[PAL/LinuxSGX] protect from both time and tsc rewind in ocall_gettime

Signed-off-by: Jonathan Shamir <jsham92@gmail.com>

pal/src/host/linux-sgx/enclave_ocalls.c

@@ -1768,49 +1768,82 @@ int ocall_shutdown(int sockfd, int how) {
 
 int ocall_gettime(uint64_t* microsec_ptr, uint64_t* tsc_ptr) {
     int retval = 0;
-    struct ocall_gettime* ocall_gettime_args;
+    struct ocall_gettime* ocall_gettime_args = NULL;
 
     void* old_ustack = sgx_prepare_ustack();
     ocall_gettime_args = sgx_alloc_on_ustack_aligned(sizeof(*ocall_gettime_args),
                                                      alignof(*ocall_gettime_args));
     if (!ocall_gettime_args) {
-        sgx_reset_ustack(old_ustack);
-        return -EPERM;
+        retval = -EPERM;
+        goto out;
     }
 
     /* Last seen time value. This guards against time rewinding. */
-    static uint64_t last_microsec = 0;
-    uint64_t last_microsec_before_ocall = __atomic_load_n(&last_microsec, __ATOMIC_ACQUIRE);
+    struct gettime_guard
+    {
+        spinlock_t lock;
+        uint64_t microsec;
+        uint64_t tsc;
+    };
+    static struct gettime_guard last_value = { .microsec = 0, .tsc = 0, };
+    if (FIRST_TIME()) {
+        spinlock_init(&last_value.lock);
+    }
+
+    spinlock_lock(&last_value.lock);
+    uint64_t last_microsec_before_ocall = last_value.microsec;
+    uint64_t last_tsc_before_ocall = last_value.tsc;
+    spinlock_unlock(&last_value.lock);
+
+    uint64_t tsc_before_ocall = 0;
+    uint64_t tsc_after_ocall = 0;
     do {
+        tsc_before_ocall = get_tsc();
         retval = sgx_exitless_ocall(OCALL_GETTIME, ocall_gettime_args);
+        tsc_after_ocall = get_tsc();
     } while (retval == -EINTR);
 
     if (retval < 0 && retval != -EINVAL && retval != -EPERM) {
         retval = -EPERM;
     }
-
     if (!retval) {
-        uint64_t microsec = COPY_UNTRUSTED_VALUE(&ocall_gettime_args->microsec);
-        if (microsec < last_microsec_before_ocall) {
-            /* Probably a malicious host. */
-            log_error("OCALL_GETTIME returned time value smaller than in the previous call");
-            _PalProcessExit(1);
-        }
-        /* Update `last_microsec`. */
-        uint64_t expected_microsec = last_microsec_before_ocall;
-        while (expected_microsec < microsec) {
-            if (__atomic_compare_exchange_n(&last_microsec, &expected_microsec, microsec,
-                                            /*weak=*/true, __ATOMIC_RELEASE, __ATOMIC_ACQUIRE)) {
-                break;
-            }
-        }
+        goto out;
+    }
 
-        *microsec_ptr = MAX(microsec, expected_microsec);
-        if (tsc_ptr != NULL) {
-            *tsc_ptr = COPY_UNTRUSTED_VALUE(&ocall_gettime_args->tsc);
-        }
+    /* detect malicious host - time and tsc must monotonically increase */
+    uint64_t new_microsec = COPY_UNTRUSTED_VALUE(&ocall_gettime_args->microsec);
+    uint64_t new_tsc = COPY_UNTRUSTED_VALUE(&ocall_gettime_args->tsc);
+    if (new_microsec < last_microsec_before_ocall) {
+        log_error("OCALL_GETTIME returned time value smaller than in the previous call");
+        _PalProcessExit(1);
+    }
+    if (new_tsc <= last_tsc_before_ocall) {
+        log_error("OCALL_GETTIME returned TSC value smaller than in previous call");
+        _PalProcessExit(1);
+    }
+    if (!((tsc_before_ocall < new_tsc) && (new_tsc < tsc_after_ocall))) {
+        log_error("OCALL_GETTIME returned TSC value inconsistent with values taken within the enclave");
+        _PalProcessExit(1);
     }
 
+    /* Update `last_value` guard. */
+    spinlock_lock(&last_value.lock);
+    if (last_value.tsc < new_tsc) {
+        last_value.microsec = new_microsec;
+        last_value.tsc = new_tsc;
+    } else {
+        /* there was a more recent ocall */
+        new_microsec = last_value.microsec;
+        new_tsc = last_value.tsc;
+    }
+    spinlock_unlock(&last_value.lock);
+
+    *microsec_ptr = new_microsec;
+    if (tsc_ptr != NULL) {
+        *tsc_ptr = new_tsc;
+    }
+
+out:
     sgx_reset_ustack(old_ustack);
     return retval;
 }