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.

(cherry picked from commit 6cc6c15a2d)

src/libstore/build.cc

@@ -9,6 +9,7 @@
 #include "archive.hh"
 #include "affinity.hh"
 #include "builtins.hh"
+#include "finally.hh"
 
 #include <algorithm>
 #include <iostream>
@@ -54,6 +55,7 @@
 #include <sys/mount.h>
 #include <sys/syscall.h>
 #include <linux/fs.h>
+#include <seccomp.h>
 #define pivot_root(new_root, put_old) (syscall(SYS_pivot_root, new_root, put_old))
 #endif
 
@@ -2251,6 +2253,42 @@ void DerivationGoal::startBuilder()
 }
 
 
+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
@@ -2262,6 +2300,8 @@ void DerivationGoal::runChild()
 
         commonChildInit(builderOut);
 
+        setupSeccomp();
+
 #if __linux__
         if (useChroot) {
 

src/libstore/local.mk

@@ -14,6 +14,10 @@ ifeq ($(OS), SunOS)
 	libstore_LDFLAGS += -lsocket
 endif
 
+ifeq ($(OS), Linux)
+	libstore_LDFLAGS += -lseccomp
+endif
+
 libstore_CXXFLAGS = \
  -DNIX_PREFIX=\"$(prefix)\" \
  -DNIX_STORE_DIR=\"$(storedir)\" \

src/libutil/finally.hh

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <functional>
+
+/* A trivial class to run a function at the end of a scope. */
+class Finally
+{
+private:
+    std::function<void()> fun;
+
+public:
+    Finally(std::function<void()> fun) : fun(fun) { }
+    ~Finally() { fun(); }
+};