@@ -877,6 +877,7 @@ Setting Up the Daemon
@section Setting Up the Daemon
@cindex daemon
+@cindex build daemon
During installation, the @dfn{build daemon} that must be running
to use Guix has already been set up and you can run @command{guix}
commands in your terminal program, @pxref{Getting Started}:
@@ -921,20 +922,38 @@ Build Environment Setup
@cindex build environment
In a standard multi-user setup, Guix and its daemon---the
@command{guix-daemon} program---are installed by the system
-administrator; @file{/gnu/store} is owned by @code{root} and
-@command{guix-daemon} runs as @code{root}. Unprivileged users may use
-Guix tools to build packages or otherwise access the store, and the
-daemon will do it on their behalf, ensuring that the store is kept in a
-consistent state, and allowing built packages to be shared among users.
+administrator. Unprivileged users may use Guix tools to build packages
+or otherwise access the store, and the daemon will do it on their
+behalf, ensuring that the store is kept in a consistent state, and
+allowing built packages to be shared among users.
+
+There are currently two ways to set up and run the build daemon:
+
+@enumerate
+@item
+running @command{guix-daemon} as ``root'', letting it run build
+processes as unprivileged users taken from a pool of build users---this
+is the historical approach;
+
+@item
+running @command{guix-daemon} as a separate unprivileged user, relying
+on Linux's @dfn{unprivileged user namespace} functionality to set up
+isolated environments---this is the option chosen when installing Guix
+on a systemd-based distribution with the installation script
+(@pxref{Binary Installation}).
+@end enumerate
+
+The sections below describe each of these two configurations in more
+detail and summarize the kind of build isolation they provide.
+
+@unnumberedsubsubsec Daemon Running as Root
@cindex build users
When @command{guix-daemon} runs as @code{root}, you may not want package
build processes themselves to run as @code{root} too, for obvious
security reasons. To avoid that, a special pool of @dfn{build users}
should be created for use by build processes started by the daemon.
-These build users need not have a shell and a home directory: they will
-just be used when the daemon drops @code{root} privileges in build
-processes. Having several such users allows the daemon to launch
+Having several such users allows the daemon to launch
distinct build processes under separate UIDs, which guarantees that they
do not interfere with each other---an essential feature since builds are
regarded as pure functions (@pxref{Introduction}).
@@ -977,11 +996,45 @@ Build Environment Setup
# guix-daemon --build-users-group=guixbuild
@end example
+In this setup, @file{/gnu/store} is owned by @code{root}.
+
+@unnumberedsubsubsec Daemon Running Without Privileges
+
+@cindex rootless build daemon
+@cindex unprivileged build daemon
+@cindex build daemon, unprivileged
+The second and preferred option is to run @command{guix-daemon}
+@emph{as an unprivileged user}. It has the advantage of reducing the
+harm that can be done should a build process manage to exploit a
+vulnerability in the daemon. This option requires the user of Linux's
+unprivileged user namespace mechanism; today it is available and enabled
+by most GNU/Linux distributions but can still be disabled. The
+installation script automatically determines whether this option is
+available on your system (@pxref{Binary Installation}).
+
+When using this option, you only need to create one user account, and
+@command{guix-daemon} will run with the authority of that account:
+
+@example
+# groupadd --system guix-daemon
+# useradd -g guix-daemon -G guix-daemon \
+ -d /var/empty -s $(which nologin) \
+ -c "Guix daemon privilege separation user" \
+ --system guix-daemon
+@end example
+
+In this configuration, @file{/gnu/store} is owned by the
+@code{guix-daemon} user.
+
+@unnumberedsubsubsec The Isolated Build Environment
+
@cindex chroot
-@noindent
-This way, the daemon starts build processes in a chroot, under one of
-the @code{guixbuilder} users. On GNU/Linux, by default, the chroot
-environment contains nothing but:
+@cindex build environment isolation
+@cindex isolated build environment
+@cindex hermetic build environment
+In both cases, the daemon starts build processes without privileges in
+an @emph{isolated} or @emph{hermetic} build environment---a ``chroot''.
+On GNU/Linux, by default, the build environment contains nothing but:
@c Keep this list in sync with libstore/build.cc! -----------------------
@itemize
@@ -1015,7 +1068,7 @@ Build Environment Setup
@file{/homeless-shelter}. This helps to highlight inappropriate uses of
@env{HOME} in the build scripts of packages.
-All this usually enough to ensure details of the environment do not
+All this is usually enough to ensure details of the environment do not
influence build processes. In some exceptional cases where more control
is needed---typically over the date, kernel, or CPU---you can resort to
a virtual build machine (@pxref{build-vm, virtual build machines}).
