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

[Jiri Olsa <jolsa@kernel.org>]

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.
+	 */
+	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)
-- 
2.38.1