linux-kernel - Re: [PATCH] fanotify: Make sure to check event

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Date:   Wed, 5 Dec 2018 13:10:36 +0100
From:   Jan Kara <jack@...e.cz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     keescook@...omium.org, Jan Kara <jack@...e.cz>, xypron.glpk@....de,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fanotify: Make sure to check event_len when copying

On Wed 05-12-18 09:15:44, Amir Goldstein wrote:
> On Wed, Dec 5, 2018 at 1:44 AM Kees Cook <keescook@...omium.org> wrote:
> >
> > As a precaution, make sure we check event_len when copying to userspace.
> > Based on old feedback: https://lkml.kernel.org/r/542D9FE5.3010009@gmx.de
> >
> 
> This precaution serves the new fanotify_event_info_fid format well:
> https://marc.info/?l=linux-fsdevel&m=154375073318106&w=2
> but also conflicts with my series.
> 
> I can weave your patch into my series and add a similar sanity check in
> copy_fid_to_user() or if Jan applies your patch and has more comments
> on my v4 series, I'll rebase on top of that.

Yeah, I don't think there's any need for you to rebase just now. Either
I'll fixup your patch or you'll do the rebase if there are further changes
to the series...

								Honza

> 
> Thanks,
> Amir.
> 
> > Signed-off-by: Kees Cook <keescook@...omium.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
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR