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

Overview

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

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

When kdamond_fn() main loop is finished, the function cancels remaining damos_walk() request and unset the damon_ctx->kdamond so that API callers and API functions themselves can show the context is terminated. damos_walk() 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, damos_walk() starts waiting for the kdamond's handling of the newly added request.

The damos_walk() requests registration and damon_ctx->kdamond unset are protected by different mutexes, though. Hence, damos_walk() could race with damon_ctx->kdamond unset, and result in deadlocks.

For example, let's suppose kdamond successfully finished the damow_walk() request cancelling. Right after that, damos_walk() 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 damos_walk() 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 damos_walk() caller thread infinitely waits.

Fix this by introducing another damon_ctx field, namely walk_control_obsolete. It is protected by the damon_ctx->walk_control_lock, which protects damos_walk() request registration. Initialize (unset) it in kdamond_fn() before letting damon_start() returns and set it just before the cancelling of the remaining damos_walk() request is executed. damos_walk() 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.

The issue is found by sashiko [1].

Affected

• Linux >= bf0eaba0ff9c9c8e6fd58ddfa1a8b6df4b813f61 < 93a7f297362deac614ce3dbbba73ff2ba31aaca3
• Linux >= bf0eaba0ff9c9c8e6fd58ddfa1a8b6df4b813f61 < 0ba956a239ba6e3fae8555d3660e22e675be63b5
• Linux >= bf0eaba0ff9c9c8e6fd58ddfa1a8b6df4b813f61 < 33c3f6c2b48cd84b441dba1ee3e62290e53930f4
• Linux 6.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.