ARM64: Add bit masks to allow some wrapping

In these cases, values are intentionally truncated, either to split them
into multiple values or to ensure that user-supplied data does not cause
Firecracker to panic with a try_from/unwrap failure.

Clippy will allow us to use "as" when we use bit masks to limit the size
of an explicitly-sized value (u16, u32, u64). It doesn't work on usize
values or on some more complicated expressions.

I've done this in a separate commit because my dev setup doesn't use
ARM. In case something ARM-specific breaks in CI, I'd like to be able to
keep those changes in their own commits so I can debug more easily.

Signed-off-by: Jonathan Browne <[email protected]>

Author: JBYoshi

src/vmm/src/arch/aarch64/vcpu.rs

@@ -42,7 +42,7 @@ pub enum VcpuError {
 pub fn get_manufacturer_id_from_state(regs: &Aarch64RegisterVec) -> Result<u32, VcpuError> {
     let midr_el1 = regs.iter().find(|reg| reg.id == MIDR_EL1);
     match midr_el1 {
-        Some(register) => Ok(register.value::<u64, 8>() as u32 >> 24),
+        Some(register) => Ok(((register.value::<u64, 8>() >> 24) & 0xFF) as u32),
         None => Err(VcpuError::GetMidrEl1(
             "Failed to find MIDR_EL1 in vCPU state!".to_string(),
         )),

src/vmm/src/cpu_config/aarch64/mod.rs

@@ -33,10 +33,16 @@ impl CpuConfiguration {
         for (modifier, mut reg) in template.reg_modifiers.iter().zip(self.regs.iter_mut()) {
             match reg.size() {
                 RegSize::U32 => {
-                    reg.set_value(modifier.bitmap.apply(u128::from(reg.value::<u32, 4>())) as u32);
+                    reg.set_value(
+                        (modifier.bitmap.apply(u128::from(reg.value::<u32, 4>())) & 0xFFFF_FFFF)
+                            as u32,
+                    );
                 }
                 RegSize::U64 => {
-                    reg.set_value(modifier.bitmap.apply(u128::from(reg.value::<u64, 8>())) as u64);
+                    reg.set_value(
+                        (modifier.bitmap.apply(u128::from(reg.value::<u64, 8>()))
+                            & 0xFFFF_FFFF_FFFF_FFFF) as u64,
+                    );
                 }
                 RegSize::U128 => {
                     reg.set_value(modifier.bitmap.apply(reg.value::<u128, 16>()));