[PATCH bpf 2/2] selftests/bpf: BPF_PSEUDO_FUNC reference to the main program

[Eduard Zingerman <eddyz87@gmail.com>]

Add a test case for a BPF_PSEUDO_FUNC load instruction that references
the entry function of the program it belongs to. W/o the previous
patch the verifier accepts this program thus allowing a runtime call
at a bogus address. See previous patch for detailed description.

Main function needs to be marked with BTF_FUNC_STATIC for the test to
trigger the bug, the patch uses test_verifier harness instead of
test_prog because libbpf has no way to convey this.

Reported-by: Nicholas Carlini <npc@anthropic.com>
Suggested-by: Nicholas Carlini <npc@anthropic.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/verifier/pseudo_func.c      | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/verifier/pseudo_func.c

diff --git a/tools/testing/selftests/bpf/verifier/pseudo_func.c b/tools/testing/selftests/bpf/verifier/pseudo_func.c
new file mode 100644
index 000000000000..63c5c67d51de
--- /dev/null
+++ b/tools/testing/selftests/bpf/verifier/pseudo_func.c
@@ -0,0 +1,45 @@
+/*
+ * Buggy verifier accepted the program below while not patching BPF_PSEUDO_FUNC
+ * load instruction to contain a real address. Which resulted in a function call
+ * to a bogus address.
+ */
+{
+	"BPF_PSEUDO_FUNC reference to the main program",
+	.insns = {
+	/* r6 = bpf_map_lookup_elem(&timer_map, &(int){0}); */
+	BPF_ST_MEM(BPF_W, BPF_REG_10, -4, 0),
+	BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4),
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem),
+	BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 10),
+	BPF_MOV64_REG(BPF_REG_6, BPF_REG_0),
+	/* bpf_timer_init(r6, &timer_map, 0); */
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
+	BPF_LD_MAP_FD(BPF_REG_2, 0),
+	BPF_MOV64_IMM(BPF_REG_3, 0),
+	BPF_EMIT_CALL(BPF_FUNC_timer_init),
+	/* bpf_timer_set_callback(r6, <insn #0>); */
+	BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
+	BPF_RAW_INSN(BPF_LD | BPF_IMM | BPF_DW, BPF_REG_2, BPF_PSEUDO_FUNC, 0, -15),
+	BPF_RAW_INSN(0, 0, 0, 0, 0),
+	BPF_EMIT_CALL(BPF_FUNC_timer_set_callback),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.prog_type = BPF_PROG_TYPE_TRACEPOINT,
+	.fixup_map_timer = { 3, 9 },
+	.result = REJECT,
+	.errstr = "callback function cannot be the main program",
+	.func_info = { { 0, 4 /* main_prog */ } },
+	.func_info_cnt = 1,
+	.btf_strings = "\0int\0ctx\0main_prog",
+	.btf_types = {
+	/* 1: int            */ BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),
+	/* 2: void*          */ BTF_PTR_ENC(0),
+	/* 3: int __(void *) */ BTF_FUNC_PROTO_ENC(1, 1),
+				BTF_FUNC_PROTO_ARG_ENC(5, 2),
+	/* 4: main_prog      */ BTF_FUNC_ENC(9, 3),
+	BTF_END_RAW
+	}
+},
-- 
2.55.0