Kernel: Stop ProcFS stack walk on bogus userspace->kernel traversal

Unsurprisingly, the /proc/PID/stacks/TID stack walk had the same
arbitrary memory read problem as the perf event stack walk.

It would be nice if the kernel had a single stack walk implementation,
but that's outside the scope of this commit.

Kernel/Arch/x86/common/Processor.cpp

@@ -471,6 +471,7 @@ Vector<FlatPtr> Processor::capture_stack_trace(Thread& thread, size_t max_frames
 
     auto walk_stack = [&](FlatPtr stack_ptr) {
         static constexpr size_t max_stack_frames = 4096;
+        bool is_walking_userspace_stack = false;
         stack_trace.append(ip);
         size_t count = 1;
         while (stack_ptr && stack_trace.size() < max_stack_frames) {
@@ -480,6 +481,15 @@ Vector<FlatPtr> Processor::capture_stack_trace(Thread& thread, size_t max_frames
             if (max_frames != 0 && count > max_frames)
                 break;
 
+            if (!Memory::is_user_address(VirtualAddress { stack_ptr })) {
+                if (is_walking_userspace_stack) {
+                    dbgln("SHENANIGANS! Userspace stack points back into kernel memory");
+                    break;
+                }
+            } else {
+                is_walking_userspace_stack = true;
+            }
+
             if (Memory::is_user_range(VirtualAddress(stack_ptr), sizeof(FlatPtr) * 2)) {
                 if (copy_from_user(&retaddr, &((FlatPtr*)stack_ptr)[1]).is_error() || !retaddr)
                     break;