kcov: detect double association with a single task

commit a77660d2 upstream.

Currently KCOV_ENABLE does not check if the current task is already
associated with another kcov descriptor.  As the result it is possible
to associate a single task with more than one kcov descriptor, which
later leads to a memory leak of the old descriptor.  This relation is
really meant to be one-to-one (task has only one back link).

Extend validation to detect such misuse.

Link: http://lkml.kernel.org/r/20180122082520.15716-1-dvyukov@google.com
Fixes: 5c9a8750 ("kernel: add kcov code coverage")
Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by: Shankara Pailoor <sp3485@columbia.edu>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/kcov.c

@@ -220,9 +220,9 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
 		if (unused != 0 || kcov->mode == KCOV_MODE_DISABLED ||
 		    kcov->area == NULL)
 			return -EINVAL;
-		if (kcov->t != NULL)
-			return -EBUSY;
 		t = current;
+		if (kcov->t != NULL || t->kcov != NULL)
+			return -EBUSY;
 		/* Cache in task struct for performance. */
 		t->kcov_size = kcov->size;
 		t->kcov_area = kcov->area;