From: Dmitrii Banshchikov <me@ubique.spb.ru>
Date: Fri, 13 Nov 2020 17:17:56 +0000
Subject: bpf: Relax return code check for subprograms
Patch-mainline: v5.10-rc5
Git-commit: f782e2c300a717233b64697affda3ea7aac00b2b
References: bsc#1177028

Currently verifier enforces return code checks for subprograms in the
same manner as it does for program entry points. This prevents returning
arbitrary scalar values from subprograms. Scalar type of returned values
is checked by btf_prepare_func_args() and hence it should be safe to
allow only scalars for now. Relax return code checks for subprograms and
allow any correct scalar values.

Fixes: 51c39bb1d5d10 (bpf: Introduce function-by-function verification)
Signed-off-by: Dmitrii Banshchikov <me@ubique.spb.ru>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201113171756.90594-1-me@ubique.spb.ru
Acked-by: Gary Lin <glin@suse.com>
---
 kernel/bpf/verifier.c                                      |   15 ++++++++--
 tools/testing/selftests/bpf/prog_tests/test_global_funcs.c |    1 
 tools/testing/selftests/bpf/progs/test_global_func8.c      |   19 +++++++++++++
 3 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_global_func8.c

--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7418,9 +7418,11 @@ static int check_return_code(struct bpf_
 	struct tnum range = tnum_range(0, 1);
 	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	int err;
+	const bool is_subprog = env->cur_state->frame[0]->subprogno;
 
 	/* LSM and struct_ops func-ptr's return type could be "void" */
-	if ((prog_type == BPF_PROG_TYPE_STRUCT_OPS ||
+	if (!is_subprog &&
+	    (prog_type == BPF_PROG_TYPE_STRUCT_OPS ||
 	     prog_type == BPF_PROG_TYPE_LSM) &&
 	    !prog->aux->attach_func_proto->type)
 		return 0;
@@ -7440,6 +7442,16 @@ static int check_return_code(struct bpf_
 		return -EACCES;
 	}
 
+	reg = cur_regs(env) + BPF_REG_0;
+	if (is_subprog) {
+		if (reg->type != SCALAR_VALUE) {
+			verbose(env, "At subprogram exit the register R0 is not a scalar value (%s)\n",
+				reg_type_str[reg->type]);
+			return -EINVAL;
+		}
+		return 0;
+	}
+
 	switch (prog_type) {
 	case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
 		if (env->prog->expected_attach_type == BPF_CGROUP_UDP4_RECVMSG ||
@@ -7493,7 +7505,6 @@ static int check_return_code(struct bpf_
 		return 0;
 	}
 
-	reg = cur_regs(env) + BPF_REG_0;
 	if (reg->type != SCALAR_VALUE) {
 		verbose(env, "At program exit the register R0 is not a known value (%s)\n",
 			reg_type_str[reg->type]);
--- a/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
@@ -60,6 +60,7 @@ void test_test_global_funcs(void)
 		{ "test_global_func5.o" , "expected pointer to ctx, but got PTR" },
 		{ "test_global_func6.o" , "modified ctx ptr R2" },
 		{ "test_global_func7.o" , "foo() doesn't return scalar" },
+		{ "test_global_func8.o" },
 	};
 	libbpf_print_fn_t old_print_fn = NULL;
 	int err, i, duration = 0;
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_global_func8.c
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2020 Facebook */
+#include <stddef.h>
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+__noinline int foo(struct __sk_buff *skb)
+{
+	return bpf_get_prandom_u32();
+}
+
+SEC("cgroup_skb/ingress")
+int test_cls(struct __sk_buff *skb)
+{
+	if (!foo(skb))
+		return 0;
+
+	return 1;
+}