kcov: properly handle subsequent mmap calls

Allocate the kcov buffer during KCOV_MODE_INIT in order to untie mmapping
of a kcov instance and the actual coverage collection process. Modify
kcov_mmap, so that it can be reliably used any number of times once
KCOV_MODE_INIT has succeeded.

These changes to the user-facing interface of the tool only weaken the
preconditions, so all existing user space code should remain compatible
with the new version.

Link: https://lkml.kernel.org/r/20220117153634.150357-3-nogikh@google.com
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Taras Madan <tarasmadan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: a-nogikh

kernel/kcov.c

@@ -459,37 +459,28 @@ void kcov_task_exit(struct task_struct *t)
 static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
 {
 	int res = 0;
-	void *area;
 	struct kcov *kcov = vma->vm_file->private_data;
 	unsigned long size, off;
 	struct page *page;
 	unsigned long flags;
 
-	area = vmalloc_user(vma->vm_end - vma->vm_start);
-	if (!area)
-		return -ENOMEM;
-
 	spin_lock_irqsave(&kcov->lock, flags);
 	size = kcov->size * sizeof(unsigned long);
-	if (kcov->mode != KCOV_MODE_INIT || vma->vm_pgoff != 0 ||
+	if (kcov->area == NULL || vma->vm_pgoff != 0 ||
 	    vma->vm_end - vma->vm_start != size) {
 		res = -EINVAL;
 		goto exit;
 	}
-	if (!kcov->area) {
-		kcov->area = area;
-		vma->vm_flags |= VM_DONTEXPAND;
-		spin_unlock_irqrestore(&kcov->lock, flags);
-		for (off = 0; off < size; off += PAGE_SIZE) {
-			page = vmalloc_to_page(kcov->area + off);
-			if (vm_insert_page(vma, vma->vm_start + off, page))
-				WARN_ONCE(1, "vm_insert_page() failed");
-		}
-		return 0;
+	spin_unlock_irqrestore(&kcov->lock, flags);
+	vma->vm_flags |= VM_DONTEXPAND;
+	for (off = 0; off < size; off += PAGE_SIZE) {
+		page = vmalloc_to_page(kcov->area + off);
+		if (vm_insert_page(vma, vma->vm_start + off, page))
+			WARN_ONCE(1, "vm_insert_page() failed");
 	}
+	return 0;
 exit:
 	spin_unlock_irqrestore(&kcov->lock, flags);
-	vfree(area);
 	return res;
 }
 
@@ -674,6 +665,7 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
 	unsigned int remote_num_handles;
 	unsigned long remote_arg_size;
 	unsigned long size, flags;
+	void *area;
 
 	kcov = filep->private_data;
 	switch (cmd) {
@@ -683,17 +675,21 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
 		 * Must happen before anything else.
 		 *
 		 * First check the size argument - it must be at least 2
-		 * to hold the current position and one PC. Later we allocate
-		 * size * sizeof(unsigned long) memory, that must not overflow.
+		 * to hold the current position and one PC.
 		 */
 		size = arg;
 		if (size < 2 || size > INT_MAX / sizeof(unsigned long))
 			return -EINVAL;
+		area = vmalloc_user(size * sizeof(unsigned long));
+		if (area == NULL)
+			return -ENOMEM;
 		spin_lock_irqsave(&kcov->lock, flags);
 		if (kcov->mode != KCOV_MODE_DISABLED) {
 			spin_unlock_irqrestore(&kcov->lock, flags);
+			vfree(area);
 			return -EBUSY;
 		}
+		kcov->area = area;
 		kcov->size = size;
 		kcov->mode = KCOV_MODE_INIT;
 		spin_unlock_irqrestore(&kcov->lock, flags);