get_user_pages_fast(): return -EFAULT on access_ok failure

From: "Michael S. Tsirkin" <[email protected]>
Subject: get_user_pages_fast(): return -EFAULT on access_ok failure

get_user_pages_fast is supposed to be a faster drop-in equivalent of get_user_pages. As such, callers expect it to return a negative return code when passed an invalid address, and never expect it to return 0 when passed a positive number of pages, since its documentation says:

* Returns number of pages pinned. This may be fewer than the number
* requested. If nr_pages is 0 or negative, returns 0. If no pages
* were pinned, returns -errno.

When get_user_pages_fast fall back on get_user_pages this is exactly what happens. Unfortunately the implementation is inconsistent: it returns 0 if passed a kernel address, confusing callers: for example, the following is pretty common but does not appear to do the right thing with a kernel address:

        ret = get_user_pages_fast(addr, 1, writeable, &page);
        if (ret < 0)
                return ret;

Change get_user_pages_fast to return -EFAULT when supplied a kernel address to make it match expectations.

All callers have been audited for consistency with the documented semantics.

---

 mm/gup.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff -puN mm/gup.c~gup-return-efault-on-access_ok-failure mm/gup.c
@@ -1806,9 +1806,12 @@ int get_user_pages_fast(unsigned long st
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
 	end = start + len;
 
+	if (nr_pages <= 0)
+		return 0;
+
 	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 					(void __user *)start, len)))
-		return 0;
+		return -EFAULT;
 
 	if (gup_fast_permitted(start, nr_pages, write)) {
 		local_irq_disable();

_