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Subject: [PATCH 0/2] NFS: limit use of ACCESS cache for negative responses
From:   NeilBrown <neilb@suse.de>
To:     Trond Myklebust <trond.myklebust@hammerspace.com>,
        Anna Schumaker <anna@kernel.org>
Cc:     linux-nfs@vger.kernel.org
Date:   Thu, 28 Apr 2022 11:37:32 +1000
Message-ID: <165110909570.7595.8578730126480600782.stgit@noble.brown>
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Since Commit 57b691819ee2 ("NFS: Cache access checks more aggressively")
(Linux 4.8) NFS has cached the results of ACCESS indefinitely while the
inode isn't changing.

This is often a good choice, but doesn't take into account the
possibility that changes out side of the inode can change effective
permissions.

Depending on configuration, some servers can map the user provided in
the RPC credential to a group list at time of request.  If the group
list for a user is changed, the result of ACCESS can change.

This is particularly a problem when extra permissions are given on the
server.  The client may make decisions based on outdated ACCESS results
and not even try operations which would in fact succeed.

These two patches change the ACCESS cache so that when the cache grants
an access, that is trusted indefinitely just as it currently does.
However when the cache denies an access, that is only trusted if the
cached data is less than acmin seconds old.  Otherwise a new ACCESS
request is made.

This allows additions to group membership to become effective with
only a modest delay.

The second patch contains even more explanatory detail.

Thanks,
NeilBrown

---

NeilBrown (2):
      NFS: change nfs_access_get_cached() to nfs_access_check_cached()
      NFS: limit use of ACCESS cache for negative responses


 fs/nfs/dir.c           | 80 +++++++++++++++++++++++++-----------------
 fs/nfs/nfs4proc.c      | 25 ++++++-------
 include/linux/nfs_fs.h |  5 +--
 3 files changed, 61 insertions(+), 49 deletions(-)

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