both the client and server appear
> to be implementing the protocol itself correctly.

Data goes in plaintext over the wire and a share that requires encryption!

metze