executor: arm64: sys/linux: introduce syzos API

Allow guest payload to call syzos API functions. The available calls
are enumerated by SYZOS_API_* constants, and have a form of:

  struct api_call {
         uint64 call;
         uint64 struct_size;
         /* arbitrary call-related data here */
  };

Complex instruction sequences are too easy to break, so most of the time
fuzzer won't be able to efficiently mutate them.
We replace kvm_text_arm64 with a sequence of `struct api_call`, making it
possible to intermix assembly instructions (SYZOS_API_CODE) with
higher-level constructs.

Right now the supported calls are:
 - SYZOS_API_UEXIT - abort from KVM_RUN (1 argument: exit code, uint64)
 - SYZOS_API_CODE - execute an ARM64 assembly blob
                    (1 argument: inline array of int32's)

1 files changed, 63 insertions, 3 deletions

diff --git a/executor/common_kvm_arm64_syzos.h b/executor/common_kvm_arm64_syzos.h
index b9edf0069..022378b34 100644
--- a/executor/common_kvm_arm64_syzos.h
+++ b/executor/common_kvm_arm64_syzos.h
@@ -11,10 +11,70 @@
 // Start/end of the guest section.
 extern char *__start_guest, *__stop_guest;
 
+typedef enum {
+	SYZOS_API_UEXIT,
+	SYZOS_API_CODE,
+	SYZOS_API_STOP, // Must be the last one
+} syzos_api_id;
+
+struct api_call_header {
+	uint64 call;
+	uint64 size;
+};
+
+struct api_call_uexit {
+	struct api_call_header header;
+	uint64 exit_code;
+};
+
+struct api_call_code {
+	struct api_call_header header;
+	uint32 insns[];
+};
+
+void guest_uexit(uint64 exit_code);
+void guest_execute_code(uint32* insns, uint64 size);
+
 // Main guest function that performs necessary setup and passes the control to the user-provided
 // payload.
-GUEST_CODE static void guest_main()
+GUEST_CODE static void guest_main(uint64 size)
+{
+	uint64 addr = ARM64_ADDR_USER_CODE;
+
+	while (size >= sizeof(struct api_call_header)) {
+		struct api_call_header* cmd = (struct api_call_header*)addr;
+		if (cmd->call >= SYZOS_API_STOP)
+			return;
+		if (cmd->size > size)
+			return;
+		switch (cmd->call) {
+		case SYZOS_API_UEXIT: {
+			struct api_call_uexit* ucmd = (struct api_call_uexit*)cmd;
+			guest_uexit(ucmd->exit_code);
+			break;
+		}
+		case SYZOS_API_CODE: {
+			struct api_call_code* ccmd = (struct api_call_code*)cmd;
+			guest_execute_code(ccmd->insns, cmd->size - sizeof(struct api_call_header));
+			break;
+		}
+		}
+		addr += cmd->size;
+		size -= cmd->size;
+	};
+}
+
+GUEST_CODE void guest_execute_code(uint32* insns, uint64 size)
+{
+	volatile void (*fn)() = (volatile void (*)())insns;
+	fn();
+}
+
+// Perform a userspace exit that can be handled by the host.
+// The host returns from ioctl(KVM_RUN) with kvm_run.exit_reason=KVM_EXIT_MMIO,
+// and can handle the call depending on the data passed as exit code.
+GUEST_CODE void guest_uexit(uint64 exit_code)
 {
-	void (*guest_payload)() = (void (*)())ARM64_ADDR_USER_CODE;
-	guest_payload();
+	volatile uint64* ptr = (volatile uint64*)ARM64_ADDR_UEXIT;
+	*ptr = exit_code;
 }