bpf: fix off-by-one error in adjust_subprog_starts

When patching in a new sequence for the first insn of a subprog, the start
 of that subprog does not change (it's the first insn of the sequence), so
 adjust_subprog_starts should check start <= off (rather than < off).
Also added a test to test_verifier.c (it's essentially the syz reproducer).

Fixes: cc8b0b92a169 ("bpf: introduce function calls (function boundaries)")
Reported-by: syzbot+4fc427c7af994b0948be@syzkaller.appspotmail.com
Signed-off-by: Edward Cree <ecree@solarflare.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -5650,7 +5650,7 @@ static void adjust_subprog_starts(struct bpf_verifier_env *env, u32 off, u32 len
 		return;
 	/* NOTE: fake 'exit' subprog should be updated as well. */
 	for (i = 0; i <= env->subprog_cnt; i++) {
-		if (env->subprog_info[i].start < off)
+		if (env->subprog_info[i].start <= off)
 			continue;
 		env->subprog_info[i].start += len - 1;
 	}

tools/testing/selftests/bpf/test_verifier.c

@@ -13896,6 +13896,25 @@ static struct bpf_test tests[] = {
 		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 		.result = ACCEPT,
 	},
+	{
+		"calls: ctx read at start of subprog",
+		.insns = {
+			BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),
+			BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 1, 0, 5),
+			BPF_JMP_REG(BPF_JSGT, BPF_REG_0, BPF_REG_0, 0),
+			BPF_MOV64_REG(BPF_REG_1, BPF_REG_6),
+			BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 1, 0, 2),
+			BPF_MOV64_REG(BPF_REG_1, BPF_REG_0),
+			BPF_EXIT_INSN(),
+			BPF_LDX_MEM(BPF_B, BPF_REG_9, BPF_REG_1, 0),
+			BPF_MOV64_IMM(BPF_REG_0, 0),
+			BPF_EXIT_INSN(),
+		},
+		.prog_type = BPF_PROG_TYPE_SOCKET_FILTER,
+		.errstr_unpriv = "function calls to other bpf functions are allowed for root only",
+		.result_unpriv = REJECT,
+		.result = ACCEPT,
+	},
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)