linux-kernel - Re: [PATCH v2 1/2] samples, selftests/seccomp: Zero out seccomp

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Date:   Sat, 28 Dec 2019 17:18:18 -0700
From:   Tycho Andersen <tycho@...ho.ws>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Kees Cook <keescook@...omium.org>,
        Aleksa Sarai <cyphar@...har.com>
Subject: Re: [PATCH v2 1/2] samples, selftests/seccomp: Zero out seccomp_notif

On Sat, Dec 28, 2019 at 07:10:29PM -0500, Sargun Dhillon wrote:
> On Sat, Dec 28, 2019 at 1:18 PM Tycho Andersen <tycho@...ho.ws> wrote:
> >
> > On Sat, Dec 28, 2019 at 01:48:39AM +0000, Sargun Dhillon wrote:
> > > The seccomp_notif structure should be zeroed out prior to calling the
> > > SECCOMP_IOCTL_NOTIF_RECV ioctl. Previously, the kernel did not check
> > > whether these structures were zeroed out or not, so these worked.
> > >
> > > Signed-off-by: Sargun Dhillon <sargun@...gun.me>
> > > Cc: Kees Cook <keescook@...omium.org>
> > > ---
> > >  samples/seccomp/user-trap.c                   | 2 +-
> > >  tools/testing/selftests/seccomp/seccomp_bpf.c | 2 ++
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
> > > index 6d0125ca8af7..0ca8fb37cd79 100644
> > > --- a/samples/seccomp/user-trap.c
> > > +++ b/samples/seccomp/user-trap.c
> > > @@ -298,7 +298,6 @@ int main(void)
> > >               req = malloc(sizes.seccomp_notif);
> > >               if (!req)
> > >                       goto out_close;
> > > -             memset(req, 0, sizeof(*req));
> > >
> > >               resp = malloc(sizes.seccomp_notif_resp);
> > >               if (!resp)
> > > @@ -306,6 +305,7 @@ int main(void)
> > >               memset(resp, 0, sizeof(*resp));
> >
> > I know it's unrelated, but it's probably worth sending a patch to fix
> > this to be sizes.seccomp_notif_resp instead of sizeof(*resp), since if
> > the kernel is older this will over-zero things. I can do that, or you
> > can add the patch to this series, just let me know which.
> 
> I was thinking about this, and initially, I chose to make the smaller
> change. I think it might make more sense to combine the patch,
> given that the memset behaviour is "incorrect" if we do it based on
> sizeof(*req), or sizeof(*resp).
> 
> I'll go ahead and respin this patch with the change to call memset
> based on sizes.

I think it would be good to keep it as a separate patch, since it's an
unrelated bug fix. That way if we have to revert these because of some
breakage, we won't lose the fix.

Cheers,

Tycho