Add a seccomp filter to prevent creating setuid/setgid binaries

This prevents builders from setting the S_ISUID or S_ISGID bits,
preventing users from using a nixbld* user to create a setuid/setgid
binary to interfere with subsequent builds under the same nixbld* uid.

This is based on aszlig's seccomp code
(47f587700d).

Reported by Linus Heckemann.

configure.ac

@@ -176,6 +176,13 @@ AC_SUBST(HAVE_SODIUM, [$have_sodium])
 PKG_CHECK_MODULES([LIBLZMA], [liblzma], [CXXFLAGS="$LIBLZMA_CFLAGS $CXXFLAGS"])
 
 
+# Look for libseccomp, required for Linux sandboxing.
+if test "$sys_name" = linux; then
+  PKG_CHECK_MODULES([LIBSECCOMP], [libseccomp],
+                    [CXXFLAGS="$LIBSECCOMP_CFLAGS $CXXFLAGS"])
+fi
+
+
 # Look for aws-cpp-sdk-s3.
 AC_LANG_PUSH(C++)
 AC_CHECK_HEADERS([aws/s3/S3Client.h],

release.nix

@@ -30,6 +30,7 @@ let
             docbook5 docbook5_xsl
             autoconf-archive
             git
+            libseccomp
           ];
 
         configureFlags = "--enable-gc";
@@ -78,6 +79,7 @@ let
             openssl pkgconfig sqlite boehmgc
 
           ]
+          ++ lib.optional stdenv.isLinux libseccomp
           ++ lib.optional (stdenv.isLinux || stdenv.isDarwin) libsodium
           ++ lib.optional (stdenv.isLinux || stdenv.isDarwin)
             (aws-sdk-cpp.override {

shell.nix

@@ -22,7 +22,8 @@ with import ./release-common.nix { inherit pkgs; };
       # For nix-perl
       perl
       perlPackages.DBDSQLite
-    ];
+    ]
+    ++ lib.optional stdenv.isLinux libseccomp;
 
   inherit configureFlags;
 

src/libstore/build.cc

@@ -46,6 +46,7 @@
 #include <sys/param.h>
 #include <sys/mount.h>
 #include <sys/syscall.h>
+#include <seccomp.h>
 #define pivot_root(new_root, put_old) (syscall(SYS_pivot_root, new_root, put_old))
 #endif
 
@@ -2298,6 +2299,42 @@ void DerivationGoal::doExportReferencesGraph()
 }
 
 
+void setupSeccomp()
+{
+#if __linux__
+    scmp_filter_ctx ctx;
+
+    if (!(ctx = seccomp_init(SCMP_ACT_ALLOW)))
+        throw SysError("unable to initialize seccomp mode 2");
+
+    Finally cleanup([&]() {
+        seccomp_release(ctx);
+    });
+
+    if (seccomp_arch_add(ctx, SCMP_ARCH_X86) != 0)
+        throw SysError("unable to add 32-bit seccomp architecture");
+
+    for (int perm : { S_ISUID, S_ISGID }) {
+        // TODO: test chmod and fchmod.
+        if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(chmod), 1,
+                SCMP_A1(SCMP_CMP_MASKED_EQ, perm, perm)) != 0)
+            throw SysError("unable to add seccomp rule");
+
+        if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(fchmod), 1,
+                SCMP_A1(SCMP_CMP_MASKED_EQ, perm, perm)) != 0)
+            throw SysError("unable to add seccomp rule");
+
+        if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(fchmodat), 1,
+                SCMP_A2(SCMP_CMP_MASKED_EQ, perm, perm)) != 0)
+            throw SysError("unable to add seccomp rule");
+    }
+
+    if (seccomp_load(ctx) != 0)
+        throw SysError("unable to load seccomp BPF program");
+#endif
+}
+
+
 void DerivationGoal::runChild()
 {
     /* Warning: in the child we should absolutely not make any SQLite
@@ -2307,6 +2344,8 @@ void DerivationGoal::runChild()
 
         commonChildInit(builderOut);
 
+        setupSeccomp();
+
         bool setUser = true;
 
         /* Make the contents of netrc available to builtin:fetchurl

src/libstore/local.mk

@@ -18,6 +18,10 @@ ifeq ($(OS), SunOS)
 	libstore_LDFLAGS += -lsocket
 endif
 
+ifeq ($(OS), Linux)
+	libstore_LDFLAGS += -lseccomp
+endif
+
 libstore_CXXFLAGS = \
  -DNIX_PREFIX=\"$(prefix)\" \
  -DNIX_STORE_DIR=\"$(storedir)\" \