libstore: add LocalDerivationGoal startChild hook
Add a platform-specific function for starting sandboxed child. Generally this just means startProcess, but on Linux we use flags for clone to start a new namespace Change-Id: I41c8aba62676a162388bbe5ab8a7518904c7b058
This commit is contained in:
Artemis Tosini
Artemis Tosini
parent
af1dcc2d5e
commit
e040b762a4
4 changed files with 176 additions and 154 deletions
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@ -1,6 +1,7 @@
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#include "local-derivation-goal.hh"
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#include "indirect-root-store.hh"
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#include "hook-instance.hh"
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#include "store-api.hh"
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#include "worker.hh"
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#include "builtins.hh"
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#include "builtins/buildenv.hh"
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@ -771,160 +772,7 @@ void LocalDerivationGoal::startBuilder()
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buildResult.startTime = time(0);
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/* Fork a child to build the package. */
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#if __linux__
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if (useChroot) {
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/* Set up private namespaces for the build:
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- The PID namespace causes the build to start as PID 1.
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Processes outside of the chroot are not visible to those
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on the inside, but processes inside the chroot are
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visible from the outside (though with different PIDs).
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- The private mount namespace ensures that all the bind
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mounts we do will only show up in this process and its
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children, and will disappear automatically when we're
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done.
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- The private network namespace ensures that the builder
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cannot talk to the outside world (or vice versa). It
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only has a private loopback interface. (Fixed-output
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derivations are not run in a private network namespace
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to allow functions like fetchurl to work.)
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- The IPC namespace prevents the builder from communicating
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with outside processes using SysV IPC mechanisms (shared
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memory, message queues, semaphores). It also ensures
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that all IPC objects are destroyed when the builder
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exits.
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- The UTS namespace ensures that builders see a hostname of
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localhost rather than the actual hostname.
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We use a helper process to do the clone() to work around
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clone() being broken in multi-threaded programs due to
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at-fork handlers not being run. Note that we use
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CLONE_PARENT to ensure that the real builder is parented to
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us.
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*/
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if (derivationType->isSandboxed())
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privateNetwork = true;
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userNamespaceSync.create();
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Pipe sendPid;
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sendPid.create();
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Pid helper = startProcess([&]() {
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sendPid.readSide.close();
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/* We need to open the slave early, before
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CLONE_NEWUSER. Otherwise we get EPERM when running as
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root. */
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openSlave();
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/* Drop additional groups here because we can't do it
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after we've created the new user namespace. */
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if (setgroups(0, 0) == -1) {
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if (errno != EPERM)
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throw SysError("setgroups failed");
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if (settings.requireDropSupplementaryGroups)
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throw Error("setgroups failed. Set the require-drop-supplementary-groups option to false to skip this step.");
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}
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ProcessOptions options;
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options.cloneFlags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
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if (privateNetwork)
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options.cloneFlags |= CLONE_NEWNET;
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if (usingUserNamespace)
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options.cloneFlags |= CLONE_NEWUSER;
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pid_t child = startProcess([&]() { runChild(); }, options).release();
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writeFull(sendPid.writeSide.get(), fmt("%d\n", child));
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_exit(0);
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});
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sendPid.writeSide.close();
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if (helper.wait() != 0)
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throw Error("unable to start build process");
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userNamespaceSync.readSide.reset();
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/* Close the write side to prevent runChild() from hanging
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reading from this. */
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Finally cleanup([&]() {
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userNamespaceSync.writeSide.reset();
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});
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auto ss = tokenizeString<std::vector<std::string>>(readLine(sendPid.readSide.get()));
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assert(ss.size() == 1);
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pid = Pid{string2Int<pid_t>(ss[0]).value()};
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if (usingUserNamespace) {
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/* Set the UID/GID mapping of the builder's user namespace
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such that the sandbox user maps to the build user, or to
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the calling user (if build users are disabled). */
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uid_t hostUid = buildUser ? buildUser->getUID() : getuid();
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uid_t hostGid = buildUser ? buildUser->getGID() : getgid();
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uid_t nrIds = buildUser ? buildUser->getUIDCount() : 1;
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writeFile("/proc/" + std::to_string(pid.get()) + "/uid_map",
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fmt("%d %d %d", sandboxUid(), hostUid, nrIds));
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if (!buildUser || buildUser->getUIDCount() == 1)
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writeFile("/proc/" + std::to_string(pid.get()) + "/setgroups", "deny");
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writeFile("/proc/" + std::to_string(pid.get()) + "/gid_map",
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fmt("%d %d %d", sandboxGid(), hostGid, nrIds));
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} else {
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debug("note: not using a user namespace");
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}
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/* Now that we now the sandbox uid, we can write
