linux-kernel - [PATCH 3.10 100/319] fix fault_in_multipages_...() on architectures with no-op access

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Message-Id: <1486322541-8206-1-git-send-email-w@1wt.eu>
Date:   Sun,  5 Feb 2017 20:18:42 +0100
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 100/319] fix fault_in_multipages_...() on architectures with no-op access_ok()

From: Al Viro <viro@...IV.linux.org.uk>

commit e23d4159b109167126e5bcd7f3775c95de7fee47 upstream.

Switching iov_iter fault-in to multipages variants has exposed an old
bug in underlying fault_in_multipages_...(); they break if the range
passed to them wraps around.  Normally access_ok() done by callers will
prevent such (and it's a guaranteed EFAULT - ERR_PTR() values fall into
such a range and they should not point to any valid objects).

However, on architectures where userland and kernel live in different
MMU contexts (e.g. s390) access_ok() is a no-op and on those a range
with a wraparound can reach fault_in_multipages_...().

Since any wraparound means EFAULT there, the fix is trivial - turn
those

    while (uaddr <= end)
	    ...
into

    if (unlikely(uaddr > end))
	    return -EFAULT;
    do
	    ...
    while (uaddr <= end);

Reported-by: Jan Stancek <jstancek@...hat.com>
Tested-by: Jan Stancek <jstancek@...hat.com>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Willy Tarreau <w@....eu>
---
 include/linux/pagemap.h | 38 +++++++++++++++++++-------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e3dea75..9497527 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -482,56 +482,56 @@ static inline int fault_in_pages_readable(const char __user *uaddr, int size)
  */
 static inline int fault_in_multipages_writeable(char __user *uaddr, int size)
 {
-	int ret = 0;
 	char __user *end = uaddr + size - 1;
 
 	if (unlikely(size == 0))
-		return ret;
+		return 0;
 
+	if (unlikely(uaddr > end))
+		return -EFAULT;
 	/*
 	 * Writing zeroes into userspace here is OK, because we know that if
 	 * the zero gets there, we'll be overwriting it.
 	 */
-	while (uaddr <= end) {
-		ret = __put_user(0, uaddr);
-		if (ret != 0)
-			return ret;
+	do {
+		if (unlikely(__put_user(0, uaddr) != 0))
+			return -EFAULT;
 		uaddr += PAGE_SIZE;
-	}
+	} while (uaddr <= end);
 
 	/* Check whether the range spilled into the next page. */
 	if (((unsigned long)uaddr & PAGE_MASK) ==
 			((unsigned long)end & PAGE_MASK))
-		ret = __put_user(0, end);
+		return __put_user(0, end);
 
-	return ret;
+	return 0;
 }
 
 static inline int fault_in_multipages_readable(const char __user *uaddr,
 					       int size)
 {
 	volatile char c;
-	int ret = 0;
 	const char __user *end = uaddr + size - 1;
 
 	if (unlikely(size == 0))
-		return ret;
+		return 0;
 
-	while (uaddr <= end) {
-		ret = __get_user(c, uaddr);
-		if (ret != 0)
-			return ret;
+	if (unlikely(uaddr > end))
+		return -EFAULT;
+
+	do {
+		if (unlikely(__get_user(c, uaddr) != 0))
+			return -EFAULT;
 		uaddr += PAGE_SIZE;
-	}
+	} while (uaddr <= end);
 
 	/* Check whether the range spilled into the next page. */
 	if (((unsigned long)uaddr & PAGE_MASK) ==
 			((unsigned long)end & PAGE_MASK)) {
-		ret = __get_user(c, end);
-		(void)c;
+		return __get_user(c, end);
 	}
 
-	return ret;
+	return 0;
 }
 
 int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
-- 
2.8.0.rc2.1.gbe9624a