File system permissions - setgid bit and Netapp NFS volumes

[Shaun Johnson]

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
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