Re: [PATCH bpf-next] bpf: Restrict attachment of bpf program to some tracepoints

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Nov 24, 2022 at 01:41:23AM +0100, Daniel Borkmann wrote:
> On 11/21/22 10:31 PM, Jiri Olsa wrote:
> > We hit following issues [1] [2] when we attach bpf program that calls
> > bpf_trace_printk helper to the contention_begin tracepoint.
> > 
> > As described in [3] with multiple bpf programs that call bpf_trace_printk
> > helper attached to the contention_begin might result in exhaustion of
> > printk buffer or cause a deadlock [2].
> > 
> > There's also another possible deadlock when multiple bpf programs attach
> > to bpf_trace_printk tracepoint and call one of the printk bpf helpers.
> > 
> > This change denies the attachment of bpf program to contention_begin
> > and bpf_trace_printk tracepoints if the bpf program calls one of the
> > printk bpf helpers.
> > 
> > Adding also verifier check for tb_btf programs, so this can be cought
> > in program loading time with error message like:
> > 
> >    Can't attach program with bpf_trace_printk#6 helper to contention_begin tracepoint.
> > 
> > [1] https://lore.kernel.org/bpf/CACkBjsakT_yWxnSWr4r-0TpPvbKm9-OBmVUhJb7hV3hY8fdCkw@mail.gmail.com/
> > [2] https://lore.kernel.org/bpf/CACkBjsaCsTovQHFfkqJKto6S4Z8d02ud1D7MPESrHa1cVNNTrw@mail.gmail.com/
> > [3] https://lore.kernel.org/bpf/Y2j6ivTwFmA0FtvY@krava/
> > 
> > Reported-by: Hao Sun <sunhao.th@gmail.com>
> > Suggested-by: Alexei Starovoitov <ast@kernel.org>
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >   include/linux/bpf.h          |  1 +
> >   include/linux/bpf_verifier.h |  2 ++
> >   kernel/bpf/syscall.c         |  3 +++
> >   kernel/bpf/verifier.c        | 46 ++++++++++++++++++++++++++++++++++++
> >   4 files changed, 52 insertions(+)
> > 
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index c9eafa67f2a2..3ccabede0f50 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1319,6 +1319,7 @@ struct bpf_prog {
> >   				enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */
> >   				call_get_stack:1, /* Do we call bpf_get_stack() or bpf_get_stackid() */
> >   				call_get_func_ip:1, /* Do we call get_func_ip() */
> > +				call_printk:1, /* Do we call trace_printk/trace_vprintk  */
> >   				tstamp_type_access:1; /* Accessed __sk_buff->tstamp_type */
> >   	enum bpf_prog_type	type;		/* Type of BPF program */
> >   	enum bpf_attach_type	expected_attach_type; /* For some prog types */
> > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> > index 545152ac136c..7118c2fda59d 100644
> > --- a/include/linux/bpf_verifier.h
> > +++ b/include/linux/bpf_verifier.h
> > @@ -618,6 +618,8 @@ bool is_dynptr_type_expected(struct bpf_verifier_env *env,
> >   			     struct bpf_reg_state *reg,
> >   			     enum bpf_arg_type arg_type);
> > +int bpf_check_tp_printk_denylist(const char *name, struct bpf_prog *prog);
> > +
> >   /* this lives here instead of in bpf.h because it needs to dereference tgt_prog */
> >   static inline u64 bpf_trampoline_compute_key(const struct bpf_prog *tgt_prog,
> >   					     struct btf *btf, u32 btf_id)
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 35972afb6850..9a69bda7d62b 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -3329,6 +3329,9 @@ static int bpf_raw_tp_link_attach(struct bpf_prog *prog,
> >   		return -EINVAL;
> >   	}
> > +	if (bpf_check_tp_printk_denylist(tp_name, prog))
> > +		return -EACCES;
> > +
> >   	btp = bpf_get_raw_tracepoint(tp_name);
> >   	if (!btp)
> >   		return -ENOENT;
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index f07bec227fef..b662bc851e1c 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -7472,6 +7472,47 @@ static void update_loop_inline_state(struct bpf_verifier_env *env, u32 subprogno
> >   				 state->callback_subprogno == subprogno);
> >   }
> > +int bpf_check_tp_printk_denylist(const char *name, struct bpf_prog *prog)
> > +{
> > +	static const char * const denylist[] = {
> > +		"contention_begin",
> > +		"bpf_trace_printk",
> > +	};
> > +	int i;
> > +
> > +	/* Do not allow attachment to denylist[] tracepoints,
> > +	 * if the program calls some of the printk helpers,
> > +	 * because there's possibility of deadlock.
> > +	 */
> 
> What if that prog doesn't but tail calls into another one which calls printk helpers?

right, I'll deny that for all BPF_PROG_TYPE_RAW_TRACEPOINT* programs,
because I don't see easy way to check on that

we can leave printk check for tracing BPF_TRACE_RAW_TP programs,
because verifier known the exact tracepoint already

thanks,
jirka

> 
> > +	if (!prog->call_printk)
> > +		return 0;
> > +
> > +	for (i = 0; i < ARRAY_SIZE(denylist); i++) {
> > +		if (!strcmp(denylist[i], name))
> > +			return 1;
> > +	}
> > +	return 0;
> > +}
> > +
> > +static int check_tp_printk_denylist(struct bpf_verifier_env *env, int func_id)
> > +{
> > +	struct bpf_prog *prog = env->prog;
> > +
> > +	if (prog->type != BPF_PROG_TYPE_TRACING ||
> > +	    prog->expected_attach_type != BPF_TRACE_RAW_TP)
> > +		return 0;
> > +
> > +	if (WARN_ON_ONCE(!prog->aux->attach_func_name))
> > +		return -EINVAL;
> > +
> > +	if (!bpf_check_tp_printk_denylist(prog->aux->attach_func_name, prog))
> > +		return 0;
> > +
> > +	verbose(env, "Can't attach program with %s#%d helper to %s tracepoint.\n",
> > +		func_id_name(func_id), func_id, prog->aux->attach_func_name);
> > +	return -EACCES;
> > +}
> > +
> >   static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> >   			     int *insn_idx_p)
> >   {
> > @@ -7675,6 +7716,11 @@ static int check_helper_call(struct bpf_verifier_env *env, struct bpf_insn *insn
> >   		err = __check_func_call(env, insn, insn_idx_p, meta.subprogno,
> >   					set_user_ringbuf_callback_state);
> >   		break;
> > +	case BPF_FUNC_trace_printk:
> > +	case BPF_FUNC_trace_vprintk:
> > +		env->prog->call_printk = 1;
> > +		err = check_tp_printk_denylist(env, func_id);
> > +		break;
> >   	}
> >   	if (err)
> > 
>