Give the Linux server 64 bits of session entropy and a pinned Steam lib

Two follow-ups to the Linux server support in #6. Both are Linux-only, so
neither reached a Windows player.

Session codes
- nonce()'s fallback for a container with no /dev/urandom read
  "i < 4 && !value", which ended the loop as soon as the first round
  produced anything: one round of sixteen bits instead of four. Over
  200k draws that is 62k distinct codes with a ceiling of 0xffff, rather
  than 200k across the full range.

libsteam_api.so
- It was fetched from a mirror's main branch with no hash, by CI and by
  the script every server operator runs, and it is loaded into the
  server process. A branch name is whatever that branch holds on the day
  it runs.
- Pinned to mirror commit df2baabf574a and checked against its SHA-256
  (385840 bytes, eb2dd015...). The size and export checks already there
  catch a wrong file; a hash is what catches a swapped one.
- --sdk-url still overrides the URL and drops the check, since a
  different URL has a different hash; --sdk-sha256 keeps it.

.github/workflows/linux-server.yml

@@ -56,8 +56,14 @@ jobs:
         run: |
           set -eux
           mkdir -p build/steamlibs
+          # Pinned and hashed: this library is loaded into the server, and a
+          # branch name is whatever that branch holds on the day CI runs. Both
+          # values also live in contrib/setup-linux-server-libs.sh.
+          SDK_COMMIT=df2baabf574a738ef1ea90a7e89339107fc0a279
+          SDK_SHA256=eb2dd015b84177cf4f4326fe578aab375fd8931bbbd719c7492420d9777007fe
           curl -fSL --retry 3 -o build/steamlibs/libsteam_api.so \
-            "https://raw.githubusercontent.com/rlabrecque/SteamworksSDK/main/redistributable_bin/linux64/libsteam_api.so"
+            "https://raw.githubusercontent.com/rlabrecque/SteamworksSDK/$SDK_COMMIT/redistributable_bin/linux64/libsteam_api.so"
+          echo "$SDK_SHA256  build/steamlibs/libsteam_api.so" | sha256sum -c -
           test "$(stat -c%s build/steamlibs/libsteam_api.so)" -gt 100000
           nm -D --defined-only build/steamlibs/libsteam_api.so \
             | grep "SteamAPI_SteamGameServerNetworkingSockets_SteamAPI_v01" > /dev/null

Server/server_host.cpp

@@ -32,7 +32,9 @@ std::uint64_t nonce() {
     std::ifstream urandom("/dev/urandom", std::ios::binary);
     if (urandom.read(reinterpret_cast<char *>(&value), sizeof(value)) && value) return value;
     std::random_device device;
-    for (int i = 0; i < 4 && !value; ++i)
+    // Four rounds of sixteen bits each. Stopping as soon as `value` was
+    // non-zero would leave a session code with sixteen bits of entropy.
+    for (int i = 0; i < 4; ++i)
         value = (value << 16) ^ static_cast<std::uint64_t>(device() & 0xFFFF);
     if (!value) throw std::runtime_error("Cannot generate a session code.");
 #endif

contrib/setup-linux-server-libs.sh

@@ -11,19 +11,26 @@
 #
 # Defaults:
 #   --server-dir: directory containing ReSkateServer (or current directory)
-#   --sdk-url:   https://raw.githubusercontent.com/rlabrecque/SteamworksSDK/main/redistributable_bin/linux64/libsteam_api.so
-#                (public SDK mirror, Valve binaries; official source: partner.steamgames.com)
+#   --sdk-url:   the pinned SDK mirror below (Valve binaries; official source:
+#                partner.steamgames.com). Pinned to a commit and checked
+#                against SDK_SHA256, because this library is loaded into the
+#                server: a branch name is whatever that branch holds today.
 set -euo pipefail
 
 SERVER_DIR=""
-SDK_URL="https://raw.githubusercontent.com/rlabrecque/SteamworksSDK/main/redistributable_bin/linux64/libsteam_api.so"
+# Mirror commit df2baabf574a, libsteam_api.so 385840 bytes.
+SDK_COMMIT="df2baabf574a738ef1ea90a7e89339107fc0a279"
+SDK_URL="https://raw.githubusercontent.com/rlabrecque/SteamworksSDK/$SDK_COMMIT/redistributable_bin/linux64/libsteam_api.so"
+# Empty, or --sdk-url given, skips the hash check: pass --sdk-sha256 to keep it.
+SDK_SHA256="eb2dd015b84177cf4f4326fe578aab375fd8931bbbd719c7492420d9777007fe"
 FORCE=0
 NO_STEAMCLIENT=0
 
 while [ $# -gt 0 ]; do
     case "$1" in
         --server-dir) SERVER_DIR="${2:?missing DIR}"; shift 2 ;;
-        --sdk-url) SDK_URL="${2:?missing URL}"; shift 2 ;;
+        --sdk-url) SDK_URL="${2:?missing URL}"; SDK_SHA256=""; shift 2 ;;
+        --sdk-sha256) SDK_SHA256="${2:?missing SHA256}"; shift 2 ;;
         --force) FORCE=1; shift ;;
         --no-steamclient) NO_STEAMCLIENT=1; shift ;;
         -h|--help) sed -n '2,14p' "$0"; exit 0 ;;
@@ -53,6 +60,19 @@ else
     if command -v nm >/dev/null && ! nm -D --defined-only "$tmp" 2>/dev/null | grep "SteamAPI_SteamGameServerNetworkingSockets_SteamAPI_v01" >/dev/null; then
         echo "error: downloaded file has no game-server sockets export" >&2; rm -f "$tmp"; exit 2
     fi
+    # The size and export checks above catch a wrong file, not a swapped one.
+    if [ -n "$SDK_SHA256" ]; then
+        command -v sha256sum >/dev/null || { echo "error: sha256sum is required" >&2; rm -f "$tmp"; exit 2; }
+        got="$(sha256sum "$tmp" | cut -d' ' -f1)"
+        if [ "$got" != "$SDK_SHA256" ]; then
+            echo "error: libsteam_api.so does not match the expected hash" >&2
+            echo "  expected $SDK_SHA256" >&2
+            echo "  got      $got" >&2
+            echo "  (a newer mirror commit needs --sdk-sha256 with its hash)" >&2
+            rm -f "$tmp"; exit 2
+        fi
+        echo "Verified libsteam_api.so against its expected hash."
+    fi
     cp "$tmp" "$SERVER_DIR/libsteam_api.so"
     chmod 644 "$SERVER_DIR/libsteam_api.so"
     rm -f "$tmp"