On 12/10/19 5:39 AM, Jan Kara wrote:
...

quoted

+void grab_page(struct page *page, unsigned int flags)
+{
+	if (flags & FOLL_GET)
+		get_page(page);
+	else if (flags & FOLL_PIN) {
+		get_page(page);
+		WARN_ON_ONCE(flags & FOLL_GET);
+		/*
+		 * Use get_page(), above, to do the refcount error
+		 * checking. Then just add in the remaining references:
+		 */
+		page_ref_add(page, GUP_PIN_COUNTING_BIAS - 1);

This is wrong for two reasons:

1) You miss compound_head() indirection from get_page() for this
page_ref_add().

whoops, yes that is missing.

2) page_ref_add() could overflow the counter without noticing.

Especially with GUP_PIN_COUNTING_BIAS being non-trivial, it is realistic
that an attacker might try to overflow the page refcount and we have to
protect the kernel against that. So I think that all the places that would
use grab_page() actually need to use try_grab_page() and then gracefully
deal with the failure.

OK, I've replaced grab_page() everywhere with try_grab_page(), with the
above issues fixed. The v7 patchset had error handling for grab_page() failures,
that had been reviewed, so relevants parts of that have reappeared.

I had initially hesitated to do this, but now I've gone ahead and added:

#define page_ref_zero_or_close_to_bias_overflow(page) \
	((unsigned int) page_ref_count(page) + \
		GUP_PIN_COUNTING_BIAS <= GUP_PIN_COUNTING_BIAS)

...which is used in the new try_grab_page() for protection.

quoted

@@ -278,11 +425,23 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 		goto retry;
 	}
 
-	if (flags & FOLL_GET) {
+	if (flags & (FOLL_PIN | FOLL_GET)) {
+		/*
+		 * Allow try_get_page() to take care of error handling, for
+		 * both cases: FOLL_GET or FOLL_PIN:
+		 */
 		if (unlikely(!try_get_page(page))) {
 			page = ERR_PTR(-ENOMEM);
 			goto out;
 		}
+
+		if (flags & FOLL_PIN) {
+			WARN_ON_ONCE(flags & FOLL_GET);
+
+			/* We got a +1 refcount from try_get_page(), above. */
+			page_ref_add(page, GUP_PIN_COUNTING_BIAS - 1);
+			__update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1);
+		}
 	}

The same problem here as above, plus this place should use the same
try_grab..() helper, shouldn't it?

Yes, now that the new try_grab_page() has behavior that matches what
this call site needs. Done.

quoted

@@ -544,8 +703,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
 	/* make this handle hugepd */
 	page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
 	if (!IS_ERR(page)) {
-		BUG_ON(flags & FOLL_GET);
-		return page;
+		WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
+		return NULL;

I agree with the change to WARN_ON_ONCE but why is correct the change of
the return value? Note that this is actually a "success branch".

Good catch, thanks! I worked through the logic...correctly at first, but then I must 
have become temporarily dazed by the raw destructive power of the pre-existing 
BUG_ON() statement, and screwed it up after all. :)


thanks,
-- 
John Hubbard
NVIDIA