harden VTL memory protection

Author: Sangho Lee

litebox_platform_lvbs/src/lib.rs

@@ -628,6 +628,11 @@ impl<Host: HostInterface> LinuxKernel<Host> {
         CPU_MHZ.store(cpu_mhz, core::sync::atomic::Ordering::Relaxed);
     }
 
+    /// Returns the physical frame range belonging to VTL1.
+    pub fn vtl1_phys_frame_range(&self) -> PhysFrameRange<Size4KiB> {
+        self.vtl1_phys_frame_range
+    }
+
     /// This function maps VTL0 physical page frames containing the physical addresses
     /// from `phys_start` to `phys_end` to the VTL1 kernel page table. It internally page aligns
     /// the input addresses to ensure the mapped memory area covers the entire input addresses

litebox_platform_lvbs/src/mshv/error.rs

@@ -41,6 +41,9 @@ pub enum VsmError {
     #[error("invalid physical address")]
     InvalidPhysicalAddress,
 
+    #[error("physical address range overlaps with VTL1 memory")]
+    Vtl1MemoryOverlap,
+
     // Memory/Data Errors
     #[error("invalid memory attributes")]
     MemoryAttributeInvalid,
@@ -172,7 +175,8 @@ impl From<VsmError> for Errno {
             | VsmError::BootSignalWriteFailed
             | VsmError::CpuOnlineMaskCopyFailed
             | VsmError::HekiPagesCopyFailed
-            | VsmError::Vtl0CopyFailed => Errno::EFAULT,
+            | VsmError::Vtl0CopyFailed
+            | VsmError::Vtl1MemoryOverlap => Errno::EFAULT,
 
             // Not found errors
             VsmError::SystemCertificatesNotFound

litebox_platform_lvbs/src/mshv/vsm.rs

@@ -85,13 +85,10 @@ pub(crate) fn init(is_bsp: bool) {
     if is_bsp {
         if let Ok((start, size)) = get_vtl1_memory_info() {
             debug_serial_println!("VSM: Protect GPAs from {:#x} to {:#x}", start, start + size);
-            if protect_physical_memory_range(
-                PhysFrame::range(
-                    PhysFrame::containing_address(PhysAddr::new(start)),
-                    PhysFrame::containing_address(PhysAddr::new(start + size)),
-                ),
-                MemAttr::empty(),
-            )
+            if protect_vtl1_physical_memory_range(PhysFrame::range(
+                PhysFrame::containing_address(PhysAddr::new(start)),
+                PhysFrame::containing_address(PhysAddr::new(start + size)),
+            ))
             .is_err()
             {
                 panic!("Failed to protect VTL1 memory");
@@ -1306,14 +1303,20 @@ fn copy_heki_pages_from_vtl0(pa: u64, nranges: u64) -> Option<Vec<HekiPage>> {
     Some(heki_pages)
 }
 
-/// This function protects a physical memory range. It is a safe wrapper for `hv_modify_vtl_protection_mask`.
-/// `phys_frame_range` specifies the physical frame range to protect
-/// `mem_attr` specifies the memory attributes to be applied to the range
+/// This function protects a VTL0 physical memory range from potentially compromised VTL0 by
+/// restricting its access permissions using VTL protection mask (e.g., kernel code integrity).
+/// It is a safe wrapper for `hv_modify_vtl_protection_mask`. `phys_frame_range` specifies the
+/// physical frame range to protect (must belong to VTL0) `mem_attr` specifies the memory
+/// attributes (VTL0's allowed access) to be applied to the range
 #[inline]
 pub(crate) fn protect_physical_memory_range(
     phys_frame_range: PhysFrameRange<Size4KiB>,
     mem_attr: MemAttr,
 ) -> Result<(), VsmError> {
+    let vtl1_range = crate::platform_low().vtl1_phys_frame_range();
+    if phys_frame_range.start < vtl1_range.end && vtl1_range.start < phys_frame_range.end {
+        return Err(VsmError::Vtl1MemoryOverlap);
+    }
     let pa = phys_frame_range.start.start_address().as_u64();
     let num_pages = phys_frame_range.count() as u64;
     if num_pages > 0 {
@@ -1323,6 +1326,30 @@ pub(crate) fn protect_physical_memory_range(
     Ok(())
 }
 
+/// This function is a variant of [`protect_physical_memory_range`] to protect a VTL1 physical memory range.
+/// Unlike [`protect_physical_memory_range`], this is intended exclusively for securing VTL1's own pages.
+/// VTL0 should never access VTL1 memory, so the memory attribute is always empty (no read, write, or execute).
+///
+/// Note. This function doesn't check whether `phys_frame_range` belongs to VTL1 because it is called by BSP
+/// before the kernel platform data structure is initialized. To this end, one might call this function with
+/// a VTL0 physical memory range which only restricts access to the range.
+#[inline]
+pub(crate) fn protect_vtl1_physical_memory_range(
+    phys_frame_range: PhysFrameRange<Size4KiB>,
+) -> Result<(), VsmError> {
+    let pa = phys_frame_range.start.start_address().as_u64();
+    let num_pages = phys_frame_range.count() as u64;
+    if num_pages > 0 {
+        hv_modify_vtl_protection_mask(
+            pa,
+            num_pages,
+            mem_attr_to_hv_page_prot_flags(MemAttr::empty()),
+        )
+        .map_err(VsmError::HypercallFailed)?;
+    }
+    Ok(())
+}
+
 /// Data structure for maintaining the memory content of a kernel module by its sections. Currently, it only maintains
 /// certain sections like `.text` and `.init.text` which are needed for module validation.
 pub struct ModuleMemory {