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nfsd: fix use-after-free due to delegation race

Benjamin Coddington requested to merge bcodding/centos-stream-9:bz1991929_use_after_free_laundromat into main

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1991929
Brew: https://brewweb.engineering.redhat.com/brew/taskinfo?taskID=42523663

commit 548ec0805c399c65ed66c6641be467f717833ab5
Author: J. Bruce Fields bfields@redhat.com
Date: Mon Nov 29 15:08:00 2021 -0500

nfsd: fix use-after-free due to delegation race

A delegation break could arrive as soon as we've called vfs_setlease.  A
delegation break runs a callback which immediately (in
nfsd4_cb_recall_prepare) adds the delegation to del_recall_lru.  If we
then exit nfs4_set_delegation without hashing the delegation, it will be
freed as soon as the callback is done with it, without ever being
removed from del_recall_lru.

Symptoms show up later as use-after-free or list corruption warnings,
usually in the laundromat thread.

I suspect aba2072f4523 "nfsd: grant read delegations to clients holding
writes" made this bug easier to hit, but I looked as far back as v3.0
and it looks to me it already had the same problem.  So I'm not sure
where the bug was introduced; it may have been there from the beginning.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

Signed-off-by: Benjamin Coddington bcodding@redhat.com

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