File system permissions - setgid bit and Netapp NFS volumes

[Aki Tuomi]

On 17.04.2018 01:42, Shaun Johnson wrote:
> On Fri, 23 Mar 2018 09:53:00 -0700
> Shaun Johnson <shaun at linuxmagic.com> wrote:
>
>> Greetings Dovecot List,
>>
>> I have a bit of an edge case I am trying to resolve.  I am currently
>> using dovecot on Ubuntu 14.04 - Ubuntu package version:
>>
>>     1:2.2.9-1ubuntu2.3
>>
>> I have attached the output of doveconf -n to this email - but to
>> describe the configuration in a nutshell:
>>
>>     my server is configured to use Maildir storage
>>
>>     I do not use dovecot delivery service (there is a separate program
>>     in my system delivering messages)
>>
>>     I am using a 'checkpassword' mechanism for authentication
>>
>>     My mail storage is mounted via NFSv3 from a Netapp Filer FAS3050
>>     configured to set anonuid=89, read write access, and root access.
>>
>>     Dovecot has been configured with:
>>
>> 	mail_fsync = always
>>         mail_nfs_index = yes
>>         mail_nfs_storage = yes
>>         mmap_disable = yes
>>         lock_method = fcntl
>>
>>     In order to function with the other components of my system, the
>>     Mail Storage directories are expected to have permissions:
>>
>> 	drwx--S---     uid 89: gid 89
>>
>> The oddity I am encountering came up recently when I set up a job to
>> monitor and correct directory permissions.  I found after observing
>> the system in action that overtime, the ~/Maildir directories for all
>> active mailboxes would mysteriously lose the setgid bit.   Now, while
>> the system still functioned, it was something unexpected so I
>> investigated further by turning on auditd to monitor what was changing
>> the permissions on the path.   After tracing through the results, I
>> found an entry indicating that the 'imap' process of dovecot was
>> performing a 'chown' call on the directory, with an end result of the
>> previous mode 2700 being changed to 0700.
>>
>> Now I understand that on some filesystems, a 'chown' call can end up
>> losing any setuid or setgid bits on an inode - and apparently Netapp's
>> WAFL file system is one of them.
>>
>> Looking at the dovecot source code, I *believe* the source of the
>> chown call is in src/lib/nfs-workarounds.c function
>> nfs_flush_chown_uid - where a 'chown' call is being used to flush NFS
>> attribute caches? for what I believe is the purposes of ascertaining
>> file lock status?
>>
>> My question for you folks is if anyone else has encountered this on
>> other file systems (or filers) - and if there is a native method in
>> dovecot - either the version I am currently running or an updated
>> newer version - that allows one to either retain the permissions
>> assigned to a directory or specify the permissions I expect on the
>> Maildir directory structures.
>>
>> Any feedback would be welcome, thanks!
>>
>>
> Attached is a quick patch that I wrote up that solves the issue I
> encountered.  Ideally I would like to have this logic take place
> dependent on if a configuration setting were set - something like
> mail_nfs_preserve_permissions = yes - but I found that in order to
> expose such a setting to the nfs-workarounds.c code base would require
> significantly more modifications than I was comfortable with.  If
> anyone has any thoughts on a simple way to make this configurable, I am
> open to suggestions.
>
>
> Thanks!
>
>  - Shaun

Hi!

Thank you for your patch, we would appreciate if you'd be able to open
it on https://github.com/dovecot/core against dovecot master branch.

Aki