1 files changed, 65 insertions, 29 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index c970fbdca..f702433a0 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -54,6 +54,7 @@
 #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
 
@@ -814,9 +815,6 @@ private:
        result. */
     ValidPathInfos prevInfos;
 
-    const uid_t sandboxUid = 1000;
-    const gid_t sandboxGid = 100;
-
 public:
     DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs,
         Worker & worker, BuildMode buildMode = bmNormal);
@@ -1644,8 +1642,56 @@ void chmod_(const Path & path, mode_t mode)
 }
 
 
+#if __linux__
+
+#define FORCE_SUCCESS(syscall) \
+    if (seccomp_rule_add(ctx, SCMP_ACT_ERRNO(0), SCMP_SYS(syscall), 0) != 0) { \
+        seccomp_release(ctx); \
+        throw SysError("unable to add seccomp rule for " #syscall); \
+    }
+
+void setupSeccomp(void) {
+    scmp_filter_ctx ctx;
+
+    if ((ctx = seccomp_init(SCMP_ACT_ALLOW)) == NULL)
+        throw SysError("unable to initialize seccomp mode 2");
+
+#if defined(__x86_64__)
+    if (seccomp_arch_add(ctx, SCMP_ARCH_X86) != 0) {
+        seccomp_release(ctx);
+        throw SysError("unable to add 32bit seccomp architecture");
+    }
+#endif
+
+    FORCE_SUCCESS(chown32);
+    FORCE_SUCCESS(fchown32);
+    FORCE_SUCCESS(lchown32);
+
+    FORCE_SUCCESS(chown);
+    FORCE_SUCCESS(fchown);
+    FORCE_SUCCESS(fchownat);
+    FORCE_SUCCESS(lchown);
+
+    FORCE_SUCCESS(setxattr);
+    FORCE_SUCCESS(lsetxattr);
+    FORCE_SUCCESS(fsetxattr);
+
+    if (seccomp_load(ctx) != 0) {
+        seccomp_release(ctx);
+        throw SysError("unable to load seccomp BPF program");
+    }
+
+    seccomp_release(ctx);
+}
+
+#undef FORCE_SUCCESS
+
+#endif
+
+
 int childEntry(void * arg)
 {
+    setupSeccomp();
     ((DerivationGoal *) arg)->runChild();
     return 1;
 }
@@ -1965,18 +2011,14 @@ void DerivationGoal::startBuilder()
         createDirs(chrootRootDir + "/etc");
 
         writeFile(chrootRootDir + "/etc/passwd",
-            (format(
-                "root:x:0:0:Nix build user:/:/noshell\n"
-                "nixbld:x:%1%:%2%:Nix build user:/:/noshell\n"
-                "nobody:x:65534:65534:Nobody:/:/noshell\n") % sandboxUid % sandboxGid).str());
+            "root:x:0:0:Nix build user:/:/noshell\n"
+            "nobody:x:65534:65534:Nobody:/:/noshell\n");
 
         /* Declare the build user's group so that programs get a consistent
            view of the system (e.g., "id -gn"). */
         writeFile(chrootRootDir + "/etc/group",
-            (format(
-                "root:x:0:\n"
-                "nixbld:!:%1%:\n"
-                "nogroup:x:65534:\n") % sandboxGid).str());
+            "root:x:0:\n"
+            "nobody:x:65534:\n");
 
         /* Create /etc/hosts with localhost entry. */
         if (!fixedOutput)
@@ -2160,12 +2202,7 @@ void DerivationGoal::startBuilder()
         Pid helper = startProcess([&]() {
 
             /* Drop additional groups here because we can't do it
-               after we've created the new user namespace.  FIXME:
-               this means that if we're not root in the parent
-               namespace, we can't drop additional groups; they will
-               be mapped to nogroup in the child namespace. There does
-               not seem to be a workaround for this. (But who can tell
-               from reading user_namespaces(7)?)*/
+               after we've created the new user namespace. */
             if (getuid() == 0 && setgroups(0, 0) == -1)
                 throw SysError("setgroups failed");
 
@@ -2198,19 +2235,19 @@ void DerivationGoal::startBuilder()
         if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), tmp)) abort();
         pid = tmp;
 
-        /* Set the UID/GID mapping of the builder's user namespace
-           such that the sandbox user maps to the build user, or to
-           the calling user (if build users are disabled). */
-        uid_t hostUid = buildUser.enabled() ? buildUser.getUID() : getuid();
-        uid_t hostGid = buildUser.enabled() ? buildUser.getGID() : getgid();
+        /* Set the UID/GID mapping of the builder's user
+           namespace such that root maps to the build user, or to the
+           calling user (if build users are disabled). */
+        uid_t targetUid = buildUser.enabled() ? buildUser.getUID() : getuid();
+        uid_t targetGid = buildUser.enabled() ? buildUser.getGID() : getgid();
 
         writeFile("/proc/" + std::to_string(pid) + "/uid_map",
-            (format("%d %d 1") % sandboxUid % hostUid).str());
+            (format("0 %d 1") % targetUid).str());
 
         writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
 
         writeFile("/proc/" + std::to_string(pid) + "/gid_map",
-            (format("%d %d 1") % sandboxGid % hostGid).str());
+            (format("0 %d 1") % targetGid).str());
 
         /* Signal the builder that we've updated its user
            namespace. */
@@ -2420,12 +2457,11 @@ void DerivationGoal::runChild()
             if (rmdir("real-root") == -1)
                 throw SysError("cannot remove real-root directory");
 
-            /* Switch to the sandbox uid/gid in the user namespace,
-               which corresponds to the build user or calling user in
-               the parent namespace. */
-            if (setgid(sandboxGid) == -1)
+            /* Become root in the user namespace, which corresponds to
+               the build user or calling user in the parent namespace. */
+            if (setgid(0) == -1)
                 throw SysError("setgid failed");
-            if (setuid(sandboxUid) == -1)
+            if (setuid(0) == -1)
                 throw SysError("setuid failed");
 
             setUser = false;