don't map boot info and memory map into bootloader's memory

Modifying the bootloaders page tables by mapping addresses that we
don't know for sure are available can lead to all sorts of problems.
Simply stop doing that.

Author: Freax13

common/src/lib.rs

@@ -463,10 +463,7 @@ pub struct Mappings {
 
 /// Allocates and initializes the boot info struct and the memory map.
 ///
-/// The boot info and memory map are mapped to both the kernel and bootloader
-/// address space at the same address. This makes it possible to return a Rust
-/// reference that is valid in both address spaces. The necessary physical frames
-/// are taken from the given `frame_allocator`.
+/// The necessary physical frames are taken from the given `frame_allocator`.
 pub fn create_boot_info<I, D>(
     config: &BootloaderConfig,
     boot_config: &BootConfig,
@@ -513,19 +510,15 @@ where
             Ok(tlb) => tlb.flush(),
             Err(err) => panic!("failed to map page {:?}: {:?}", page, err),
         }
-        // we need to be able to access it too
-        match unsafe {
-            page_tables
-                .bootloader
-                .map_to(page, frame, flags, &mut frame_allocator)
-        } {
-            Ok(tlb) => tlb.flush(),
-            Err(err) => panic!("failed to map page {:?}: {:?}", page, err),
-        }
     }
 
-    let boot_info: &'static mut MaybeUninit<BootInfo> =
-        unsafe { &mut *boot_info_addr.as_mut_ptr() };
+    let boot_info: &'static mut MaybeUninit<BootInfo> = unsafe {
+        // SAFETY: This is technically UB because the current page tables don't
+        // map `memory_map_regions_addr`, so we have to be careful to not
+        // access any elements.
+        // We have to do this because `BootInfo` needs a `MemoryRegions`.
+        &mut *boot_info_addr.as_mut_ptr()
+    };
 
     log::info!("Create Memory Map");
 
@@ -546,38 +539,43 @@ where
     log::info!("Create bootinfo");
 
     // create boot info
-    let boot_info = boot_info.write({
-        let mut info = BootInfo::new(memory_regions.into());
-        info.framebuffer = mappings
-            .framebuffer
-            .map(|addr| unsafe {
-                FrameBuffer::new(
-                    addr.as_u64(),
-                    system_info
-                        .framebuffer
-                        .expect(
-                            "there shouldn't be a mapping for the framebuffer if there is \
-                            no framebuffer",
-                        )
-                        .info,
-                )
-            })
-            .into();
-        info.physical_memory_offset = mappings.physical_memory_offset.map(VirtAddr::as_u64).into();
-        info.recursive_index = mappings.recursive_index.map(Into::into).into();
-        info.rsdp_addr = system_info.rsdp_addr.map(|addr| addr.as_u64()).into();
-        info.tls_template = mappings.tls_template.into();
-        info.ramdisk_addr = mappings
-            .ramdisk_slice_start
-            .map(|addr| addr.as_u64())
-            .into();
-        info.ramdisk_len = mappings.ramdisk_slice_len;
-        info.kernel_addr = mappings.kernel_slice_start.as_u64();
-        info.kernel_len = mappings.kernel_slice_len as _;
-        info.kernel_image_offset = mappings.kernel_image_offset.as_u64();
-        info._test_sentinel = boot_config._test_sentinel;
-        info
-    });
+    let mut info = BootInfo::new(memory_regions.into());
+    info.framebuffer = mappings
+        .framebuffer
+        .map(|addr| unsafe {
+            FrameBuffer::new(
+                addr.as_u64(),
+                system_info
+                    .framebuffer
+                    .expect(
+                        "there shouldn't be a mapping for the framebuffer if there is \
+                        no framebuffer",
+                    )
+                    .info,
+            )
+        })
+        .into();
+    info.physical_memory_offset = mappings.physical_memory_offset.map(VirtAddr::as_u64).into();
+    info.recursive_index = mappings.recursive_index.map(Into::into).into();
+    info.rsdp_addr = system_info.rsdp_addr.map(|addr| addr.as_u64()).into();
+    info.tls_template = mappings.tls_template.into();
+    info.ramdisk_addr = mappings
+        .ramdisk_slice_start
+        .map(|addr| addr.as_u64())
+        .into();
+    info.ramdisk_len = mappings.ramdisk_slice_len;
+    info.kernel_addr = mappings.kernel_slice_start.as_u64();
+    info.kernel_len = mappings.kernel_slice_len as _;
+    info.kernel_image_offset = mappings.kernel_image_offset.as_u64();
+    info._test_sentinel = boot_config._test_sentinel;
+
+    // Write to boot info directly to the identity-mapped frame.
+    let boot_info_frame = page_tables.kernel.translate_page(start_page).unwrap();
+    assert!(size_of::<BootInfo>() <= Size4KiB::SIZE as usize);
+    let ptr = boot_info_frame.start_address().as_u64() as *mut BootInfo;
+    unsafe {
+        ptr.write(info);
+    }
 
     boot_info_addr
 }