linux-kernel - Re: [PATCH 4.19 105/116] bpf: Explicitly memset the bpf

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Message-ID: <20200402191936.GA3243295@kroah.com>
Date:   Thu, 2 Apr 2020 21:19:36 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Pavel Machek <pavel@...x.de>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Maciej Żenczykowski <maze@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Alexander Potapenko <glider@...gle.com>,
        Alistair Delva <adelva@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH 4.19 105/116] bpf: Explicitly memset the bpf_attr
 structure

On Thu, Apr 02, 2020 at 08:53:21PM +0200, Pavel Machek wrote:
> Hi!
> 
> > From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > 
> > commit 8096f229421f7b22433775e928d506f0342e5907 upstream.
> > 
> > For the bpf syscall, we are relying on the compiler to properly zero out
> > the bpf_attr union that we copy userspace data into. Unfortunately that
> > doesn't always work properly, padding and other oddities might not be
> > correctly zeroed, and in some tests odd things have been found when the
> > stack is pre-initialized to other values.
> > 
> > Fix this by explicitly memsetting the structure to 0 before using
> > it.
> 
> Is not that a gcc bug?

No.

> I mean, that's seriously unhelpful behaviour from security
> perspective.

I totally agree, and it is something we have been playing whack-a-mole
over for a number of years now.

Nothing new, but we do have a config option to zero out the stack all
the time if you are feeling paranoid and can take the performance hit.

> Is there any reason to believe this is not causing problems elsewhere?

It probably is, please feel free to audit and fix up the remaining
issues that you find.

thanks,

greg k-h