Kernel: Prevent execve/ptrace race

Add a per-process ptrace lock and use it to prevent ptrace access to a
process after it decides to commit to a new executable in sys$execve().

Fixes #5230.

Kernel/Process.h

@@ -408,10 +408,8 @@ public:
         return m_thread_count.load(AK::MemoryOrder::memory_order_relaxed);
     }
 
-    Lock& big_lock()
+    Lock& big_lock() { return m_big_lock; }
-    {
+    Lock& ptrace_lock() { return m_ptrace_lock; }
-        return m_big_lock;
-    }
 
     Custody& root_directory();
     Custody& root_directory_relative_to_global_root();
@@ -579,6 +577,7 @@ private:
     size_t m_master_tls_alignment { 0 };
 
     Lock m_big_lock { "Process" };
+    Lock m_ptrace_lock { "ptrace" };
 
     RefPtr<Timer> m_alarm_timer;
 

Kernel/Syscalls/execve.cpp

@@ -483,6 +483,9 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
 
     // We commit to the new executable at this point. There is no turning back!
 
+    // Prevent other processes from attaching to us with ptrace while we're doing this.
+    Locker ptrace_locker(ptrace_lock());
+
     // Disable profiling temporarily in case it's running on this process.
     TemporaryChange profiling_disabler(m_profiling, false);
 

Kernel/Syscalls/ptrace.cpp

@@ -57,6 +57,8 @@ static KResultOr<u32> handle_ptrace(const Kernel::Syscall::SC_ptrace_params& par
     if (!peer)
         return ESRCH;
 
+    Locker ptrace_locker(peer->process().ptrace_lock());
+
     if ((peer->process().uid() != caller.euid())
         || (peer->process().uid() != peer->process().euid())) // Disallow tracing setuid processes
         return EACCES;