linux-kernel - [PATCH] mm: tighten fault_in_pages

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Date:	Tue, 26 Apr 2016 13:59:12 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] mm: tighten fault_in_pages_writeable()

From: Eric Dumazet <edumazet@...gle.com>

copy_page_to_iter_iovec() is currently the only user of
fault_in_pages_writeable(), and it definitely can use fragments
from high order pages.

Make sure fault_in_pages_writeable() is only touching two
adjacent pages at most, as claimed.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/linux/pagemap.h |   24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 7e1ab155c67c..83eb89e8d276 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -518,33 +518,27 @@ void page_endio(struct page *page, int rw, int err);
 extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
 
 /*
- * Fault a userspace page into pagetables.  Return non-zero on a fault.
- *
- * This assumes that two userspace pages are always sufficient.
+ * Fault one or two userspace pages into pagetables.
+ * Return -EINVAL if more than two pages would be needed.
+ * Return non-zero on a fault.
  */
 static inline int fault_in_pages_writeable(char __user *uaddr, int size)
 {
-	int ret;
+	int span, ret;
 
 	if (unlikely(size == 0))
 		return 0;
 
+	span = offset_in_page(uaddr) + size;
+	if (span > 2 * PAGE_SIZE)
+		return -EINVAL;
 	/*
 	 * Writing zeroes into userspace here is OK, because we know that if
 	 * the zero gets there, we'll be overwriting it.
 	 */
 	ret = __put_user(0, uaddr);
-	if (ret == 0) {
-		char __user *end = uaddr + size - 1;
-
-		/*
-		 * If the page was already mapped, this will get a cache miss
-		 * for sure, so try to avoid doing it.
-		 */
-		if (((unsigned long)uaddr & PAGE_MASK) !=
-				((unsigned long)end & PAGE_MASK))
-			ret = __put_user(0, end);
-	}
+	if (ret == 0 && span > PAGE_SIZE)
+		ret = __put_user(0, uaddr + size - 1);
 	return ret;
 }