Lock decryption in CtrEncryptedBacking

This prevents a race where two threads could read at the same time and
end up using the wrong IV leading to garbage data being read. This
caused crashes in several games including Celeste.

app/src/main/cpp/skyline/crypto/aes_cipher.h

@@ -9,6 +9,7 @@
 namespace skyline::crypto {
     /**
      * @brief Wrapper for mbedtls for AES decryption using a cipher
+     * @note The IV state must be appropriately locked during multi-threaded usage
      */
     class AesCipher {
       private:

app/src/main/cpp/skyline/vfs/ctr_encrypted_backing.cpp

@@ -22,11 +22,14 @@ namespace skyline::vfs {
 
         size_t sectorOffset{offset % SectorSize};
         if (sectorOffset == 0) {
-            UpdateCtr(baseOffset + offset);
             size_t read{backing->Read(output, offset)};
             if (read != size)
                 return 0;
-            cipher.Decrypt(output);
+            {
+                std::lock_guard guard(mutex);
+                UpdateCtr(baseOffset + offset);
+                cipher.Decrypt(output);
+            }
             return size;
         }
 
@@ -35,8 +38,11 @@ namespace skyline::vfs {
         size_t read{backing->Read(blockBuf, sectorStart)};
         if (read != SectorSize)
             return 0;
-        UpdateCtr(baseOffset + sectorStart);
-        cipher.Decrypt(blockBuf);
+        {
+            std::lock_guard guard(mutex);
+            UpdateCtr(baseOffset + sectorStart);
+            cipher.Decrypt(blockBuf);
+        }
         if (size + sectorOffset < SectorSize) {
             std::memcpy(output.data(), blockBuf.data() + sectorOffset, size);
             return size;

app/src/main/cpp/skyline/vfs/ctr_encrypted_backing.h

@@ -16,6 +16,7 @@ namespace skyline::vfs {
         crypto::KeyStore::Key128 ctr;
         crypto::AesCipher cipher;
         std::shared_ptr<Backing> backing;
+        std::mutex mutex; //!< Synchronize all AES-CTR cipher state modifications
         size_t baseOffset; //!< The offset of the backing into the file is used to calculate the IV
 
         /**