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/etc/passwd. */
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writeFile(chrootRootDir + "/etc/passwd", fmt(
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"root:x:0:0:Nix build user:%3%:/noshell\n"
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"nixbld:x:%1%:%2%:Nix build user:%3%:/noshell\n"
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"nobody:x:65534:65534:Nobody:/:/noshell\n",
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sandboxUid(), sandboxGid(), settings.sandboxBuildDir));
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/* Declare the build user's group so that programs get a consistent
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view of the system (e.g., "id -gn"). */
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writeFile(chrootRootDir + "/etc/group",
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fmt("root:x:0:\n"
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"nixbld:!:%1%:\n"
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"nogroup:x:65534:\n", sandboxGid()));
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/* Save the mount- and user namespace of the child. We have to do this
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*before* the child does a chroot. */
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sandboxMountNamespace = AutoCloseFD{open(fmt("/proc/%d/ns/mnt", pid.get()).c_str(), O_RDONLY)};
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if (sandboxMountNamespace.get() == -1)
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throw SysError("getting sandbox mount namespace");
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if (usingUserNamespace) {
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sandboxUserNamespace = AutoCloseFD{open(fmt("/proc/%d/ns/user", pid.get()).c_str(), O_RDONLY)};
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if (sandboxUserNamespace.get() == -1)
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throw SysError("getting sandbox user namespace");
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}
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/* Move the child into its own cgroup. */
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if (cgroup)
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writeFile(*cgroup + "/cgroup.procs", fmt("%d", pid.get()));
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/* Signal the builder that we've updated its user namespace. */
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writeFull(userNamespaceSync.writeSide.get(), "1");
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} else
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#endif
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{
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pid = startProcess([&]() {
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openSlave();
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runChild();
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});
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}
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pid = startChild(openSlave);
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/* parent */
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pid.setSeparatePG(true);
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@ -958,6 +806,14 @@ void LocalDerivationGoal::startBuilder()
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}
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Pid LocalDerivationGoal::startChild(std::function<void()> openSlave) {
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return startProcess([&]() {
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openSlave();
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runChild();
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});
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}
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void LocalDerivationGoal::initTmpDir() {
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/* In a sandbox, for determinism, always use the same temporary
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directory. */
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@ -333,6 +333,12 @@ protected:
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throw Error("sandboxing builds is not supported on this platform");
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};
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/**
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* Create a new process that runs `openSlave` and `runChild`
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* On some platforms this process is created with sandboxing flags.
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*/
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virtual Pid startChild(std::function<void()> openSlave);
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/**
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* Execute the builder, replacing the current process.
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* Generally this means an `execve` call.
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@ -1,10 +1,12 @@
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#include "build/worker.hh"
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#include "cgroup.hh"
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#include "finally.hh"
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#include "gc-store.hh"
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#include "signals.hh"
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#include "platform/linux.hh"
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#include "regex.hh"
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#include <grp.h>
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#include <regex>
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namespace nix {
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@ -204,6 +206,158 @@ void LinuxLocalDerivationGoal::prepareSandbox()
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}
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}
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Pid LinuxLocalDerivationGoal::startChild(std::function<void()> openSlave)
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{
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// If we're not sandboxing no need to faff about, use the fallback
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if (!useChroot) {
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return LocalDerivationGoal::startChild(openSlave);
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}
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/* Set up private namespaces for the build:
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- The PID namespace causes the build to start as PID 1.
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Processes outside of the chroot are not visible to those
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on the inside, but processes inside the chroot are
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visible from the outside (though with different PIDs).
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- The private mount namespace ensures that all the bind
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mounts we do will only show up in this process and its
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children, and will disappear automatically when we're
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done.
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- The private network namespace ensures that the builder
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cannot talk to the outside world (or vice versa). It
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only has a private loopback interface. (Fixed-output
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derivations are not run in a private network namespace
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to allow functions like fetchurl to work.)
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- The IPC namespace prevents the builder from communicating
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with outside processes using SysV IPC mechanisms (shared
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memory, message queues, semaphores). It also ensures
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that all IPC objects are destroyed when the builder
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exits.
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- The UTS namespace ensures that builders see a hostname of
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localhost rather than the actual hostname.
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We use a helper process to do the clone() to work around
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clone() being broken in multi-threaded programs due to
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at-fork handlers not being run. Note that we use
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CLONE_PARENT to ensure that the real builder is parented to
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us.
