fix fault_in_multipages_...() on architectures with no-op access_ok()

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@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Cc: stable@vger.kernel.org # v3.5+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 19 deletions

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 66a1260b33de..7e3d53753612 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -571,56 +571,56 @@ 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,