linux-kernel - Re: [Patch v7 21/22] CIFS: SMBD: Upper layer performs SMB read via RDMA write through memory registration

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Message-ID: <a7680efb-6ecf-0f70-2fde-55c2b69b534b@samba.org>
Date:   Sat, 22 Sep 2018 05:56:26 +0200
From:   Stefan Metzmacher <metze@...ba.org>
To:     Tom Talpey <tom@...pey.com>,
        Long Li <longli@...hange.microsoft.com>,
        Steve French <sfrench@...ba.org>, linux-cifs@...r.kernel.org,
        samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org,
        linux-rdma@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Tom Talpey <ttalpey@...rosoft.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [Patch v7 21/22] CIFS: SMBD: Upper layer performs SMB read via
 RDMA write through memory registration

Hi,

>> +        req->Channel = SMB2_CHANNEL_RDMA_V1_INVALIDATE;
>> +        if (need_invalidate)
>> +            req->Channel = SMB2_CHANNEL_RDMA_V1;
>> +        req->ReadChannelInfoOffset =
>> +            offsetof(struct smb2_read_plain_req, Buffer);
>> +        req->ReadChannelInfoLength =
>> +            sizeof(struct smbd_buffer_descriptor_v1);
>> +        v1 = (struct smbd_buffer_descriptor_v1 *) &req->Buffer[0];
>> +        v1->offset = rdata->mr->mr->iova;
> 
> It's unnecessary, and possibly leaking kernel information, to use
> the IOVA as the offset of a memory region which is registered using
> an FRWR. Because such regions are based on the exact bytes targeted
> by the memory handle, the offset can be set to any value, typically
> zero, but nearly arbitrary. As long as the (offset + length) does
> not wrap or otherwise overflow, offset can be set to anything
> convenient.
> 
> Since SMB reads and writes range up to 8MB, I'd suggest zeroing the
> least significant 23 bits, which should guarantee it. The other 41
> bits, party on. You could randomize them, pass some clever identifier
> such as MID sequence, whatever.

I just tested that setting:

mr->iova &= (PAGE_SIZE - 1);
mr->iova |= 0xFFFFFFFF00000000;

after the ib_map_mr_sg() and before doing the IB_WR_REG_MR, seems to work.

metze



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