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*/
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if (derivationType->isSandboxed())
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privateNetwork = true;
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userNamespaceSync.create();
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Pipe sendPid;
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sendPid.create();
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Pid helper = startProcess([&]() {
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sendPid.readSide.close();
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/* We need to open the slave early, before
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CLONE_NEWUSER. Otherwise we get EPERM when running as
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root. */
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openSlave();
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/* Drop additional groups here because we can't do it
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after we've created the new user namespace. */
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if (setgroups(0, 0) == -1) {
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if (errno != EPERM)
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throw SysError("setgroups failed");
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if (settings.requireDropSupplementaryGroups)
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throw Error("setgroups failed. Set the require-drop-supplementary-groups option to false to skip this step.");
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}
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ProcessOptions options;
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options.cloneFlags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
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if (privateNetwork)
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options.cloneFlags |= CLONE_NEWNET;
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if (usingUserNamespace)
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options.cloneFlags |= CLONE_NEWUSER;
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pid_t child = startProcess([&]() { runChild(); }, options).release();
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writeFull(sendPid.writeSide.get(), fmt("%d\n", child));
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_exit(0);
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});
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sendPid.writeSide.close();
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if (helper.wait() != 0)
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throw Error("unable to start build process");
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userNamespaceSync.readSide.reset();
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/* Close the write side to prevent runChild() from hanging
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reading from this. */
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Finally cleanup([&]() {
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userNamespaceSync.writeSide.reset();
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});
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auto ss = tokenizeString<std::vector<std::string>>(readLine(sendPid.readSide.get()));
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assert(ss.size() == 1);
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Pid pid = Pid{string2Int<pid_t>(ss[0]).value()};
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if (usingUserNamespace) {
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/* Set the UID/GID mapping of the builder's user namespace
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such that the sandbox user maps to the build user, or to
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the calling user (if build users are disabled). */
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uid_t hostUid = buildUser ? buildUser->getUID() : getuid();
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uid_t hostGid = buildUser ? buildUser->getGID() : getgid();
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uid_t nrIds = buildUser ? buildUser->getUIDCount() : 1;
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writeFile("/proc/" + std::to_string(pid.get()) + "/uid_map",
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fmt("%d %d %d", sandboxUid(), hostUid, nrIds));
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if (!buildUser || buildUser->getUIDCount() == 1)
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writeFile("/proc/" + std::to_string(pid.get()) + "/setgroups", "deny");
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writeFile("/proc/" + std::to_string(pid.get()) + "/gid_map",
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fmt("%d %d %d", sandboxGid(), hostGid, nrIds));
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} else {
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debug("note: not using a user namespace");
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}
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/* Now that we now the sandbox uid, we can write
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/etc/passwd. */
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writeFile(chrootRootDir + "/etc/passwd", fmt(
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"root:x:0:0:Nix build user:%3%:/noshell\n"
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"nixbld:x:%1%:%2%:Nix build user:%3%:/noshell\n"
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"nobody:x:65534:65534:Nobody:/:/noshell\n",
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sandboxUid(), sandboxGid(), settings.sandboxBuildDir));
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/* Declare the build user's group so that programs get a consistent
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view of the system (e.g., "id -gn"). */
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writeFile(chrootRootDir + "/etc/group",
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fmt("root:x:0:\n"
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"nixbld:!:%1%:\n"
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"nogroup:x:65534:\n", sandboxGid()));
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/* Save the mount- and user namespace of the child. We have to do this
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*before* the child does a chroot. */
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sandboxMountNamespace = AutoCloseFD{open(fmt("/proc/%d/ns/mnt", pid.get()).c_str(), O_RDONLY)};
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if (sandboxMountNamespace.get() == -1)
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throw SysError("getting sandbox mount namespace");
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if (usingUserNamespace) {
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sandboxUserNamespace = AutoCloseFD{open(fmt("/proc/%d/ns/user", pid.get()).c_str(), O_RDONLY)};
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if (sandboxUserNamespace.get() == -1)
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throw SysError("getting sandbox user namespace");
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}
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/* Move the child into its own cgroup. */
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if (cgroup)
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writeFile(*cgroup + "/cgroup.procs", fmt("%d", pid.get()));
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/* Signal the builder that we've updated its user namespace. */
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writeFull(userNamespaceSync.writeSide.get(), "1");
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return pid;
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}
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void LinuxLocalDerivationGoal::killSandbox(bool getStats)
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{
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if (cgroup) {
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@ -47,6 +47,12 @@ private:
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*/
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void prepareSandbox() override;
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/**
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* Start child process in new namespaces and cgroup,
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* create /etc/passwd and /etc/group based on discovered uid/gid
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*/
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Pid startChild(std::function<void()> openSlave) override;
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/**
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* Kill all processes by build user, possibly using a reused
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* cgroup if we have one
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