Re: WARNING: bad usercopy in fanotify_read

[Jan Kara <jack@suse.cz>]

On Tue 12-03-19 23:26:22, Kees Cook wrote:
> 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.

Do you mean to protect it from a situation when some other code (i.e. not
copy_fid_to_user()) would be tricked into copying ext_fh containing slab
pointer to userspace?

								Honza

> 
> 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
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR