Alternative way to detect AMD bug

Author: josephlr

src/rdrand.rs

@@ -23,13 +23,6 @@ unsafe fn rdrand() -> Result<[u8; WORD_SIZE], Error> {
     for _ in 0..RETRY_LIMIT {
         let mut el = mem::uninitialized();
         if _rdrand64_step(&mut el) == 1 {
-            // AMD CPUs from families 14h to 16h (pre Ryzen) will sometimes give
-            // bogus random data. Discard these values and warn the user.
-            // See https://github.com/systemd/systemd/issues/11810#issuecomment-489727505
-            if cfg!(not(target_env = "sgx")) && (el == 0 || el == !0) {
-                error!("RDRAND returned suspicious value {}, CPU RNG is broken", el);
-                return Err(Error::UNKNOWN)
-            }
             return Ok(el.to_ne_bytes());
         }
     }
@@ -43,25 +36,53 @@ compile_error!(
     "SGX targets require 'rdrand' target feature. Enable by using -C target-feature=+rdrnd."
 );
 
-#[cfg(target_feature = "rdrand")]
+#[cfg(target_env = "sgx")]
 fn is_rdrand_supported() -> bool {
     true
 }
 
 // TODO use is_x86_feature_detected!("rdrand") when that works in core. See:
 // https://github.com/rust-lang-nursery/stdsimd/issues/464
-#[cfg(not(target_feature = "rdrand"))]
+#[cfg(not(target_env = "sgx"))]
 fn is_rdrand_supported() -> bool {
-    use core::arch::x86_64::__cpuid;
     use lazy_static::lazy_static;
-    // SAFETY: All x86_64 CPUs support CPUID leaf 1
-    const FLAG: u32 = 1 << 30;
     lazy_static! {
-        static ref HAS_RDRAND: bool = unsafe { __cpuid(1).ecx & FLAG != 0 };
+        static ref HAS_RDRAND: bool = has_rdrand();
     }
     *HAS_RDRAND
 }
 
+#[cfg(not(target_env = "sgx"))]
+pub fn has_rdrand() -> bool {
+    use core::arch::x86_64::__cpuid;
+    // SAFETY: All x86_64 CPUs support CPUID leaf 0 and 1
+    let leaf_0 = unsafe { __cpuid(0) };
+    let vendor_id: [u8; 12] = unsafe { mem::transmute([leaf_0.ebx, leaf_0.edx, leaf_0.ecx]) };
+    let is_amd = &vendor_id == b"AuthenticAMD";
+
+    if cfg!(target_feature = "rdrand") && !is_amd {
+        return true;
+    }
+    let leaf_1 = unsafe { __cpuid(1) };
+
+    // Early AMD CPUs have a broken RDRAND. They return nonrandom data when
+    // resuming from sleep. We will ignore RDRAND support on such platfroms.
+    // See: https://github.com/systemd/systemd/issues/11810#issuecomment-489727505
+    if is_amd {
+        let mut family_id = (leaf_1.eax & 0x00000f00) >> 8;
+        if family_id == 0xF {
+            let extended_id = (leaf_1.eax & 0x0ff00000) >> 20;
+            family_id += extended_id;
+        }
+        // This issue is only present before Zen (family 17h).
+        if family_id < 0x17 {
+            return false;
+        }
+    }
+    const RDRAND_FLAG: u32 = 1 << 30;
+    leaf_1.ecx & RDRAND_FLAG != 0
+}
+
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     if !is_rdrand_supported() {
         return Err(Error::UNAVAILABLE);