2 files changed, 20 insertions, 14 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index 807a463fd5ed..1ed4be2a7654 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1892,7 +1892,7 @@ int remove_suid(struct dentry *dentry)
 EXPORT_SYMBOL(remove_suid);
 
 size_t
-__filemap_copy_from_user_iovec(char *vaddr, 
+__filemap_copy_from_user_iovec_inatomic(char *vaddr,
                         const struct iovec *iov, size_t base, size_t bytes)
 {
         size_t copied = 0, left = 0;
@@ -1908,12 +1908,8 @@ __filemap_copy_from_user_iovec(char *vaddr,
                 vaddr += copy;
                 iov++;
 
-                if (unlikely(left)) {
-                        /* zero the rest of the target like __copy_from_user */
-                        if (bytes)
-                                memset(vaddr, 0, bytes);
+                if (unlikely(left))
                         break;
-                }
         }
         return copied - left;
 }
diff --git a/mm/filemap.h b/mm/filemap.h
index 5683cde22055..536979fb4ba7 100644
--- a/mm/filemap.h
+++ b/mm/filemap.h
@@ -16,15 +16,23 @@
 #include <linux/uaccess.h>
 
 size_t
-__filemap_copy_from_user_iovec(char *vaddr,
-                               const struct iovec *iov,
-                               size_t base,
-                               size_t bytes);
+__filemap_copy_from_user_iovec_inatomic(char *vaddr,
+                                        const struct iovec *iov,
+                                        size_t base,
+                                        size_t bytes);
 
 /*
  * Copy as much as we can into the page and return the number of bytes which
  * were sucessfully copied.  If a fault is encountered then clear the page
  * out to (offset+bytes) and return the number of bytes which were copied.
+ *
+ * NOTE: For this to work reliably we really want copy_from_user_inatomic_nocache
+ * to *NOT* zero any tail of the buffer that it failed to copy.  If it does,
+ * and if the following non-atomic copy succeeds, then there is a small window
+ * where the target page contains neither the data before the write, nor the
+ * data after the write (it contains zero).  A read at this time will see
+ * data that is inconsistent with any ordering of the read and the write.
+ * (This has been detected in practice).
  */
 static inline size_t
 filemap_copy_from_user(struct page *page, unsigned long offset,
@@ -60,13 +68,15 @@ filemap_copy_from_user_iovec(struct page *page, unsigned long offset,
         size_t copied;
 
         kaddr = kmap_atomic(page, KM_USER0);
-        copied = __filemap_copy_from_user_iovec(kaddr + offset, iov,
-                                                base, bytes);
+        copied = __filemap_copy_from_user_iovec_inatomic(kaddr + offset, iov,
+                                                         base, bytes);
         kunmap_atomic(kaddr, KM_USER0);
         if (copied != bytes) {
                 kaddr = kmap(page);
-                copied = __filemap_copy_from_user_iovec(kaddr + offset, iov,
-                                                        base, bytes);
+                copied = __filemap_copy_from_user_iovec_inatomic(kaddr + offset, iov,
+                                                                 base, bytes);
+                if (bytes - copied)
+                        memset(kaddr + offset + copied, 0, bytes - copied);
                 kunmap(page);
         }
         return copied;