Re: [PATCH bpf-next v2 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM

From: KP Singh <kpsingh@chromium.org>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: "KP Singh" <kpsingh@chromium.org>,
	linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"James Morris" <jmorris@namei.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Thomas Garnier" <thgarnie@chromium.org>,
	"Michael Halcrow" <mhalcrow@google.com>,
	"Paul Turner" <pjt@google.com>,
	"Brendan Gregg" <brendan.d.gregg@gmail.com>,
	"Jann Horn" <jannh@google.com>,
	"Matthew Garrett" <mjg59@google.com>,
	"Christian Brauner" <christian@brauner.io>,
	"Mickaël Salaün" <mic@digikod.net>,
	"Florent Revest" <revest@chromium.org>,
	"Brendan Jackman" <jackmanb@chromium.org>,
	"Martin KaFai Lau" <kafai@fb.com>,
	"Song Liu" <songliubraving@fb.com>, "Yonghong Song" <yhs@fb.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	"Mauro Carvalho Chehab" <mchehab+samsung@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Nicolas Ferre" <nicolas.ferre@microchip.com>,
	"Stanislav Fomichev" <sdf@google.com>,
	"Quentin Monnet" <quentin.monnet@netronome.com>,
	"Andrey Ignatov" <rdna@fb.com>, "Joe Stringer" <joe@wand.net.nz>
Subject: Re: [PATCH bpf-next v2 04/10] bpf: lsm: Add mutable hooks list for the BPF LSM
Date: Thu, 16 Jan 2020 10:48:47 +0100
Message-ID: <20200116094847.GB240584@google.com> (raw)
In-Reply-To: <cd1d9d9f-1b68-8d2c-118a-334e4c71eb57@tycho.nsa.gov>

On 15-Jan 12:30, Stephen Smalley wrote:
> On 1/15/20 12:13 PM, KP Singh wrote:
> > From: KP Singh <kpsingh@google.com>
> > 
> > - The list of hooks registered by an LSM is currently immutable as they
> >    are declared with __lsm_ro_after_init and they are attached to a
> >    security_hook_heads struct.
> > - For the BPF LSM we need to de/register the hooks at runtime. Making
> >    the existing security_hook_heads mutable broadens an
> >    attack vector, so a separate security_hook_heads is added for only
> >    those that ~must~ be mutable.
> > - These mutable hooks are run only after all the static hooks have
> >    successfully executed.
> > 
> > This is based on the ideas discussed in:
> > 
> >    https://lore.kernel.org/lkml/20180408065916.GA2832@ircssh-2.c.rugged-nimbus-611.internal
> > 
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > ---
> [...]
> > diff --git a/security/security.c b/security/security.c
> > index cd2d18d2d279..4a2eb4c089b2 100644
> > --- a/security/security.c
> > +++ b/security/security.c
> > @@ -652,20 +653,21 @@ static void __init lsm_early_task(struct task_struct *task)
> >   								\
> >   		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) \
> >   			P->hook.FUNC(__VA_ARGS__);		\
> > +		CALL_BPF_LSM_VOID_HOOKS(FUNC, __VA_ARGS__);	\
> >   	} while (0)
> > -#define call_int_hook(FUNC, IRC, ...) ({			\
> > -	int RC = IRC;						\
> > -	do {							\
> > -		struct security_hook_list *P;			\
> > -								\
> > +#define call_int_hook(FUNC, IRC, ...) ({				\
> > +	int RC = IRC;							\
> > +	do {								\
> > +		struct security_hook_list *P;				\
> >   		hlist_for_each_entry(P, &security_hook_heads.FUNC, list) { \
> > -			RC = P->hook.FUNC(__VA_ARGS__);		\
> > -			if (RC != 0)				\
> > -				break;				\
> > -		}						\
> > -	} while (0);						\
> > -	RC;							\
> > +			RC = P->hook.FUNC(__VA_ARGS__);			\
> > +			if (RC != 0)					\
> > +				break;					\
> > +		}							\
> > +		RC = CALL_BPF_LSM_INT_HOOKS(RC, FUNC, __VA_ARGS__);	\
> 
> Let's not clobber the return code from the other LSMs with the bpf one.

Good catch and thanks for pointing it out. Should be fixed in v3.

- KP

> 
> > +	} while (0);							\
> > +	RC;								\
> >   })
> >   /* Security operations */
> > 
> 