Improve pKVM hypercalls

The pKVM hypervisor forwards PKVM_GHC_IOWRITE and PKVM_GHC_IOWREAD to
pKVM host for emulation. In such case, guest registers values in RAX,
RBX, RCX, RDX and RSI might become visible to the host.

To prevent potential unintended information leakage, introduce vmcall
macro which zeros not used hyperacall arguments, and use it for
PKVM_GHC_IOWRITE/PKVM_GHC_IOWREAD hypercall to make sure that unused RDX
or RSI are set to 0.

Author: jaszczyk-grzegorz

src/transport/x86_64/hypercalls.rs

@@ -9,9 +9,9 @@ const KVM_CPUID_SIGNATURE: u32 = 0x40000000;
 
 // See `include/uapi/linux/kvm_para.h`. (These hypercalls numbers can change depending on the
 // upstream progress.)
-const KVM_HC_PKVM_OP: u32 = 20;
-const PKVM_GHC_IOREAD: u32 = KVM_HC_PKVM_OP + 3;
-const PKVM_GHC_IOWRITE: u32 = KVM_HC_PKVM_OP + 4;
+const KVM_HC_PKVM_OP: u64 = 20;
+const PKVM_GHC_IOREAD: u64 = KVM_HC_PKVM_OP + 3;
+const PKVM_GHC_IOWRITE: u64 = KVM_HC_PKVM_OP + 4;
 
 /// The maximum number of bytes that can be read or written by a single IO hypercall.
 const HYP_IO_MAX: usize = 8;
@@ -42,48 +42,58 @@ pub fn cpuid_signature() -> [u8; 4] {
     signature.to_le_bytes()
 }
 
-/// Asks the hypervisor to perform an IO read at the given physical address.
-pub fn hyp_io_read(address: u64, size: usize) -> u64 {
-    let data;
+fn __vmcall_impl(hypcall: u64, a1: u64, a2: u64, a3: u64, a4: u64) -> u64 {
+    let ret: u64;
 
-    // SAFETY: Assembly call. Arguments for vmcall are passed via rax, rbx, rcx and rdx. Ideally
-    // using a named argument in the inline asm for rbx would be more straightforward, but when
-    // "rbx" is used directly LLVM complains that it is used internally.
+    // SAFETY: Assembly call. Arguments for vmcall are passed via rax, rbx, rcx, rdx and rsi.
+    // Ideally using a named argument in the inline asm for rbx would be more straightforward,
+    // but when "rbx" is used directly LLVM complains that it is used internally.
     //
-    // Therefore use r8 temporary, push rbx to the stack, perform proper call and pop rbx again.
+    // Therefore use temp register to store rbx content and restore it back afterwards.
     unsafe {
-        asm!(
-            "push rbx",
-            "mov rbx, r8",
-            "vmcall",
-            "pop rbx",
-            inout("rax") u64::from(PKVM_GHC_IOREAD) => data,
-            in("r8") address,
-            in("rcx") size,
-        );
-    }
-    data
+        core::arch::asm!(
+                "xchg %rbx, {0:r}",
+                "vmcall",
+                "xchg %rbx, {0:r}",
+                in(reg) a1,
+                inout("rax") hypcall => ret,
+                in("rcx") a2,
+                in("rdx") a3,
+                in("rsi") a4,
+                options(att_syntax));
+    };
+    ret
+}
+
+// This block uses inline assembly to perform a vmcall, interacting directly with the hypervisor.
+// The pKVM hypervisor can share RAX/RBX/RCX/RDX/RSI with pKVM host during hypercall
+// handling.
+macro_rules! vmcall {
+    ($hypcall:expr) => {
+        __vmcall_impl($hypcall, 0, 0, 0, 0)
+    };
+    ($hypcall:expr, $a1:expr) => {
+        __vmcall_impl($hypcall, $a1, 0, 0, 0)
+    };
+    ($hypcall:expr, $a1:expr, $a2:expr) => {
+        __vmcall_impl($hypcall, $a1, $a2, 0, 0)
+    };
+    ($hypcall:expr, $a1:expr, $a2:expr, $a3:expr) => {
+        __vmcall_impl($hypcall, $a1, $a2, $a3, 0)
+    };
+    ($hypcall:expr, $a1:expr, $a2:expr, $a3:expr, $a4:expr) => {
+        __vmcall_impl($hypcall, $a1, $a2, $a3, $a4)
+    };
+}
+
+/// Asks the hypervisor to perform an IO read at the given physical address.
+pub fn hyp_io_read(address: u64, size: usize) -> u64 {
+    vmcall!(PKVM_GHC_IOREAD, address, size as u64)
 }
 
 /// Asks the hypervisor to perform an IO write at the given physical address.
 pub fn hyp_io_write(address: u64, size: usize, data: u64) {
-    // SAFETY: Assembly call. Arguments for vmcall are passed via rax, rbx, rcx and rdx. Ideally
-    // using a named argument in the inline asm for rbx would be more straightforward but when
-    // "rbx" is used directly used LLVM complains that it is used internally.
-    //
-    // Therefore use r8 temporary, push rbx to the stack, perform proper call and pop rbx again.
-    unsafe {
-        asm!(
-            "push rbx",
-            "mov rbx, r8",
-            "vmcall",
-            "pop rbx",
-            in("rax") PKVM_GHC_IOWRITE,
-            in("r8") address,
-            in("rcx") size,
-            in("rdx") data,
-        );
-    }
+    vmcall!(PKVM_GHC_IOWRITE, address, size as u64, data);
 }
 
 /// A region of physical address space which may be accessed by IO read and/or write hypercalls.