kernel: Mask away unsafe bits in rflags when restoring state after a signal

kernel/src/arch/x86_64/CPU.h

@@ -18,6 +18,16 @@ struct FPData
     void save();
     void restore();
 
+    void* data()
+    {
+        return (void*)m_data;
+    }
+
+    usize size() const
+    {
+        return 512;
+    }
+
   private:
     char m_data[512] alignas(16);
     bool m_already_saved;

kernel/src/arch/x86_64/Thread.cpp

@@ -111,8 +111,11 @@ bool Thread::deliver_signal(int signo, Registers* current_regs)
 
     regs.rsp -= 128; // Skip the red zone
 
+    fp_data.save();
+
     if (push_mem_on_stack((u8*)current_regs, sizeof(*current_regs)).has_error()) return false;
     if (push_mem_on_stack((u8*)&signal_mask, sizeof(signal_mask)).has_error()) return false;
+    if (push_mem_on_stack((u8*)fp_data.data(), fp_data.size()).has_error()) return false;
 
     u64 rsp = regs.rsp;
 
@@ -147,16 +150,20 @@ void Thread::sigreturn(Registers* current_regs)
 {
     memcpy(&regs, current_regs, sizeof(regs));
 
+    u64 rflags = current_regs->rflags;
+
     u64 rsp;
     pop_mem_from_stack((u8*)&rsp, sizeof(rsp));
     regs.rsp = rsp;
+    pop_mem_from_stack((u8*)fp_data.data(), fp_data.size());
     pop_mem_from_stack((u8*)&signal_mask, sizeof(signal_mask));
     pop_mem_from_stack((u8*)current_regs, sizeof(*current_regs));
     memcpy(&regs, current_regs, sizeof(regs));
     regs.cs = 0x18 | 3;
     regs.ss = 0x20 | 3;
-    // FIXME: Using this, a program can craft a special RFLAGS that gives them a higher IOPL or other stuff. Find out
+    regs.rflags = (rflags & ~0xdff) | (regs.rflags & 0xdff);
-    // exactly what bits to block from modifying.
+
+    fp_data.restore();
 
     kinfoln("sigreturn: restored program state, sp=%p, ip=%p", (void*)regs.rsp, (void*)regs.rip);