fix: Validate guest address ranges for overlapping regions in map_region

Add overlap validation to HyperlightVm::map_region to enforce the
safety contract documented on VirtualMachine::map_memory, which
requires that memory regions must not overlap.

The check validates the new region against:
- Existing dynamically mapped regions (mmap_regions)
- The snapshot region (starting at BASE_ADDRESS)
- The scratch region (at the top of the GPA space)

Add Overlapping variant to MapRegionError with a descriptive message
showing both the new and conflicting ranges.

Add four tests covering:
- Exact duplicate mapping rejection
- Partial overlap rejection
- Adjacent non-overlapping regions (should succeed)
- Overlap with the snapshot region

Signed-off-by: Richard Durkee <Richard-Durkee@users.noreply.github.com>

Author: Richard-Durkee

src/hyperlight_host/src/hypervisor/hyperlight_vm/mod.rs

@@ -243,6 +243,10 @@ pub enum MapRegionError {
     MapMemory(#[from] MapMemoryError),
     #[error("Region is not page-aligned (page size: {0:#x})")]
     NotPageAligned(usize),
+    #[error(
+        "Region [{0:#x}..{1:#x}) overlaps existing region [{2:#x}..{3:#x})"
+    )]
+    Overlapping(usize, usize, usize, usize),
 }
 
 /// Errors that can occur when unmapping a memory region
@@ -420,6 +424,44 @@ impl HyperlightVm {
             return Err(MapRegionError::NotPageAligned(self.page_size));
         }
 
+        let new_start = region.guest_region.start;
+        let new_end = region.guest_region.end;
+
+        // Check against existing dynamically mapped regions
+        for (_, existing) in &self.mmap_regions {
+            if new_start < existing.guest_region.end && new_end > existing.guest_region.start {
+                return Err(MapRegionError::Overlapping(
+                    new_start,
+                    new_end,
+                    existing.guest_region.start,
+                    existing.guest_region.end,
+                ));
+            }
+        }
+
+        // Check against the snapshot region
+        if let Some(ref snapshot) = self.snapshot_memory {
+            let snap_start = crate::mem::layout::SandboxMemoryLayout::BASE_ADDRESS;
+            let snap_end = snap_start + snapshot.mem_size();
+            if new_start < snap_end && new_end > snap_start {
+                return Err(MapRegionError::Overlapping(
+                    new_start, new_end, snap_start, snap_end,
+                ));
+            }
+        }
+
+        // Check against the scratch region
+        if let Some(ref scratch) = self.scratch_memory {
+            let scratch_start =
+                hyperlight_common::layout::scratch_base_gpa(scratch.mem_size()) as usize;
+            let scratch_end = scratch_start + scratch.mem_size();
+            if new_start < scratch_end && new_end > scratch_start {
+                return Err(MapRegionError::Overlapping(
+                    new_start, new_end, scratch_start, scratch_end,
+                ));
+            }
+        }
+
         // Try to reuse a freed slot first, otherwise use next_slot
         let slot = if let Some(freed_slot) = self.freed_slots.pop() {
             freed_slot

src/hyperlight_host/src/sandbox/initialized_multi_use.rs

@@ -2551,4 +2551,101 @@ mod tests {
         }
         let _ = std::fs::remove_file(&path);
     }
+
+    #[test]
+    fn map_region_rejects_overlapping_regions() {
+        let mut sbox: MultiUseSandbox = {
+            let path = simple_guest_as_string().unwrap();
+            let u_sbox = UninitializedSandbox::new(GuestBinary::FilePath(path), None).unwrap();
+            u_sbox.evolve().unwrap()
+        };
+
+        let mem1 = allocate_guest_memory();
+        let mem2 = allocate_guest_memory();
+        let guest_base: usize = 0x200000000;
+        let region1 = region_for_memory(&mem1, guest_base, MemoryRegionFlags::READ);
+
+        // First mapping should succeed
+        unsafe { sbox.map_region(&region1).unwrap() };
+
+        // Exact same range should fail
+        let region2 = region_for_memory(&mem2, guest_base, MemoryRegionFlags::READ);
+        let err = unsafe { sbox.map_region(&region2) }.unwrap_err();
+        assert!(
+            format!("{err:?}").contains("Overlapping"),
+            "Expected Overlapping error, got: {err:?}"
+        );
+    }
+
+    #[test]
+    fn map_region_rejects_partial_overlap() {
+        let mut sbox: MultiUseSandbox = {
+            let path = simple_guest_as_string().unwrap();
+            let u_sbox = UninitializedSandbox::new(GuestBinary::FilePath(path), None).unwrap();
+            u_sbox.evolve().unwrap()
+        };
+
+        let mem1 = allocate_guest_memory();
+        let mem2 = allocate_guest_memory();
+        let guest_base: usize = 0x200000000;
+        let region1 = region_for_memory(&mem1, guest_base, MemoryRegionFlags::READ);
+        let region_size = mem1.mem_size();
+
+        unsafe { sbox.map_region(&region1).unwrap() };
+
+        // Overlapping from below (starts before, ends inside)
+        let overlap_base = guest_base - region_size / 2;
+        // Align to page boundary
+        let overlap_base = overlap_base & !(0x1000 - 1);
+        let region2 = region_for_memory(&mem2, overlap_base, MemoryRegionFlags::READ);
+        let err = unsafe { sbox.map_region(&region2) }.unwrap_err();
+        assert!(
+            format!("{err:?}").contains("Overlapping"),
+            "Expected Overlapping error for partial overlap, got: {err:?}"
+        );
+    }
+
+    #[test]
+    fn map_region_allows_adjacent_non_overlapping() {
+        let mut sbox: MultiUseSandbox = {
+            let path = simple_guest_as_string().unwrap();
+            let u_sbox = UninitializedSandbox::new(GuestBinary::FilePath(path), None).unwrap();
+            u_sbox.evolve().unwrap()
+        };
+
+        let mem1 = allocate_guest_memory();
+        let mem2 = allocate_guest_memory();
+        let guest_base: usize = 0x200000000;
+        let region1 = region_for_memory(&mem1, guest_base, MemoryRegionFlags::READ);
+        let region_size = mem1.mem_size();
+
+        unsafe { sbox.map_region(&region1).unwrap() };
+
+        // Adjacent region (starts right after the first one ends) should succeed
+        let adjacent_base = guest_base + region_size;
+        let region2 = region_for_memory(&mem2, adjacent_base, MemoryRegionFlags::READ);
+        unsafe { sbox.map_region(&region2).unwrap() };
+    }
+
+    #[test]
+    fn map_region_rejects_overlap_with_snapshot() {
+        let mut sbox: MultiUseSandbox = {
+            let path = simple_guest_as_string().unwrap();
+            let u_sbox = UninitializedSandbox::new(GuestBinary::FilePath(path), None).unwrap();
+            u_sbox.evolve().unwrap()
+        };
+
+        // Try to map at BASE_ADDRESS (0x1000) which overlaps the snapshot region
+        let mem = allocate_guest_memory();
+        let region = region_for_memory(
+            &mem,
+            crate::mem::layout::SandboxMemoryLayout::BASE_ADDRESS,
+            MemoryRegionFlags::READ,
+        );
+        let err = unsafe { sbox.map_region(&region) }.unwrap_err();
+        assert!(
+            format!("{err:?}").contains("Overlapping"),
+            "Expected Overlapping error for snapshot overlap, got: {err:?}"
+        );
+    }
 }