loader: Verify alignment of the LOAD segment for the x86-64 kernel

For the x86-64 kernel, the loaded address must be aligned to 2MB;
otherwise, the check in __startup_64() in the Linux kernel would fail
and cause the kernel to hang. Therefore, verify the alignment of the
LOAD segment during the loading process.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>

Author: bysui

docs/elfio_files/bad_align_writer.cpp

@@ -56,10 +56,10 @@ int main( void )
     // Create a loadable segment
     segment* text_seg = writer.segments.add();
     text_seg->set_type( PT_LOAD );
-    text_seg->set_virtual_address( 0x400 );
-    text_seg->set_physical_address( 0x400 );
+    text_seg->set_virtual_address( 0x200000 );
+    text_seg->set_physical_address( 0x200000 );
     text_seg->set_flags( PF_X | PF_R );
-    text_seg->set_align( 0x100 );
+    text_seg->set_align( 0x200000 );
 
     // Add code section into program segment
     text_seg->add_section_index( text_sec->get_index(),
@@ -80,10 +80,10 @@ int main( void )
     // Create a read/write segment
     segment* data_seg = writer.segments.add();
     data_seg->set_type( PT_NOTE );
-    data_seg->set_virtual_address( 0x8888048020 );
-    data_seg->set_physical_address( 0x8888048020 );
+    data_seg->set_virtual_address( 0x8888400000 );
+    data_seg->set_physical_address( 0x888400000 );
     data_seg->set_flags( PF_W | PF_R );
-    data_seg->set_align( 13 );
+    data_seg->set_align( 0x200000 );
 
     // Add code section into program segment
     data_seg->add_section_index( data_sec->get_index(),
@@ -103,7 +103,7 @@ int main( void )
     // In this example, the code starts at the first address of the
     // 'text_seg' segment. Therefore, the start address is set
     // to be equal to the segment location
-    writer.set_entry( 0x400 );
+    writer.set_entry( 0x200000 );
 
     // Create ELF file
     writer.save( "test_bad_align.bin" );

docs/elfio_files/basic_elf.cpp

@@ -56,10 +56,10 @@ int main( void )
     // Create a loadable segment
     segment* text_seg = writer.segments.add();
     text_seg->set_type( PT_LOAD );
-    text_seg->set_virtual_address( 0x400 );
-    text_seg->set_physical_address( 0x0 );
+    text_seg->set_virtual_address( 0x200000 );
+    text_seg->set_physical_address( 0x200000 );
     text_seg->set_flags( PF_X | PF_R );
-    text_seg->set_align( 0x1 );
+    text_seg->set_align( 0x200000 );
 
     // Add code section into program segment
     text_seg->add_section_index( text_sec->get_index(),
@@ -80,10 +80,10 @@ int main( void )
     // Create a read/write segment
     segment* data_seg = writer.segments.add();
     data_seg->set_type( PT_LOAD );
-    data_seg->set_virtual_address( 0x0420 );
-    data_seg->set_physical_address( 0x0420 );
+    data_seg->set_virtual_address( 0x400000 );
+    data_seg->set_physical_address( 0x400000 );
     data_seg->set_flags( PF_W | PF_R );
-    data_seg->set_align( 0x10 );
+    data_seg->set_align( 0x200000 );
 
     // Add code section into program segment
     data_seg->add_section_index( data_sec->get_index(),
@@ -103,7 +103,7 @@ int main( void )
     // In this example, the code starts at the first address of the
     // 'text_seg' segment. Therefore, the start address is set
     // to be equal to the segment location
-    writer.set_entry( 0x400 );
+    writer.set_entry( 0x200000 );
 
     // Create ELF file
     writer.save( "test_elf.bin" );

src/loader/elf/mod.rs

@@ -159,6 +159,9 @@ impl Elf {
     }
 }
 
+// x86-64 kernel must be aligned to 2MB.
+const KERNEL_SEGMENT_ALIGN: u64 = 0x200000; // 2 MiB
+
 impl KernelLoader for Elf {
     /// Loads a kernel from a vmlinux elf image into guest memory.
     ///
@@ -280,6 +283,12 @@ impl KernelLoader for Elf {
                 None => GuestAddress(phdr.p_paddr),
             };
 
+            // Ensure that mem_offset is aligned to 2M, otherwise, the kernel may hang during
+            // booting. Refer to the check in __startup_64() in the Linux kernel source code.
+            if mem_offset.raw_value() & (KERNEL_SEGMENT_ALIGN - 1) != 0 {
+                return Err(Error::Align.into());
+            }
+
             guest_mem
                 .read_exact_volatile_from(mem_offset, kernel_image, phdr.p_filesz as usize)
                 .map_err(|_| Error::ReadKernelImage)?;
@@ -479,10 +488,10 @@ mod tests {
             Some(highmem_start_address),
         )
         .unwrap();
-        assert_eq!(loader_result.kernel_load.raw_value(), 0x200400);
+        assert_eq!(loader_result.kernel_load.raw_value(), 0x400000);
 
         loader_result = Elf::load(&gm, Some(kernel_addr), &mut Cursor::new(&image), None).unwrap();
-        assert_eq!(loader_result.kernel_load.raw_value(), 0x200400);
+        assert_eq!(loader_result.kernel_load.raw_value(), 0x400000);
 
         loader_result = Elf::load(
             &gm,
@@ -491,7 +500,7 @@ mod tests {
             Some(highmem_start_address),
         )
         .unwrap();
-        assert_eq!(loader_result.kernel_load.raw_value(), 0x400);
+        assert_eq!(loader_result.kernel_load.raw_value(), 0x200000);
 
         highmem_start_address = GuestAddress(0xa00000);
         assert_eq!(
@@ -632,4 +641,20 @@ mod tests {
             .err()
         );
     }
+
+    #[test]
+    fn test_unaligned_loadaddr() {
+        let gm = create_guest_mem();
+        let image = make_elf_bin();
+        assert_eq!(
+            Some(KernelLoaderError::Elf(Error::Align)),
+            Elf::load(
+                &gm,
+                Some(GuestAddress(0x1000)),
+                &mut Cursor::new(&image),
+                None
+            )
+            .err()
+        );
+    }
 }