bpf: Relax return code check for subprograms

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

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6204ec705d809952e3c4a84ec1e7c68ee070b2a7..1388bf73307191c6cfea96c0b7c68db020c9cca9 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -7786,9 +7786,11 @@ static int check_return_code(struct bpf_verifier_env *env)
        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;
@@ -7808,6 +7810,16 @@ static int check_return_code(struct bpf_verifier_env *env)
                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 ||
@@ -7861,7 +7873,6 @@ static int check_return_code(struct bpf_verifier_env *env)
                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]);

diff --git a/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c b/tools/testing/selftests/bpf/prog_tests/test_global_funcs.c
index 193002b14d7f66ac9c1b6942f79d662760278ac7..32e4348b714bcef612d9ce9687b09993a84fdb6f 100644 (file)
--- 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;

diff --git a/tools/testing/selftests/bpf/progs/test_global_func8.c b/tools/testing/selftests/bpf/progs/test_global_func8.c
new file mode 100644 (file)
index 0000000..d55a654
--- /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;
+}