From: andros@netapp.com
To: trond.myklebust@netapp.com
Cc: linux-nfs@vger.kernel.org, Andy Adamson <andros@netapp.com>
Subject: [PATCH 0/4] RFC Avoid expired credential keys for buffered writes
Date: Thu,  6 Sep 2012 15:54:07 -0400
Message-Id: <1346961251-2554-1-git-send-email-andros@netapp.com>
Sender: linux-nfs-owner@vger.kernel.org

From: Andy Adamson <andros@netapp.com>

This patch set is a Request for Comments: I think it does a fair job of
handling the issue - but it is complicated, and other brains might find a
better way to get the job done.

We must avoid buffering a WRITE that is using a credential key (e.g. a GSS
context key) that is about to expire or has expired.  We currently will
paint ourselves into a corner by returning success to the applciation
for such a buffered WRITE, only to discover that we do not have permission when
we attempt to flush the WRITE (and potentially associated COMMIT) to disk.
This results in data corruption.

First, a couple of "setup" patches:
1) Patch SUNRPC handle EKEYEXPIRED in call_refreshresult returns EACCES to the
application on an expired or non-existent gss context when the users (Kerberos)
credentials have also expired or are non-existent. Current behavior is to
retry upcalls to GSSD forever. Please see patch comment for detail.

2) Patch SUNRPC set gss gc_expiry to full lifetime works in conjunction with
the gssd patch "0001-GSSD-Pass-GSS_context-lifetime-to-the-kernel". The
gssd patch passes the actual remaining TGT lifetime in the downcall, and
this kernel patch sets the gss context gc_expiry to this lifetime.

Then the two patches that avoid using an expired credential key:
3) Patch SUNRPC new rpc_credops to test credential expiry is the heart of this
work. It provides the RPC layer helper functions to allow NFS to manage
data in the face of expired credentials

4) Patch NFS avoid expired credential keys for buffered writes calls the
RPC helper functions.

Pages for buffered WRITEs are allocated in nfs_write_begin where we have an
nfs_open_context and associated rpc_cred. This is a generic rpc_cred, NOT
the gss_cred used in the actual WRITE RPC. Each WRITE RPC call takes the generic
rpc_cred (or uses the 'current_cred') uid and uses it to lookup the associated
gss_cred and gss_context in the call_refresh RPC state. So, there is a
one-to-one association between the nfs_open_context generic_cred and a
gss_cred with a matching uid and a valid non expired gss context.

We need to check the nfs_open_context generic cred 'underlying' gss_cred
gss_context gc_expiry in nfs_write_begin to determine if there is enough time
left in the gss_context lifetime to complete the buffered WRITE.

I started by adding a "key_timeout" rpc_authops routine only set by the generic
auth to do this work, called by rpcauth_key_timeout_notify, called from
nfs_write_begin. It does the lookup of the gss_cred (see the patch for
fast-tracking of non-gss underlying creds) and then tests the gss_context
gc_expiry against timeouts by calling a new crkey_timeout rpc_credops set
only for the gss_cred.

I came up with the idea of two credential key timeout watermarks:
1) A high watermark, RPC_KEY_EXPIRE_TIMEO set to 90 seconds.
2) A low watermark, RPC_KEY_EXPIRE_FAIL set to 30 seconds.
NOTE: these timeouts are guesses that work in a VM environment. We may want to
make them adjustable via module parameters.

If key_timeout is called on a credential with an underlying credential key that
will expire within high watermark seconds, we set the RPC_CRED_KEY_EXPIRE_SOON
flag in the generic_cred acred so that the NFS layer can clean up prior to
key expiration It does this by calling a new crkey_to_expire rpc_credop set
only for generic creds that tests for the RPC_CRED_KEY_EXPIRE_SOON flag.

If the RPC_CRED_KEY_EXPIRE_SOON flag is set in the nfs_open_context generic
cred (the acred portion), then nfs_write_end will call nfs_wb_all
on EVERY WRITE CALL it sees, and nfs_write_rpcsetup will send NFS_FILE_SYNC
WRITEs.  The idea of the high watermark is to give time to flush all buffered
WRITEs and COMMITs, as well as to continue to WRITE, but only with
NFS_FILE_SYNC, allowing the application to try to finish writing before the
gss context expires. NOTE that this means EACH WRITE within the high watermark
timeout is a singe PAGE of NFS_FILE_SYNC. I think this is fine, because we are
in a failure mode - the most important thing is to NOT fail a buffered WRITE..

If key_timeout is called on a credential key that will expire within low
watermark seconds, we return EACCES from nfs_write_begin to stop
the buffered WRITE using the credential key.

Checking a generic credential's underlying credential involves a cred lookup.
To avoid this lookup in the normal case when the underlying credential has
a key that is valid (before the high watermark), a notify flag is set in
the generic credential the first time the key_timeout is called. The
generic credential then stops checking the underlying credential key expiry, and
the underlying credential (gss_cred) match routine then checks the key
expiration upon each normal use and sets a flag in the associated generic
credential only when the key expiration is within the high watermark.
This in turn signals the generic credential key_timeout to perform the extra
credetial lookup thereafter.

TING:

I have tested these patches with a TGT of 5 minutes, and a modified
Connectathon special test: bigfile.c that only writes and never flushes.
I also increased the file size to 524288000 bytes to give me time.
I named the test buffered-write.

I kinit, and run 6 instances of buffered write with the first and maybe second
test completing prior to TGT expiration, and the third/fourth tests spanning
the high water mark. The fifth test starts within the high water mark, and the
sixth test starts within the low water mark.

I have seen the expected behavior: test 1,2 succeed. Tests 3-6 fail with
Permission Denied. Tests 3 and 4 start with normal buffered UNSTABLE writes
then switch to single page NFS_FILE_SYNC writes with a COMMIT for the
normal buffered writes. Test 5 has only single page NFS_FILE_SYNC writes,
test 6 simply fails.

None of the WRITEs fail.

ISSUES
1) Once in a while I see "short write" instead of "Permission
denied". I cannot reproduce this. Even then wireshark shows all writes succeeding.

2) Although I have tested re-establishing Kerberos credentials after the
test run, I have not done any testing re-estabishing Kerberos credentials 
during the a test run that is within the high watermark.

3) General corner case and other testing

-->Andy

Andy Adamson (4):
  SUNRPC handle EKEYEXPIRED in call_refreshresult
  SUNRPC set gss gc_expiry to full lifetime
  SUNRPC new rpc_credops to test credential expiry
  NFS avoid expired credential keys for buffered writes

 fs/nfs/file.c                  |   10 ++++
 fs/nfs/internal.h              |    2 +
 fs/nfs/nfs3proc.c              |    6 +-
 fs/nfs/nfs4filelayout.c        |    1 -
 fs/nfs/nfs4proc.c              |   18 --------
 fs/nfs/nfs4state.c             |   22 ---------
 fs/nfs/proc.c                  |   43 ------------------
 fs/nfs/write.c                 |   29 ++++++++++++
 include/linux/sunrpc/auth.h    |   27 ++++++++++++
 net/sunrpc/auth.c              |   21 +++++++++
 net/sunrpc/auth_generic.c      |   93 ++++++++++++++++++++++++++++++++++++++++
 net/sunrpc/auth_gss/auth_gss.c |   51 +++++++++++++++++++---
 net/sunrpc/clnt.c              |    1 +
 13 files changed, 231 insertions(+), 93 deletions(-)

-- 
1.7.7.6