Re: WARNING: bad usercopy in fanotify_read

[Kees Cook <keescook@chromium.org>]

On Mon, Mar 11, 2019 at 1:42 PM syzbot
<syzbot+2c49971e251e36216d1f@syzkaller.appspotmail.com> wrote:
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17ee410b200000
> [...]
> ------------[ cut here ]------------
> Bad or missing usercopy whitelist? Kernel memory exposure attempt detected
> from SLAB object 'fanotify_event' (offset 40, size 8)!
> [...]
>   copy_to_user include/linux/uaccess.h:151 [inline]
>   copy_fid_to_user fs/notify/fanotify/fanotify_user.c:236 [inline]
>   copy_event_to_user fs/notify/fanotify/fanotify_user.c:294 [inline]

Looks like this is the fh/ext_fh union in struct fanotify_fid, field
"fid" in struct fanotify_event. Given that "fid" is itself in a union
against a struct path, I think instead of a whitelist using
KMEM_CACHE_USERCOPY(), this should just use a bounce buffer to avoid
leaving a whitelist open for path or ext_fh exposure.

Maybe something like this (untested):

diff --git a/fs/notify/fanotify/fanotify_user.c
b/fs/notify/fanotify/fanotify_user.c
index 56992b32c6bb..b87da9580b3c 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -207,6 +207,7 @@ static int process_access_response(struct
fsnotify_group *group,
 static int copy_fid_to_user(struct fanotify_event *event, char __user *buf)
 {
        struct fanotify_event_info_fid info = { };
+       unsigned char bounce[FANOTIFY_INLINE_FH_LEN], *fh;
        struct file_handle handle = { };
        size_t fh_len = event->fh_len;
        size_t len = fanotify_event_info_len(event);
@@ -233,7 +234,18 @@ static int copy_fid_to_user(struct fanotify_event
*event, char __user *buf)

        buf += sizeof(handle);
        len -= sizeof(handle);
-       if (copy_to_user(buf, fanotify_event_fh(event), fh_len))
+
+       /*
+        * For an inline fh, copy through stack to exclude the copy from
+        * usercopy hardening protections.
+        */
+       fh = fanotify_event_fh(event);
+       if (fh_len <= sizeof(bounce)) {
+               memcpy(bounce, fh, fh_len);
+               fh = bounce;
+       }
+
+       if (copy_to_user(buf, fh, fh_len))
                return -EFAULT;

        /* Pad with 0's */


-- 
Kees Cook