mm/damon/core: clear walk_control on inactive context in damos_walk()

From: Raul Pazemecxas De Andrade <[email protected]>

damos_walk() sets ctx->walk_control to the caller-provided control structure before checking whether the context is running. If the context is inactive (damon_is_running() returns false), the function returns -EINVAL without clearing ctx->walk_control. This leaves a dangling pointer to a stack-allocated structure that will be freed when the caller returns.

This is structurally identical to the bug fixed in commit f9132fbc2e83 ("mm/damon/core: remove call_control in inactive contexts") for damon_call(), which had the same pattern of linking a control object and returning an error without unlinking it.

The dangling walk_control pointer can cause:
1. Use-after-free if the context is later started and kdamond dereferences ctx->walk_control (e.g., in damos_walk_cancel() which writes to control->canceled and calls complete())
2. Permanent -EBUSY from subsequent damos_walk() calls, since the stale pointer is non-NULL

Nonetheless, the real user impact is quite restrictive. The use-after-free is impossible because there is no damos_walk() callers who starts the context later. The permanent -EBUSY can actually confuse users, as DAMON is not running. But the symptom is kept only while the context is turned off. Turning it on again will make DAMON internally uses a newly generated damon_ctx object that doesn't have the invalid damos_walk_control pointer, so everything will work fine again.

Fix this by clearing ctx->walk_control under walk_control_lock before returning -EINVAL, mirroring the fix pattern from f9132fbc2e83.

---
Changes from v1
(https://lore.kernel.org/all/CPUPR80MB81718B12798BCF9959535B05956CA@CPUPR80MB8171.lamprd80.prod.outlook.com/)
- Rebase to latest mm-new
- Add notes about real user impacts

 mm/damon/core.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index 6be8c4cc14162..eae387d4e0786 100644
@@ -1562,8 +1562,13 @@ int damos_walk(struct damon_ctx *ctx, struct damos_walk_control *control)
 	}
 	ctx->walk_control = control;
 	mutex_unlock(&ctx->walk_control_lock);
-	if (!damon_is_running(ctx))
+	if (!damon_is_running(ctx)) {
+		mutex_lock(&ctx->walk_control_lock);
+		if (ctx->walk_control == control)
+			ctx->walk_control = NULL;
+		mutex_unlock(&ctx->walk_control_lock);
 		return -EINVAL;
+	}
 	wait_for_completion(&control->completion);
 	if (control->canceled)
 		return -ECANCELED;

base-commit: 252ff65aa9536c88ef66dedfbdafe63a479a4b3d

2.47.3