mm: kmemleak: allow safe memory scanning during kmemleak disabling

The kmemleak scanning thread can run for minutes.  Callbacks like
kmemleak_free() are allowed during this time, the race being taken care
of by the object->lock spinlock.  Such lock also prevents a memory block
from being freed or unmapped while it is being scanned by blocking the
kmemleak_free() -> ...  -> __delete_object() function until the lock is
released in scan_object().

When a kmemleak error occurs (e.g.  it fails to allocate its metadata),
kmemleak_enabled is set and __delete_object() is no longer called on
freed objects.  If kmemleak_scan is running at the same time,
kmemleak_free() no longer waits for the object scanning to complete,
allowing the corresponding memory block to be freed or unmapped (in the
case of vfree()).  This leads to kmemleak_scan potentially triggering a
page fault.

This patch separates the kmemleak_free() enabling/disabling from the
overall kmemleak_enabled nob so that we can defer the disabling of the
object freeing tracking until the scanning thread completed.  The
kmemleak_free_part() is deliberately ignored by this patch since this is
only called during boot before the scanning thread started.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>
Tested-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/kmemleak.c

@@ -195,6 +195,8 @@ static struct kmem_cache *scan_area_cache;
 
 /* set if tracing memory operations is enabled */
 static int kmemleak_enabled;
+/* same as above but only for the kmemleak_free() callback */
+static int kmemleak_free_enabled;
 /* set in the late_initcall if there were no errors */
 static int kmemleak_initialized;
 /* enables or disables early logging of the memory operations */
@@ -942,7 +944,7 @@ void __ref kmemleak_free(const void *ptr)
 {
 	pr_debug("%s(0x%p)\n", __func__, ptr);
 
-	if (kmemleak_enabled && ptr && !IS_ERR(ptr))
+	if (kmemleak_free_enabled && ptr && !IS_ERR(ptr))
 		delete_object_full((unsigned long)ptr);
 	else if (kmemleak_early_log)
 		log_early(KMEMLEAK_FREE, ptr, 0, 0);
@@ -982,7 +984,7 @@ void __ref kmemleak_free_percpu(const void __percpu *ptr)
 
 	pr_debug("%s(0x%p)\n", __func__, ptr);
 
-	if (kmemleak_enabled && ptr && !IS_ERR(ptr))
+	if (kmemleak_free_enabled && ptr && !IS_ERR(ptr))
 		for_each_possible_cpu(cpu)
 			delete_object_full((unsigned long)per_cpu_ptr(ptr,
 								      cpu));
@@ -1750,6 +1752,13 @@ static void kmemleak_do_cleanup(struct work_struct *work)
 	mutex_lock(&scan_mutex);
 	stop_scan_thread();
 
+	/*
+	 * Once the scan thread has stopped, it is safe to no longer track
+	 * object freeing. Ordering of the scan thread stopping and the memory
+	 * accesses below is guaranteed by the kthread_stop() function.
+	 */
+	kmemleak_free_enabled = 0;
+
 	if (!kmemleak_found_leaks)
 		__kmemleak_do_cleanup();
 	else
@@ -1776,6 +1785,8 @@ static void kmemleak_disable(void)
 	/* check whether it is too early for a kernel thread */
 	if (kmemleak_initialized)
 		schedule_work(&cleanup_work);
+	else
+		kmemleak_free_enabled = 0;
 
 	pr_info("Kernel memory leak detector disabled\n");
 }
@@ -1840,8 +1851,10 @@ void __init kmemleak_init(void)
 	if (kmemleak_error) {
 		local_irq_restore(flags);
 		return;
-	} else
+	} else {
 		kmemleak_enabled = 1;
+		kmemleak_free_enabled = 1;
+	}
 	local_irq_restore(flags);
 
 	/*