[cifs-protocol] [MS-SMB 2.2.1.1.1 <6>] Opening previous version of a file with WRITE access

[Richard Sharpe]

On Fri, Nov 16, 2018 at 11:04 AM Ralph Böhme via cifs-protocol
<cifs-protocol at lists.samba.org> wrote:
>
> Hello dochelp,
>
> I was debuging an interesting problem in the context previous versions of a file
> on a Samba server, opened from a Windows client.
>
> The unexpected client behaviour I obverved was a Windows 2016 client opening a
> previous version of a file over SMB2 for reading *and* writing (Access Mask:
> 0x0012019f). pcap available on request.
>
> With Samba this fails, in the specific configuration involving the VFS module
> shadow_copy2 with ZFS snapshots on FreeBSD, as we map the SMB layer access mask
> to O_RDWR for the POSIX open and when calling open() on the file in a ZFS
> snapshot with mode=O_RDWR, unsurprisingly this fails with EROFS.
>
> Now the interesting part.
>
> The same open of a previous version of a file over SMB2 against a Windows 2016
> server succeeds, *but* then a subsequent write on the handle files with
> STATUS_MEDIA_WRITE_PROTECTED, pcap attached. I've reproduced this with a Samba
> smbtorture test as client, cf attached pcap "twrp_write_w16.pcapng" packet 18.

In my experience this is standard Windows behavior. In a previous life
we handled this by changing the open to a RO open and then failing
with the correct response on the WRITE-like requests.

> The only section I could find in the protocol documentation dealing with opens
> of previous versions and the requested access mask is [MS-SMB 2.2.1.1.1 <6>]
> where we read:
>
>   When enabled previous versions of files are accessible as read-only.
>
> Related to this question is how a server is supposed to return effective
> permissions of a previous version of a file. In the attached pcap the Windows
> 2016 server return "Access Mask: 0x001f01ff".
>
> Questions:
>
> 1. What is the expected behaviour for an SMB server for opens of previous
> version of a file with regard to the desired access bits? Is this documented
> anywhere in the protocol documentation?
>
> 2. What is the expected behaviour for an SMB server for opens of previous
> version of a file with regard to queries for effective permissions? Is this
> documented anywhere in the protocol documentation?
>
> Thanks a lot!
> -slow
>
> --
> Ralph Boehme, Samba Team       https://samba.org/
> Samba Developer, SerNet GmbH   https://sernet.de/en/samba/
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-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)(传说杜康是酒的发明者)