mm/damon/core: fix damon_call() vs kdamond_fn() exit race

In the Linux kernel, the following vulnerability has been resolved:

mm/damon/core: fix damon_call() vs kdamond_fn() exit race

Patch series "mm/damon/core: fix damon_call()/damos_walk() vs kdmond exit

race".

damon_call() and damos_walk() can leak memory and/or deadlock when they

race with kdamond terminations. Fix those.

This patch (of 2);

When kdamond_fn() main loop is finished, the function cancels all

remaining damon_call() requests and unset the damon_ctx->kdamond so that

API callers and API functions themselves can know the context is

terminated. damon_call() adds the caller's request to the queue first.

After that, it shows if the kdamond of the damon_ctx is still running

(damon_ctx->kdamond is set). Only if the kdamond is running, damon_call()

starts waiting for the kdamond's handling of the newly added request.

The damon_call() requests registration and damon_ctx->kdamond unset are

protected by different mutexes, though. Hence, damon_call() could race

with damon_ctx->kdamond unset, and result in deadlocks.

For example, let's suppose kdamond successfully finished the damon_call()

requests cancelling. Right after that, damon_call() is called for the

context. It registers the new request, and shows the context is still

running, because damon_ctx->kdamond unset is not yet done. Hence the

damon_call() caller starts waiting for the handling of the request.

However, the kdamond is already on the termination steps, so it never

handles the new request. As a result, the damon_call() caller threads

infinitely waits.

Fix this by introducing another damon_ctx field, namely

call_controls_obsolete. It is protected by the

damon_ctx->call_controls_lock, which protects damon_call() requests

registration. Initialize (unset) it in kdamond_fn() before letting

damon_start() returns and set it just before the cancelling of remaining

damon_call() requests is executed. damon_call() reads the obsolete field

under the lock and avoids adding a new request.

After this change, only requests that are guaranteed to be handled or

cancelled are registered. Hence the after-registration DAMON context

termination check is no longer needed. Remove it together.

Note that the deadlock will not happen when damon_call() is called for

repeat mode request. In tis case, damon_call() returns instead of waiting

for the handling when the request registration succeeds and it shows the

kdamond is running. However, if the request also has dealloc_on_cancel,

the request memory would be leaked.

The issue is found by sashiko [1].