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Date:   Fri, 31 Jul 2020 10:00:30 +0300
From:   Leon Romanovsky <leon@kernel.org>
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peilin Ye <yepeilin.cs@gmail.com>,
        Santosh Shilimkar <santosh.shilimkar@oracle.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Arnd Bergmann <arnd@arndb.de>,
        "linux-kernel-mentees@lists.linuxfoundation.org" 
        <linux-kernel-mentees@lists.linuxfoundation.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>,
        "rds-devel@oss.oracle.com" <rds-devel@oss.oracle.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak
 in rds_notify_queue_get()
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On Fri, Jul 31, 2020 at 09:29:27AM +0300, Andy Shevchenko wrote:
> On Friday, July 31, 2020, Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> wrote:
>
> > On Fri, Jul 31, 2020 at 07:33:06AM +0200, Greg Kroah-Hartman wrote:
> > > On Fri, Jul 31, 2020 at 07:53:01AM +0300, Leon Romanovsky wrote:
> > > > On Thu, Jul 30, 2020 at 03:20:26PM -0400, Peilin Ye wrote:
> > > > > rds_notify_queue_get() is potentially copying uninitialized kernel
> > stack
> > > > > memory to userspace since the compiler may leave a 4-byte hole at
> > the end
> > > > > of `cmsg`.
> > > > >
> > > > > In 2016 we tried to fix this issue by doing `= { 0 };` on `cmsg`,
> > which
> > > > > unfortunately does not always initialize that 4-byte hole. Fix it by
> > using
> > > > > memset() instead.
> > > >
> > > > Of course, this is the difference between "{ 0 }" and "{}"
> > initializations.
> > >
> > > Really?  Neither will handle structures with holes in it, try it and
> > > see.
>
>
> {} is a GCC extension, but I never thought it works differently.

Yes, this is GCC extension and kernel relies on them very heavily.

Thanks

>
>
>
> >
> > And if true, where in the C spec does it say that?
> >
> > thanks,
> >
> > greg k-h
> >
>
>
> --
> With Best Regards,
> Andy Shevchenko