@@ -1035,14 +1088,6 @@ Build Environment Setup
for fixed-output derivations (@pxref{Derivations}) or for substitutes
(@pxref{Substitutes}).
-If you are installing Guix as an unprivileged user, it is still possible
-to run @command{guix-daemon} provided you pass @option{--disable-chroot}.
-However, build processes will not be isolated from one another, and not
-from the rest of the system. Thus, build processes may interfere with
-each other, and may access programs, libraries, and other files
-available on the system---making it much harder to view them as
-@emph{pure} functions.
-
@node Daemon Offload Setup
@subsection Using the Offload Facility
@@ -1567,10 +1612,17 @@ Invoking guix-daemon
@item --disable-chroot
Disable chroot builds.
-Using this option is not recommended since, again, it would allow build
-processes to gain access to undeclared dependencies. It is necessary,
-though, when @command{guix-daemon} is running under an unprivileged user
-account.
+@quotation Warning
+Using this option is not recommended since it allows build processes to
+gain access to undeclared dependencies, to interfere with one another,
+and more generally to do anything that can be done with the authority of
+the daemon---which includes at least the ability to tamper with any file
+in the store!
+
+You may find it necessary, though, when support for Linux unprivileged
+user namespaces is missing (@pxref{Build Environment Setup}). Use at
+your own risk!
+@end quotation
@item --log-compression=@var{type}
Compress build logs according to @var{type}, one of @code{gzip},
@@ -79,7 +79,7 @@ (define %narinfo-cache-directory
;; time, 'guix substitute' is called by guix-daemon as root and stores its
;; cached data in /var/guix/…. However, when invoked from 'guix challenge'
;; as a user, it stores its cache in ~/.cache.
- (if (zero? (getuid))
+ (if (getenv "_NIX_OPTIONS") ;invoked by guix-daemon
(or (and=> (getenv "XDG_CACHE_HOME")
(cut string-append <> "/guix/substitute"))
(string-append %state-directory "/substitute/cache"))
@@ -744,6 +744,10 @@ private:
friend int childEntry(void *);
+ /* Pipe to notify readiness to the child process when using unprivileged
+ user namespaces. */
+ Pipe readiness;
+
/* Check that the derivation outputs all exist and register them
as valid. */
void registerOutputs();
@@ -1619,6 +1623,24 @@ int childEntry(void * arg)
}
+/* UID and GID of the build user inside its own user namespace. */
+static const uid_t guestUID = 30001;
+static const gid_t guestGID = 30000;
+
+/* Initialize the user namespace of CHILD. */
+static void initializeUserNamespace(pid_t child,
+ uid_t hostUID = getuid(),
+ gid_t hostGID = getgid())
+{
+ writeFile("/proc/" + std::to_string(child) + "/uid_map",
+ (format("%d %d 1") % guestUID % hostUID).str());
+
+ writeFile("/proc/" + std::to_string(child) + "/setgroups", "deny");
+
+ writeFile("/proc/" + std::to_string(child) + "/gid_map",
+ (format("%d %d 1") % guestGID % hostGID).str());
+}
+
void DerivationGoal::startBuilder()
{
auto f = format(
@@ -1682,7 +1704,7 @@ void DerivationGoal::startBuilder()
then an attacker could create in it a hardlink to a root-owned file
such as /etc/shadow. If 'keepFailed' is true, the daemon would
then chown that hardlink to the user, giving them write access to
- that file. */
+ that file. See CVE-2021-27851. */
tmpDir += "/top";
if (mkdir(tmpDir.c_str(), 0700) == 1)
throw SysError("creating top-level build directory");
@@ -1799,7 +1821,7 @@ void DerivationGoal::startBuilder()
if (mkdir(chrootRootDir.c_str(), 0750) == -1)
throw SysError(format("cannot create ‘%1%’") % chrootRootDir);
- if (chown(chrootRootDir.c_str(), 0, buildUser.getGID()) == -1)
+ if (buildUser.enabled() && chown(chrootRootDir.c_str(), 0, buildUser.getGID()) == -1)
throw SysError(format("cannot change ownership of ‘%1%’") % chrootRootDir);
