std: sys: random: uefi: Provide rdrand based fallback

Some UEFI systems based on American Megatrends Inc. v3.3 do not provide
RNG support [1]. So fallback to rdrand in such cases.

[1]: #138252 (comment)

Signed-off-by: Ayush Singh <[email protected]>

Author: Ayush1325

library/std/src/sys/random/uefi.rs

@@ -23,5 +23,39 @@ pub fn fill_bytes(bytes: &mut [u8]) {
         }
     }
 
+    // Fallback to rdrand if rng protocol missing.
+    //
+    // For real-world example, see [issue-13825](https://github.com/rust-lang/rust/issues/138252#issuecomment-2891270323)
+    #[cfg(any(target_arch = "x86_64", target_arch = "x86"))]
+    if crate::is_x86_feature_detected!("rdrand") {
+        #[cfg(target_arch = "x86_64")]
+        for chunk in bytes.chunks_mut(core::mem::size_of::<u64>()) {
+            let mut rand_val: u64 = 0;
+            unsafe {
+                if core::arch::x86_64::_rdrand64_step(&mut rand_val) == 0 {
+                    panic!("failed to generate random data using rdrand");
+                }
+            }
+
+            let bytes = rand_val.to_le_bytes();
+            chunk.copy_from_slice(&bytes[..chunk.len()]);
+        }
+
+        #[cfg(target_arch = "x86")]
+        for chunk in bytes.chunks_mut(core::mem::size_of::<u32>()) {
+            let mut rand_val: u32 = 0;
+            unsafe {
+                if core::arch::x86::_rdrand32_step(&mut rand_val) == 0 {
+                    panic!("failed to generate random data using rdrand");
+                }
+            }
+
+            let bytes = rand_val.to_le_bytes();
+            chunk.copy_from_slice(&bytes[..chunk.len()]);
+        }
+
+        return;
+    }
+
     panic!("failed to generate random data");
 }