fix: remove cached scratch_base_gpa from HyperlightVm

The scratch_base_gpa field was computed once at construction time but
update_scratch_mapping() can be called with different-sized scratch
regions. This is a correctness hazard. Revert to upstream's approach
of computing the GPA inline from the scratch region's actual size.

Signed-off-by: danbugs <danilochiarlone@gmail.com>

Author: danbugs

src/hyperlight_host/src/hypervisor/hyperlight_vm.rs

@@ -166,8 +166,6 @@ pub(crate) struct HyperlightVm {
     // The current scratch region, used to keep it alive as long as it
     // is used & when unmapping
     scratch_memory: Option<GuestSharedMemory>,
-    // The resolved base GPA of the scratch region
-    scratch_base_gpa: u64,
 
     mmap_regions: Vec<(u32, MemoryRegion)>, // Later mapped regions (slot number, region)
 
@@ -474,8 +472,6 @@ impl HyperlightVm {
 
         let snapshot_slot = 0u32;
         let scratch_slot = 1u32;
-        let scratch_base_gpa =
-            hyperlight_common::layout::scratch_base_gpa(scratch_mem.mem_size());
         #[cfg_attr(not(gdb), allow(unused_mut))]
         let mut ret = Self {
             vm,
@@ -491,7 +487,6 @@ impl HyperlightVm {
             snapshot_memory: None,
             scratch_slot,
             scratch_memory: None,
-            scratch_base_gpa,
 
             mmap_regions: Vec::new(),
 
@@ -670,11 +665,11 @@ impl HyperlightVm {
         &mut self,
         scratch: GuestSharedMemory,
     ) -> Result<(), UpdateRegionError> {
-        let guest_base = self.scratch_base_gpa;
+        let guest_base = hyperlight_common::layout::scratch_base_gpa(scratch.mem_size());
         let rgn = scratch.mapping_at(guest_base, MemoryRegionType::Scratch);
 
         if let Some(old_scratch) = self.scratch_memory.replace(scratch) {
-            let old_base = self.scratch_base_gpa;
+            let old_base = hyperlight_common::layout::scratch_base_gpa(old_scratch.mem_size());
             let old_rgn = old_scratch.mapping_at(old_base, MemoryRegionType::Scratch);
             self.vm.unmap_memory((self.scratch_slot, &old_rgn))?;
         }