/* Create a writable /tmp in the chroot. Many builders need
@@ -1818,8 +1840,8 @@ void DerivationGoal::startBuilder()
(format(
"nixbld:x:%1%:%2%:Nix build user:/:/noshell\n"
"nobody:x:65534:65534:Nobody:/:/noshell\n")
- % (buildUser.enabled() ? buildUser.getUID() : getuid())
- % (buildUser.enabled() ? buildUser.getGID() : getgid())).str());
+ % (buildUser.enabled() ? buildUser.getUID() : guestUID)
+ % (buildUser.enabled() ? buildUser.getGID() : guestGID)).str());
/* Declare the build user's group so that programs get a consistent
view of the system (e.g., "id -gn"). */
@@ -1854,7 +1876,7 @@ void DerivationGoal::startBuilder()
createDirs(chrootStoreDir);
chmod_(chrootStoreDir, 01775);
- if (chown(chrootStoreDir.c_str(), 0, buildUser.getGID()) == -1)
+ if (buildUser.enabled() && chown(chrootStoreDir.c_str(), 0, buildUser.getGID()) == -1)
throw SysError(format("cannot change ownership of ‘%1%’") % chrootStoreDir);
foreach (PathSet::iterator, i, inputPaths) {
@@ -1960,14 +1982,34 @@ void DerivationGoal::startBuilder()
if (useChroot) {
char stack[32 * 1024];
int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | SIGCHLD;
- if (!fixedOutput) flags |= CLONE_NEWNET;
+ if (!fixedOutput) {
+ flags |= CLONE_NEWNET;
+ }
+ if (!buildUser.enabled() || getuid() != 0) {
+ flags |= CLONE_NEWUSER;
+ readiness.create();
+ }
+
/* Ensure proper alignment on the stack. On aarch64, it has to be 16
bytes. */
- pid = clone(childEntry,
+ pid = clone(childEntry,
(char *)(((uintptr_t)stack + sizeof(stack) - 8) & ~(uintptr_t)0xf),
flags, this);
- if (pid == -1)
- throw SysError("cloning builder process");
+ if (pid == -1) {
+ if ((flags & CLONE_NEWUSER) != 0 && getuid() != 0)
+ /* 'clone' fails with EPERM on distros where unprivileged user
+ namespaces are disabled. Error out instead of giving up on
+ isolation. */
+ throw SysError("cannot create process in unprivileged user namespace");
+ else
+ throw SysError("cloning builder process");
+ }
+
+ if ((flags & CLONE_NEWUSER) != 0) {
+ /* Initialize the UID/GID mapping of the child process. */
+ initializeUserNamespace(pid);
+ writeFull(readiness.writeSide, (unsigned char*)"go\n", 3);
+ }
} else
#endif
{
@@ -2013,23 +2055,34 @@ void DerivationGoal::runChild()
_writeToStderr = 0;
+ if (readiness.readSide > 0) {
+ /* Wait for the parent process to initialize the UID/GID mapping
+ of our user namespace. */
+ char str[20] = { '\0' };
+ readFull(readiness.readSide, (unsigned char*)str, 3);
+ if (strcmp(str, "go\n") != 0)
+ throw Error("failed to initialize process in unprivileged user namespace");
+ }
+
restoreAffinity();
commonChildInit(builderOut);
#if CHROOT_ENABLED
if (useChroot) {
- /* Initialise the loopback interface. */
- AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
- if (fd == -1) throw SysError("cannot open IP socket");
+ if (!fixedOutput) {
+ /* Initialise the loopback interface. */
+ AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
+ if (fd == -1) throw SysError("cannot open IP socket");
- struct ifreq ifr;
- strcpy(ifr.ifr_name, "lo");
- ifr.ifr_flags = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
- if (ioctl(fd, SIOCSIFFLAGS, &ifr) == -1)
- throw SysError("cannot set loopback interface flags");
+ struct ifreq ifr;
+ strcpy(ifr.ifr_name, "lo");
+ ifr.ifr_flags = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
+ if (ioctl(fd, SIOCSIFFLAGS, &ifr) == -1)
+ throw SysError("cannot set loopback interface flags");
- fd.close();
+ fd.close();
+ }
/* Set the hostname etc. to fixed values. */
char hostname[] = "localhost";
@@ -2180,6 +2233,27 @@ void DerivationGoal::runChild()
/* Remount root as read-only. */
if (mount("/", "/", 0, MS_BIND | MS_REMOUNT | MS_RDONLY, 0) == -1)
throw SysError(format("read-only remount of build root '%1%' failed") % chrootRootDir);
+
+ if (getuid() != 0) {
+ /* Create a new mount namespace to "lock" previous mounts.
