fix(guest): prefault exception handler counter

A guest exception handler runs on the IST1 stack. If the handler writes
a copy-on-write page, the first write faults. The page fault also uses
IST1, so the CPU resets RSP to the top of IST1 and writes the fault
frame over the live handler frame. The handler returns to a bad address
and the guest aborts.

The test exception handler increments a counter that starts on a
copy-on-write page. Write the counter once in install_handler so its
page copies into scratch before the handler runs. The handler then
increments a writable page and does not fault.

Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>

Author: ludfjig

src/tests/rust_guests/simpleguest/src/main.rs

@@ -134,6 +134,11 @@ fn test_exception_handler(
 /// Install handler for a specific vector
 #[guest_function("InstallHandler")]
 fn install_handler(vector: i32) {
+    // The exception handler increments a counter that starts on a
+    // copy-on-write page. Write the counter once here so its page copies
+    // into scratch. The handler then increments a writable page and does
+    // not fault while it runs on the exception stack.
+    HANDLER_INVOCATION_COUNT.fetch_add(0, Ordering::SeqCst);
     hyperlight_guest_bin::exception::arch::HANDLERS[vector as usize].store(
         test_exception_handler as *const () as usize as u64,
         Ordering::Release,