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

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Message-ID: <4b94b915-e3cb-01a7-92be-70d291f67f4a@talpey.com>
Date:   Wed, 16 Nov 2022 10:41:30 -0500
From:   Tom Talpey <tom@...pey.com>
To:     Stefan Metzmacher <metze@...ba.org>,
        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

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.

Tom.