[PATCH] fanotify: Make sure to check event_len when copying

[Kees Cook]

As a precaution, make sure we check event_len when copying to userspace.
Based on old feedback: https://lkml.kernel.org/r/542d9fe5.3010...@gmx.de

Signed-off-by: Kees Cook <keesc...@chromium.org>
---
 fs/notify/fanotify/fanotify_user.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c 
b/fs/notify/fanotify/fanotify_user.c
index e03be5071362..d9484a0ac6b3 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -206,7 +206,7 @@ static int process_access_response(struct fsnotify_group 
*group,
 
 static ssize_t copy_event_to_user(struct fsnotify_group *group,
                                  struct fsnotify_event *event,
-                                 char __user *buf)
+                                 char __user *buf, size_t count)
 {
        struct fanotify_event_metadata fanotify_event_metadata;
        struct file *f;
@@ -220,6 +220,12 @@ static ssize_t copy_event_to_user(struct fsnotify_group 
*group,
 
        fd = fanotify_event_metadata.fd;
        ret = -EFAULT;
+       /*
+        * Sanity check copy size in case get_one_event() and
+        * fill_event_metadata() event_len sizes ever get out of sync.
+        */
+       if (WARN_ON_ONCE(fanotify_event_metadata.event_len > count))
+               goto out_close_fd;
        if (copy_to_user(buf, &fanotify_event_metadata,
                         fanotify_event_metadata.event_len))
                goto out_close_fd;
@@ -295,7 +301,7 @@ static ssize_t fanotify_read(struct file *file, char __user 
*buf,
                        continue;
                }
 
-               ret = copy_event_to_user(group, kevent, buf);
+               ret = copy_event_to_user(group, kevent, buf, count);
                if (unlikely(ret == -EOPENSTALE)) {
                        /*
                         * We cannot report events with stale fd so drop it.
-- 
2.17.1


-- 
Kees Cook