mm/damon/core: remove call_control in inactive contexts

mm/damon/core: remove call_control in inactive contexts

If damon_call() is executed against a DAMON context that is not running,
the function returns error while keeping the damon_call_control object
linked to the context's call_controls list.  Let's suppose the object is
deallocated after the damon_call(), and yet another damon_call() is
executed against the same context.  The function tries to add the new
damon_call_control object to the call_controls list, which still has the
pointer to the previous damon_call_control object, which is deallocated.
As a result, use-after-free happens.

This can actually be triggered using the DAMON sysfs interface.  It is not
easily exploitable since it requires the sysfs write permission and making
a definitely weird file writes, though.  Please refer to the report for
more details about the issue reproduction steps.

Fix the issue by making two changes.  Firstly, move the final
kdamond_call() for cancelling all existing damon_call() requests from
terminating DAMON context to be done before the ctx->kdamond reset.  This
makes any code that sees NULL ctx->kdamond can safely assume the context
may not access damon_call() requests anymore.  Secondly, let damon_call()
to cleanup the damon_call_control objects that were added to the
already-terminated DAMON context, before returning the error.

Link: https://lkml.kernel.org/r/20251231012315.75835-1-sj@kernel.org
Fixes: 004ded6bee11 ("mm/damon: accept parallel damon_call() requests")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: JaeJoon Jung <rgbi3307@gmail.com>
Closes: https://lore.kernel.org/20251224094401.20384-1-rgbi3307@gmail.com
Cc: <stable@vger.kernel.org> # 6.17.x
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>