[PATCH] cachefiles: fix multiple-put race.

[NeilBrown]

cachefiles_read_waiter() owns a reference to a 'monitor'
object and adds the object to a 'to_do' list *before*
calling fscache_enqueue_retrieval() on it.

The reference is passed to the to_do list, so
cachefiles_read_waiter() no longer owns a reference and shouldn't be
accessing the monitor.
cachefiles_read_copier(), which handles the to_do list, might call
fscache_put_retrieval() *before* fscache_enqueue_retrieval()
takes its own reference.

This can result in fscache_put_retrieval() trying to discard the op
twice, which triggers an assertion failure, and can result in freed
memory be accessed.

The former looks like:

 FS-Cache:
 FS-Cache: Assertion failed
 FS-Cache: 6 == 5 is false
 ------------[ cut here ]------------
 kernel BUG at fs/fscache/operation.c:494!

A previous patch from Lei Xue moved the fscache_enqueue_retrieval()
call inside the spin_locked region.  I think it is cleaner to move it
before.

Reported-by: Lei Xue <carmark.dlut@xxxxxxxxx>
Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxx>
Reported-by: Anthony DeRobertis <aderobertis@xxxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---

hi Andrew,
 this issue was first mentioned in
   https://lkml.org/lkml/2018/2/22/82
 in February, but David Howells doesn't seem to have responded.
 Can you take the patch?

Thanks,
NeilBrown



 fs/cachefiles/rdwr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 5082c8a49686..553a71a2c9cb 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -56,11 +56,12 @@ static int cachefiles_read_waiter(wait_queue_entry_t *wait, unsigned mode,
 	object = container_of(monitor->op->op.object,
 			      struct cachefiles_object, fscache);
 
+	fscache_enqueue_retrieval(monitor->op);
+
 	spin_lock(&object->work_lock);
 	list_add_tail(&monitor->op_link, &monitor->op->to_do);
 	spin_unlock(&object->work_lock);
 
-	fscache_enqueue_retrieval(monitor->op);
 	return 0;
 }
 
-- 
2.14.0.rc0.dirty

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