+ See mount_namespaces(7). */
+ auto uid = getuid();
+ auto gid = getgid();
+
+ if (unshare(CLONE_NEWNS | CLONE_NEWUSER) == -1)
+ throw SysError(format("creating new user and mount namespaces"));
+
+ initializeUserNamespace(getpid(), uid, gid);
+
+ /* Check that mounts within the build environment are "locked"
+ together and cannot be separated from within the build
+ environment namespace. Since
+ umount(2) is documented to fail with EINVAL when attempting
+ to unmount one of the mounts that are locked together,
+ check that this is what we get. */
+ int ret = umount(tmpDirInSandbox.c_str());
+ assert(ret == -1 && errno == EINVAL);
+ }
}
#endif
@@ -2476,8 +2550,16 @@ void DerivationGoal::registerOutputs()
if (buildMode == bmRepair)
replaceValidPath(path, actualPath);
else
- if (buildMode != bmCheck && rename(actualPath.c_str(), path.c_str()) == -1)
- throw SysError(format("moving build output `%1%' from the chroot to the store") % path);
+ if (buildMode != bmCheck) {
+ if (S_ISDIR(st.st_mode))
+ /* Change mode on the directory to allow for
+ rename(2). */
+ chmod(actualPath.c_str(), st.st_mode | 0700);
+ if (rename(actualPath.c_str(), path.c_str()) == -1)
+ throw SysError(format("moving build output `%1%' from the chroot to the store") % path);
+ if (S_ISDIR(st.st_mode) && chmod(path.c_str(), st.st_mode) == -1)
+ throw SysError(format("restoring permissions on directory `%1%'") % actualPath);
+ }
}
if (buildMode != bmCheck) actualPath = path;
}
@@ -2736,16 +2818,46 @@ void DerivationGoal::deleteTmpDir(bool force)
// Change the ownership if clientUid is set. Never change the
// ownership or the group to "root" for security reasons.
if (settings.clientUid != (uid_t) -1 && settings.clientUid != 0) {
- _chown(tmpDir, settings.clientUid,
- settings.clientGid != 0 ? settings.clientGid : -1);
+ uid_t uid = settings.clientUid;
+ gid_t gid = settings.clientGid != 0 ? settings.clientGid : -1;
+ bool reown = false;
+
+ /* First remove setuid/setgid bits. */
+ secureFilePerms(tmpDir);
+
+ try {
+ _chown(tmpDir, uid, gid);
+
+ if (getuid() != 0) {
+ /* If, without being root, the '_chown' call above
+ succeeded, then it means we have CAP_CHOWN. Retake
+ ownership of tmpDir itself so it can be renamed
+ below. */
+ reown = true;
+ }
+
+ } catch (SysError & e) {
+ /* When running as an unprivileged user and without
+ CAP_CHOWN, we cannot chown the build tree. Print a
+ message and keep going. */
+ printMsg(lvlInfo, format("cannot change ownership of build directory '%1%': %2%")
+ % tmpDir % strerror(e.errNo));
+ }
if (top != tmpDir) {
+ if (reown) chown(tmpDir.c_str(), getuid(), getgid());
+
// Rename tmpDir to its parent, with an intermediate step.
string pivot = top + ".pivot";
if (rename(top.c_str(), pivot.c_str()) == -1)
throw SysError("pivoting failed build tree");
if (rename((pivot + "/top").c_str(), top.c_str()) == -1)
throw SysError("renaming failed build tree");
+
+ if (reown)
+ /* Running unprivileged but with CAP_CHOWN. */
+ chown(top.c_str(), uid, gid);
+
rmdir(pivot.c_str());
}
}
@@ -1614,11 +1614,19 @@ void LocalStore::createUser(const std::string & userName, uid_t userId)
{
auto dir = settings.nixStateDir + "/profiles/per-user/" + userName;
- createDirs(dir);
- if (chmod(dir.c_str(), 0755) == -1)
- throw SysError(format("changing permissions of directory '%s'") % dir);
- if (chown(dir.c_str(), userId, -1) == -1)
- throw SysError(format("changing owner of directory '%s'") % dir);
+ auto created = createDirs(dir);
+ if (!created.empty()) {
+ if (chmod(dir.c_str(), 0755) == -1)
+ throw SysError(format("changing permissions of directory '%s'") % dir);
+
+ /* The following operation requires CAP_CHOWN or can be handled
+ manually by a user with CAP_CHOWN. */
+ if (chown(dir.c_str(), userId, -1) == -1) {
+ rmdir(dir.c_str());
+ string message = strerror(errno);
+ printMsg(lvlInfo, format("failed to change owner of directory '%1%' to %2%: %3%") % dir % userId % message);
+ }
+ }
}