src: allow CAP_NET_BIND_SERVICE in SafeGetenv

This commit updates SafeGetenv to check if the current process has the
effective capability cap_net_bind_service set, and if so allows
environment variables to be read.

The motivation for this change is a use-case where Node is run in a
container, and the is a requirement to be able to listen to ports
below 1024. This is done by setting the capability of
cap_net_bind_service. In addition there is a need to set the
environment variable `NODE_EXTRA_CA_CERTS`. But currently this
environment variable will not be read when the capability has been set
on the executable.

Author: danbev

.github/workflows/build-tarball.yml

@@ -25,6 +25,8 @@ jobs:
           python-version: ${{ env.PYTHON_VERSION }}
       - name: Environment Information
         run: npx envinfo
+      - name: Install libcap-dev
+        run: sudo apt-get install -y libcap-dev
       - name: Make tarball
         run: |
           export DISTTYPE=nightly
@@ -51,6 +53,8 @@ jobs:
           python-version: ${{ env.PYTHON_VERSION }}
       - name: Environment Information
         run: npx envinfo
+      - name: Install libcap-dev
+        run: sudo apt-get install -y libcap-dev
       - name: Download tarball
         uses: actions/download-artifact@v1
         with:

.github/workflows/coverage-linux.yml

@@ -34,6 +34,8 @@ jobs:
         run: npx envinfo
       - name: Install gcovr
         run: pip install gcovr==4.2
+      - name: Install libcap-dev
+        run: sudo apt-get install -y libcap-dev
       - name: Build
         run: make build-ci -j2 V=1 CONFIG_FLAGS="--error-on-warn --coverage"
       # TODO(bcoe): fix the couple tests that fail with the inspector enabled.

.github/workflows/test-asan.yml

@@ -34,6 +34,8 @@ jobs:
           python-version: ${{ env.PYTHON_VERSION }}
       - name: Environment Information
         run: npx envinfo
+      - name: Install libcap-dev
+        run: sudo apt-get install -y libcap-dev
       - name: Build
         run: make build-ci -j2 V=1
       - name: Test

.github/workflows/test-linux.yml

@@ -25,6 +25,8 @@ jobs:
           python-version: ${{ env.PYTHON_VERSION }}
       - name: Environment Information
         run: npx envinfo
+      - name: Install libcap-dev
+        run: sudo apt-get install -y libcap-dev
       - name: Build
         run: make build-ci -j2 V=1 CONFIG_FLAGS="--error-on-warn"
       - name: Test

common.gypi

@@ -373,6 +373,9 @@
         'cflags': [ '-pthread' ],
         'ldflags': [ '-pthread' ],
       }],
+      [ 'OS in "linux"', {
+        'ldflags': [ '-Wl,-Bstatic -Wl,--whole-archive -lcap -Wl,--no-whole-archive -Wl,-Bdynamic' ],
+      }],
       [ 'OS in "linux freebsd openbsd solaris android aix cloudabi"', {
         'cflags': [ '-Wall', '-Wextra', '-Wno-unused-parameter', ],
         'cflags_cc': [ '-fno-rtti', '-fno-exceptions', '-std=gnu++1y' ],

src/node_credentials.cc

@@ -11,6 +11,9 @@
 #if !defined(_MSC_VER)
 #include <unistd.h>  // setuid, getuid
 #endif
+#ifdef __linux__
+#include <sys/capability.h>
+#endif  // __linux__
 
 namespace node {
 
@@ -33,11 +36,48 @@ bool linux_at_secure = false;
 
 namespace credentials {
 
-// Look up environment variable unless running as setuid root.
+#if defined(__linux__)
+// Returns true if the current process only has the passed-in capability.
+bool HasOnly(cap_value_t capability) {
+  DCHECK(cap_valid(capability));
+
+  cap_t cap = cap_get_proc();
+  if (cap == nullptr) {
+    return false;
+  }
+
+  // Create a cap_t and set the capability passed in.
+  cap_t cap_cmp = cap_init();
+  if (cap_cmp == nullptr) {
+    cap_free(cap);
+    return false;
+  }
+  cap_value_t cap_list[] = { capability };
+  cap_set_flag(cap_cmp, CAP_EFFECTIVE, 1, cap_list, CAP_SET);
+  cap_set_flag(cap_cmp, CAP_PERMITTED, 1, cap_list, CAP_SET);
+  // Compare this to the process's effective capabilities
+  bool ret = cap_compare(cap, cap_cmp) == 0;
+
+  cap_free(cap);
+  cap_free(cap_cmp);
+
+  return ret;
+}
+#endif
+
+// Look up the environment variable and allow the lookup if the current
+// process only has the capability CAP_NET_BIND_SERVICE set. If the current
+// process does not have any capabilities set and the process is running as
+// setuid root then lookup will not be allowed.
 bool SafeGetenv(const char* key, std::string* text, Environment* env) {
 #if !defined(__CloudABI__) && !defined(_WIN32)
+#if defined(__linux__)
+  if ((!HasOnly(CAP_NET_BIND_SERVICE) && per_process::linux_at_secure) ||
+      getuid() != geteuid() || getgid() != getegid())
+#else
   if (per_process::linux_at_secure || getuid() != geteuid() ||
       getgid() != getegid())
+#endif
     goto fail;
 #endif