fscache: Fix oops due to race with cookie_lru and use_cookie

If a cookie expires from the LRU and the LRU_DISCARD flag is set, but
the state machine has not run yet, it's possible another thread can call
fscache_use_cookie and begin to use it.

When the cookie_worker finally runs, it will see the LRU_DISCARD flag
set, transition the cookie->state to LRU_DISCARDING, which will then
withdraw the cookie.  Once the cookie is withdrawn the object is removed
the below oops will occur because the object associated with the cookie
is now NULL.

Fix the oops by clearing the LRU_DISCARD bit if another thread uses the
cookie before the cookie_worker runs.

  BUG: kernel NULL pointer dereference, address: 0000000000000008
  ...
  CPU: 31 PID: 44773 Comm: kworker/u130:1 Tainted: G     E    6.0.0-5.dneg.x86_64 #1
  Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022
  Workqueue: events_unbound netfs_rreq_write_to_cache_work [netfs]
  RIP: 0010:cachefiles_prepare_write+0x28/0x90 [cachefiles]
  ...
  Call Trace:
    netfs_rreq_write_to_cache_work+0x11c/0x320 [netfs]
    process_one_work+0x217/0x3e0
    worker_thread+0x4a/0x3b0
    kthread+0xd6/0x100

Fixes: 12bb21a2 ("fscache: Implement cookie user counting and resource pinning")
Reported-by: Daire Byrne <daire.byrne@gmail.com>
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Daire Byrne <daire@dneg.com>
Link: https://lore.kernel.org/r/20221117115023.1350181-1-dwysocha@redhat.com/ # v1
Link: https://lore.kernel.org/r/20221117142915.1366990-1-dwysocha@redhat.com/ # v2
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fscache/cookie.c

@@ -605,6 +605,14 @@ void __fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify)
 			set_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags);
 			queue = true;
 		}
+		/*
+		 * We could race with cookie_lru which may set LRU_DISCARD bit
+		 * but has yet to run the cookie state machine.  If this happens
+		 * and another thread tries to use the cookie, clear LRU_DISCARD
+		 * so we don't end up withdrawing the cookie while in use.
+		 */
+		if (test_and_clear_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags))
+			fscache_see_cookie(cookie, fscache_cookie_see_lru_discard_clear);
 		break;
 
 	case FSCACHE_COOKIE_STATE_FAILED:

include/trace/events/fscache.h

@@ -66,6 +66,7 @@ enum fscache_cookie_trace {
 	fscache_cookie_put_work,
 	fscache_cookie_see_active,
 	fscache_cookie_see_lru_discard,
+	fscache_cookie_see_lru_discard_clear,
 	fscache_cookie_see_lru_do_one,
 	fscache_cookie_see_relinquish,
 	fscache_cookie_see_withdraw,
@@ -149,6 +150,7 @@ enum fscache_access_trace {
 	EM(fscache_cookie_put_work,		"PQ  work ")		\
 	EM(fscache_cookie_see_active,		"-   activ")		\
 	EM(fscache_cookie_see_lru_discard,	"-   x-lru")		\
+	EM(fscache_cookie_see_lru_discard_clear,"-   lrudc")            \
 	EM(fscache_cookie_see_lru_do_one,	"-   lrudo")		\
 	EM(fscache_cookie_see_relinquish,	"-   x-rlq")		\
 	EM(fscache_cookie_see_withdraw,		"-   x-wth")		\