mm/gup: protect unpin_user_pages() against npages==-ERRNO

As suggested by Dan Carpenter, fortify unpin_user_pages() just a bit,
against a typical caller mistake: check if the npages arg is really a
-ERRNO value, which would blow up the unpinning loop: WARN and return.

If this new WARN_ON() fires, then the system *might* be leaking pages (by
leaving them pinned), but probably not.  More likely, gup/pup returned a
hard -ERRNO error to the caller, who erroneously passed it here.

Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Dan Carpenter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Souptick Joarder <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

mm/gup.c

@@ -328,6 +328,13 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
 {
 	unsigned long index;
 
+	/*
+	 * If this WARN_ON() fires, then the system *might* be leaking pages (by
+	 * leaving them pinned), but probably not. More likely, gup/pup returned
+	 * a hard -ERRNO error to the caller, who erroneously passed it here.
+	 */
+	if (WARN_ON(IS_ERR_VALUE(npages)))
+		return;
 	/*
 	 * TODO: this can be optimized for huge pages: if a series of pages is
 	 * physically contiguous and part of the same compound page, then a