netdev - Re: [PATCH] lsm: make security_socket_getpeersec_stream() sockptr

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Date:   Fri, 14 Oct 2022 08:51:32 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        selinux@...r.kernel.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH] lsm: make security_socket_getpeersec_stream() sockptr_t safe

On Thu, Oct 13, 2022 at 8:59 AM Paul Moore <paul@...l-moore.com> wrote:
>
> On Thu, Oct 13, 2022 at 11:53 AM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Mon, 10 Oct 2022 17:58:29 -0400 Paul Moore wrote:
> > > Commit 4ff09db1b79b ("bpf: net: Change sk_getsockopt() to take the
> > > sockptr_t argument") made it possible to call sk_getsockopt()
> > > with both user and kernel address space buffers through the use of
> > > the sockptr_t type.  Unfortunately at the time of conversion the
> > > security_socket_getpeersec_stream() LSM hook was written to only
> > > accept userspace buffers, and in a desire to avoid having to change
> > > the LSM hook the commit author simply passed the sockptr_t's
> > > userspace buffer pointer.  Since the only sk_getsockopt() callers
> > > at the time of conversion which used kernel sockptr_t buffers did
> > > not allow SO_PEERSEC, and hence the
> > > security_socket_getpeersec_stream() hook, this was acceptable but
> > > also very fragile as future changes presented the possibility of
> > > silently passing kernel space pointers to the LSM hook.
> > >
> > > There are several ways to protect against this, including careful
> > > code review of future commits, but since relying on code review to
> > > catch bugs is a recipe for disaster and the upstream eBPF maintainer
> > > is "strongly against defensive programming", this patch updates the
> > > LSM hook, and all of the implementations to support sockptr_t and
> > > safely handle both user and kernel space buffers.
> >
> > Code seems sane, FWIW, but the commit message sounds petty,
> > which is likely why nobody is willing to ack it.
>
> Heh, feel free to look at Alexei's comments to my original email; the
> commit description seems spot on to me.

Paul,

The commit message:
"
also very fragile as future changes presented the possibility of
silently passing kernel space pointers to the LSM hook.
"
shows that you do not understand how copy_from_user works.

Martin's change didn't introduce any fragility.
Do you realize that user space can pass any 64-bit value
as 'user pointer' via syscall, right?
And that value may just as well be a valid kernel address.
copy_from_user always had a check to prevent reading kernel
memory. It will simply return an error when it sees
kernel address.

Your patch itself is not wrong per-se, but it's doing
not what you think it's doing.
Right now the patch is useless, but
if switch statement in sol_socket_sockopt() would be relaxed
the bpf progs would be able to pass kernel pointers
to security_socket_getpeersec which makes little sense at this point.
So the code you're adding will be a dead code without a test
for the foreseeable future.
Because of that I can only add my Nack.