Kernel: Demonstrate race condition in clock_nanosleep

This adds a test for the race condition in clock_nanosleep.
The crux is that clock_nanosleep verifies that the output buffer
is writable *before* sleeping, and writes to it *after* sleeping.
In the meantime, a concurrent thread can make the output buffer
unwritable, e.g. by deallocating it.

This testcase is needlessly complex because pthread_kill is
not implemented yet.  I tried to keep it as simple as possible.

Here is the relevant part of dmesg:
[nanosleep-race-outbuf-munmap(22:22)]: Unblock nanosleep-race-outbuf-munmap(20:20) due to signal
nanosleep-race-outbuf-munmap(20:20) Unrecoverable page fault, write to address 0x02130016
CRASH: Page Fault. Process: nanosleep-race-outbuf-munmap(20)
[nanosleep-race-outbuf-munmap(20:20)]: 0xc01160ff  memcpy +44
[nanosleep-race-outbuf-munmap(20:20)]: 0xc014de64  Kernel::Process::crash(int, unsigned int) +782
[nanosleep-race-outbuf-munmap(20:20)]: 0xc01191b5  illegal_instruction_handler +0
[nanosleep-race-outbuf-munmap(20:20)]: 0xc011965b  page_fault_handler +649
[nanosleep-race-outbuf-munmap(20:20)]: 0xc0117233  page_fault_asm_entry +22
[nanosleep-race-outbuf-munmap(20:20)]: 0xc011616b  copy_to_user +102
[nanosleep-race-outbuf-munmap(20:20)]: 0xc015911f  Kernel::Process::sys(Kernel::Syscall::SC_clock_nanosleep_params const*) +457
[nanosleep-race-outbuf-munmap(20:20)]: 0xc015daad  syscall_handler +1130
[nanosleep-race-outbuf-munmap(20:20)]: 0xc015d597  syscall_asm_entry +29
[nanosleep-race-outbuf-munmap(20:20)]: 0x08048437  main +146
[nanosleep-race-outbuf-munmap(20:20)]: 0x08048573  _start +94

Most importantly, note that it crashes *inside*
Kernel::Process::sys.
Instead, the correct behavior is to return -EFAULT.

Tests/Kernel/nanosleep-race-outbuf-munmap.cpp

@@ -0,0 +1,155 @@
+/*
+ * Copyright (c) 2020, Ben Wiederhake <BenWiederhake.GitHub@gmx.de>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <pthread.h>
+#include <assert.h>
+#include <signal.h>
+#include <stdio.h>
+#include <time.h>
+#include <unistd.h>
+
+static void signal_printer(int)
+{
+    // no-op
+}
+
+typedef struct yank_shared_t {
+    timespec* remaining_sleep;
+    // TODO: Be nice and use thread ID
+    //pthread_t sleeper_thread;
+} yank_shared_t;
+
+static void* yanker_fn(void* shared_)
+{
+    yank_shared_t* shared = static_cast<yank_shared_t*>(shared_);
+
+    timespec requested_sleep = { 1, 0 };
+    int rc = clock_nanosleep(CLOCK_MONOTONIC, 0, &requested_sleep, nullptr);
+    if (rc != 0) {
+        printf("Yanker: Failed during sleep: %d\n", rc);
+        return nullptr;
+    }
+
+    delete shared->remaining_sleep; // T2
+
+    // Send SIGUSR1.
+
+    // Use pthread:
+    // pthread_kill(somewhere, SIGUSR1);
+    // But wait!  pthread_kill isn't implemented yet, and therefore causes
+    // a linker error.  It also looks like the corresponding syscall is missing.
+
+    // Use normal IPC syscall:
+    // kill(getpid(), SIGUSR1);
+    // But wait!  If destination_pid == own_pid, then the signal is sent
+    // to the calling thread, *no matter what*.
+
+    // So, we have to go the very ugly route of fork():
+    // (Thank goodness this is only a demo of a kernel bug!)
+    pid_t pid_to_kill = getpid();
+
+    pid_t child_pid = fork();
+    if (child_pid < 0) {
+        printf("Yanker: Fork failed: %d\n", child_pid);
+        pthread_exit(nullptr); // See below
+        return nullptr;
+    }
+
+    if (child_pid > 0) {
+        // Success.  Terminate quickly.  T3
+        // FIXME: LibPthread bug: returning during normal operation causes nullptr deref.
+        // Workaround: Exit manually.
+        pthread_exit(nullptr);
+        return nullptr;
+    }
+
+    // Give parent *thread* a moment to die.
+    requested_sleep = { 1, 0 };
+    rc = clock_nanosleep(CLOCK_MONOTONIC, 0, &requested_sleep, nullptr);
+    if (rc != 0) {
+        printf("Yanker-child: Failed during sleep: %d\n", rc);
+        return nullptr;
+    }
+
+    // Prod the parent *process*
+    kill(pid_to_kill, SIGUSR1); // T4
+
+    // Wait a moment, to ensure the log output is as well-separated as possible.
+    requested_sleep = { 2, 0 };
+    rc = clock_nanosleep(CLOCK_MONOTONIC, 0, &requested_sleep, nullptr);
+    if (rc != 0) {
+        printf("Yanker-child: Failed during after-sleep: %d\n", rc);
+        return nullptr;
+    }
+
+    pthread_exit(nullptr);
+    assert(false);
+    // FIXME: return nullptr;
+}
+
+int main()
+{
+    // Chronological order:
+    // T0: Main thread allocates region for the outvalue of clock_nanosleep
+    // T1: Main thread enters clock_nanosleep
+    // T2: Side thread deallocates that region
+    // T3: Side thread dies
+    // T4: A different process sends SIGUSR1, waking up the main thread,
+    //     forcing the kernel to write to the deallocated Region.
+
+    // I'm sorry that both a side *thread* and a side *process* are necessary.
+    // Maybe in the future this test can be simplified, see above.
+
+    yank_shared_t shared = { nullptr };
+    shared.remaining_sleep = new timespec({ 0xbad, 0xf00d }); // T0
+
+    pthread_t yanker_thread;
+    int rc = pthread_create(&yanker_thread, nullptr, yanker_fn, &shared);
+    if (rc != 0) {
+        perror("pthread");
+        printf("FAIL\n");
+        return 1;
+    }
+
+    // Set an action for SIGUSR1, so that the sleep can be interrupted:
+    signal(SIGUSR1, signal_printer);
+
+    // T1: Go to sleep.
+    const timespec requested_sleep = { 3, 0 };
+    rc = clock_nanosleep(CLOCK_MONOTONIC, 0, &requested_sleep, shared.remaining_sleep);
+    // Now we are beyond T4.
+
+    if (rc == 0) {
+        // We somehow weren't interrupted.  Bad.
+        printf("Not interrupted.\n");
+        printf("FAIL\n");
+        return 1;
+    }
+
+    // nanosleep was interrupted and the kernel didn't crash.  Good!
+    printf("PASS\n");
+    return 0;
+}