** Changed in: linux (Ubuntu Eoan)
Status: In Progress => Fix Committed
** Changed in: linux (Ubuntu Disco)
Status: In Progress => Fix Committed
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https://bugs.launchpad.net/bugs/1854887
Title:
cifs: DFS Caching feature causing problems traversing multi-tier DFS
setups
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Disco:
Fix Committed
Status in linux source package in Eoan:
Fix Committed
Status in linux source package in Focal:
Fix Released
Bug description:
BugLink: https://bugs.launchpad.net/bugs/1854887
[Impact]
There is a problem where kernels 5.0-rc1 and onwards cannot mount a
multi tier cifs DFS setup, while kernels 4.20 and below can mount the
share fine.
The DFS tiering structure looks like this:
Domain virtual DFS (i.e. \\company.com\folders\share)
|-- Domain controller DFS (i.e. \\regional-dc.company.com\folders\share)
|-- Regional DFS Server (i.e. \\regional-dfs.company.com\folders\share)
|-- Actual file server (i.e. \\regional-svr.company.com\share)
On the 5.x series kernels, after getting the DFS referrals list
through to the Regional DFS Server, which responds with the correct
server/share, instead of going to the Actual file server, the kernel
backtracks from the Regional DFS Server back to the Domain controller
and requests the share there. Of course, this share does not exist on
the Domain controller, as it only exists on the Actual file server,
and the connection dies.
We have collected a packet capture, and the flow looks like this:
Legend:
--------------------------------------------------
DC = Domain Controller / Domain DFS Root
RDC = Regional Domain Controller / Domain DFS Root
RDS = Regional DFS Server
AFS = Actual File Server
4.18.0-21-generic Ubuntu kernel - Good
Host: request/response
--------------------------------------------------------------------
DC: company.com\folders
DC: Referral List
RDC: start convo
RDC: <Regional Domain Controller>\Folders\Country\<Share> referral
RDC: <Regional Domain Controller>\Folders\Country\<Share> referral
RDS: start convo
RDS: <Regional DFS Server>\Root\Country\<Share>
RDS: STATUS_PATH_NOT_COVERED
RDS: request referrals
RDS: Referral List
AFS: convo started
AFS: <Actual File Server>\<Share>
AFS: Good response
5.0.0-26-generic Ubuntu kernel - Bad
Host: request/response
------------------------------------------------------------
DC: company.com\folders
RDC: start convo
RDC: <Regional Domain Controller>\Folders\Country\<Share>
RDC: STATUS_PATH_NOT_COVERED
RDS: start convo
RDS: <Regional DFS Server>\Root\Country\<Share>
RDS: STATUS_PATH_NOT_COVERED
RDC: <Regional DFS Server>\Root\Country\<Share>
RDC: STATUS_PATH_NOT_COVERED
From there the debugging output was more or less the same between the
two kernel versions, until the problematic area:
Linux 4.18:
Full log: https://paste.ubuntu.com/p/D9XwBbvTXc/
Status code returned 0xc0000257 STATUS_PATH_NOT_COVERED
fs/cifs/smb2maperror.c: Mapping SMB2 status code 0xc0000257 to POSIX err -66
fs/cifs/connect.c: build_unc_path_to_root: full_path=\\<Regional DFS
Server>\Root\Country\<Share>
fs/cifs/smb2ops.c: smb2_get_dfs_refer path <\<Regional DFS
Server>\Root\Country\<Share>>
fs/cifs/misc.c: num_referrals: 1 dfs flags: 0x2 ...
fs/cifs/dns_resolve.c: dns_resolve_server_name_to_ip: resolved: <Actual File
Server> to <IPV4 Address>
fs/cifs/connect.c: Username: XXX
// mounts the share successfully
Linux 5.0:
Full log: https://paste.ubuntu.com/p/9sXPj7WMQv/
Status code returned 0xc0000257 STATUS_PATH_NOT_COVERED
fs/cifs/smb2maperror.c: Mapping SMB2 status code 0xc0000257 to POSIX err -66
fs/cifs/connect.c: build_unc_path_to_root: full_path=\\<Regional DFS
Server>\Root\Country\<Share>
fs/cifs/connect.c: build_unc_path_to_root: full_path=\\<Regional DFS
Server>\Root\Country\<Share>
fs/cifs/dfs_cache.c: do_dfs_cache_find: search path: \<Regional DFS
Server>\Root\Country\<Share>
fs/cifs/dfs_cache.c: do_dfs_cache_find: cache miss
fs/cifs/dfs_cache.c: do_dfs_cache_find: DFS referral request for \<Regional
DFS Server>\Root\Country\<Share>
fs/cifs/smb2ops.c: smb2_get_dfs_refer path <\<Regional DFS
Server>\Root\Country\<Share>>
fs/cifs/smb2pdu.c: SMB2 IOCTL
Status code returned 0xc0000225 STATUS_NOT_FOUND
fs/cifs/smb2maperror.c: Mapping SMB2 status code 0xc0000225 to POSIX err -2
// mounting the share fails shortly after
This has quite a big impact to customers who need to mount their
multi-tier DFS mounts, as they have to remain on the 4.15 bionic
kernel and cannot use the HWE kernel for their machines.
[Fix]
After some debugging, I narrowed the cause down to a new DFS caching
feature introduced in 5.0-rc1. I started a discussion with the
upstream maintainer of cifs, which you can read here:
https://lore.kernel.org/linux-cifs/05aa2995-e85e-
[email protected]/T/#u
This discussion resulted in the below upstream commit, which was
merged in the 5.5 development window:
commit 5bb30a4dd60e2a10a4de9932daff23e503f1dd2b
Author: Paulo Alcantara (SUSE) <[email protected]>
Date: Fri Nov 22 12:30:56 2019 -0300
Subject: cifs: Fix retrieval of DFS referrals in cifs_mount()
You can read it here:
https://github.com/torvalds/linux/commit/5bb30a4dd60e2a10a4de9932daff23e503f1dd2b
This commit sets referrals to be passed to the newest resolved root
server, instead of older ones up the order. This ensures that we keep
descending down the tree instead of backtracking, which what was
happening.
This commit has been submitted for upstream -stable, and is still
being processed. The commit is needed on kernels 5.0 and up. I will
update this section if it is accepted for -stable.
[Testcase]
To test this commit you need a multi-tier cifs DFS with a similar
structure as the tree mentioned in the Impact section. From there, you
simply try and mount a cifs share.
On patched kernels, the mount will succeed. On broken kernels, the
mount will fail.
I have prepared a test kernel for Bionic HWE, based on
5.0.0-37.40~18.04 which you can find here:
https://launchpad.net/~mruffell/+archive/ubuntu/sf245466-test
This test kernel has been tested by the customer and mounts the cifs
DFS correctly.
[Regression Potential]
I believe the risk of regression for this commit is low. All changes
are limited to DFS within cifs, and only change the behaviour of what
server is the root server referrals are sent to.
The commit is a clean cherry pick for disco, eoan and focal. The
maintainer has submitted the commit for upstream -stable, and we have
tested the commit with the customer, and things are now working as
intended.
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