fw_meta: refactor parse_fw_meta_data() to take a &[u8]

Refactor the public function parse_fw_meta_data() so that the caller
can provide an slice containing the firmware metadata. This will help
building a fuzzing harness around this interface

Signed-off-by: Carlos López <[email protected]>

src/fw_meta.rs

@@ -10,7 +10,6 @@ use crate::address::{Address, PhysAddr};
 use crate::cpu::percpu::this_cpu_mut;
 use crate::error::SvsmError;
 use crate::mm::PerCPUPageMappingGuard;
-use crate::mm::SIZE_1G;
 use crate::sev::ghcb::PageStateChangeOp;
 use crate::sev::{pvalidate, rmp_adjust, PvalidateOp, RMPFlags};
 use crate::types::{PageSize, PAGE_SIZE};
@@ -245,16 +244,18 @@ fn find_table<'a>(uuid: &Uuid, mem: &'a [u8]) -> Option<&'a [u8]> {
     None
 }
 
-pub fn parse_fw_meta_data() -> Result<SevFWMetaData, SvsmError> {
+/// Parse the firmware metadata from the given slice.
+pub fn parse_fw_meta_data(mem: &[u8]) -> Result<SevFWMetaData, SvsmError> {
     let mut meta_data = SevFWMetaData::new();
-    // Map meta-data location, it starts at 32 bytes below 4GiB
-    let pstart = PhysAddr::from((4 * SIZE_1G) - PAGE_SIZE);
-    let guard = PerCPUPageMappingGuard::create_4k(pstart)?;
-    let vstart = guard.virt_addr();
-
-    // Safety: RawMetaBuffer has a size of one page and it has no invalid
-    // representations.
-    let raw_meta = unsafe { &*vstart.as_ptr::<RawMetaBuffer>() };
+
+    if mem.len() != size_of::<RawMetaBuffer>() {
+        return Err(SvsmError::Firmware);
+    }
+
+    // Safety: `RawMetaBuffer` has no invalid representations and is
+    // `repr(C, packed)`, which means there are no alignment requirements.
+    // We have also verified that the size of the slice matches.
+    let raw_meta = unsafe { &*mem.as_ptr().cast::<RawMetaBuffer>() };
 
     // Check the UUID
     let raw_uuid = raw_meta.header.uuid;

src/svsm.rs

@@ -37,7 +37,7 @@ use svsm::mm::alloc::{memory_info, print_memory_info, root_mem_init};
 use svsm::mm::memory::init_memory_map;
 use svsm::mm::pagetable::paging_init;
 use svsm::mm::virtualrange::virt_log_usage;
-use svsm::mm::{init_kernel_mapping_info, PerCPUPageMappingGuard};
+use svsm::mm::{init_kernel_mapping_info, PerCPUPageMappingGuard, SIZE_1G};
 use svsm::requests::{request_loop, update_mappings};
 use svsm::serial::SerialPort;
 use svsm::serial::SERIAL_PORT;
@@ -310,6 +310,17 @@ fn mapping_info_init(launch_info: &KernelLaunchInfo) {
     );
 }
 
+fn map_and_parse_fw_meta() -> Result<SevFWMetaData, SvsmError> {
+    // Map meta-data location, it starts at 32 bytes below 4GiB
+    let pstart = PhysAddr::from((4 * SIZE_1G) - PAGE_SIZE);
+    let guard = PerCPUPageMappingGuard::create_4k(pstart)?;
+    let vstart = guard.virt_addr().as_ptr::<u8>();
+    // Safety: we just mapped a page, so the size must hold. The type
+    // of the slice elements is `u8` so there are no alignment requirements.
+    let metadata = unsafe { slice::from_raw_parts(vstart, PAGE_SIZE) };
+    parse_fw_meta_data(metadata)
+}
+
 #[no_mangle]
 pub extern "C" fn svsm_start(li: &KernelLaunchInfo, vb_addr: usize) {
     let launch_info: KernelLaunchInfo = *li;
@@ -443,7 +454,7 @@ pub extern "C" fn svsm_main() {
 
     start_secondary_cpus(&cpus);
 
-    let fw_meta = parse_fw_meta_data()
+    let fw_meta = map_and_parse_fw_meta()
         .unwrap_or_else(|e| panic!("Failed to parse FW SEV meta-data: {:#?}", e));
 
     print_fw_meta(&fw_meta);