Package: guix-patches;
Reported by: Ludovic Courtès <ludo <at> gnu.org>
Date: Fri, 24 Jan 2025 17:24:02 UTC
Severity: normal
Tags: patch
Done: Ludovic Courtès <ludo <at> gnu.org>
Bug is archived. No further changes may be made.
View this message in rfc822 format
From: Ludovic Courtès <ludo <at> gnu.org> To: 75810 <at> debbugs.gnu.org Cc: Ludovic Courtès <ludovic.courtes <at> inria.fr>, Christopher Baines <guix <at> cbaines.net>, Josselin Poiret <dev <at> jpoiret.xyz>, Ludovic Courtès <ludo <at> gnu.org>, Mathieu Othacehe <othacehe <at> gnu.org>, Simon Tournier <zimon.toutoune <at> gmail.com>, Tobias Geerinckx-Rice <me <at> tobias.gr> Subject: [bug#75810] [PATCH 1/6] daemon: Allow running as non-root with unprivileged user namespaces. Date: Fri, 24 Jan 2025 18:24:51 +0100
From: Ludovic Courtès <ludovic.courtes <at> inria.fr>
* nix/libstore/build.cc (guestUID, guestGID): New variables.
(initializeUserNamespace): New function.
(DerivationGoal::startBuilder): Call ‘chown’
only when ‘buildUser.enabled()’ is true. Pass CLONE_NEWUSER to ‘clone’
when ‘buildUser.enabled()’ is false or not running as root. Retry
‘clone’ without CLONE_NEWUSER upon EPERM.
(DerivationGoal::registerOutputs): Make ‘actualPath’ writable before
‘rename’.
(DerivationGoal::deleteTmpDir): Catch ‘SysError’ around ‘_chown’ call.
* nix/libstore/local-store.cc (LocalStore::createUser): Do nothing if
‘dirs’ already exists. Warn instead of failing when failing to chown
‘dir’.
* guix/substitutes.scm (%narinfo-cache-directory): Check for
‘_NIX_OPTIONS’ rather than getuid() == 0 to determine the cache
location.
Change-Id: I38fbe01f80fb45a99cd8a391e55a39a54d64fcb7
---
guix/substitutes.scm | 4 +-
nix/libstore/build.cc | 123 +++++++++++++++++++++++++++++-------
nix/libstore/local-store.cc | 22 +++++--
3 files changed, 118 insertions(+), 31 deletions(-)
diff --git a/guix/substitutes.scm b/guix/substitutes.scm
index e31b394020..2761a3dafb 100644
--- a/guix/substitutes.scm
+++ b/guix/substitutes.scm
@@ -1,5 +1,5 @@
;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013-2021, 2023-2024 Ludovic Courtès <ludo <at> gnu.org>
+;;; Copyright © 2013-2021, 2023-2025 Ludovic Courtès <ludo <at> gnu.org>
;;; Copyright © 2014 Nikita Karetnikov <nikita <at> karetnikov.org>
;;; Copyright © 2018 Kyle Meyer <kyle <at> kyleam.com>
;;; Copyright © 2020 Christopher Baines <mail <at> cbaines.net>
@@ -76,7 +76,7 @@ (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"))
diff --git a/nix/libstore/build.cc b/nix/libstore/build.cc
index edd01bab34..727472c77f 100644
--- a/nix/libstore/build.cc
+++ b/nix/libstore/build.cc
@@ -1622,6 +1622,25 @@ 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)
+{
+ auto hostUID = getuid();
+ auto 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(
@@ -1685,7 +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");
@@ -1802,7 +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
@@ -1821,8 +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"). */
@@ -1859,7 +1878,7 @@ 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) {
@@ -1971,14 +1990,42 @@ 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;
+ Pipe readiness;
+ 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 guest. */
+ if (pid == 0) {
+ 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");
+ } else {
+ initializeUserNamespace(pid);
+ writeFull(readiness.writeSide, (unsigned char*)"go\n", 3);
+ }
+ }
} else
#endif
{
@@ -2030,17 +2077,19 @@ void DerivationGoal::runChild()
#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";
@@ -2463,8 +2512,16 @@ 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;
}
@@ -2723,8 +2780,25 @@ 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;
+ 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. */
+ chown(tmpDir.c_str(), getuid(), getgid());
+ }
+ } 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) {
// Rename tmpDir to its parent, with an intermediate step.
@@ -2733,6 +2807,11 @@ void DerivationGoal::deleteTmpDir(bool force)
throw SysError("pivoting failed build tree");
if (rename((pivot + "/top").c_str(), top.c_str()) == -1)
throw SysError("renaming failed build tree");
+
+ if (getuid() != 0)
+ /* Running unprivileged but with CAP_CHOWN. */
+ chown(top.c_str(), uid, gid);
+
rmdir(pivot.c_str());
}
}
diff --git a/nix/libstore/local-store.cc b/nix/libstore/local-store.cc
index 0883a4bbce..4308264a4f 100644
--- a/nix/libstore/local-store.cc
+++ b/nix/libstore/local-store.cc
@@ -306,14 +306,14 @@ void LocalStore::openDB(bool create)
void LocalStore::makeStoreWritable()
{
#if HAVE_UNSHARE && HAVE_STATVFS && HAVE_SYS_MOUNT_H && defined(MS_BIND) && defined(MS_REMOUNT)
- if (getuid() != 0) return;
/* Check if /nix/store is on a read-only mount. */
struct statvfs stat;
if (statvfs(settings.nixStore.c_str(), &stat) != 0)
throw SysError("getting info about the store mount point");
if (stat.f_flag & ST_RDONLY) {
- if (unshare(CLONE_NEWNS) == -1)
+ int flags = CLONE_NEWNS | (getpid() == 0 ? 0 : CLONE_NEWUSER);
+ if (unshare(flags) == -1)
throw SysError("setting up a private mount namespace");
if (mount(0, settings.nixStore.c_str(), "none", MS_REMOUNT | MS_BIND, 0) == -1)
@@ -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);
+ }
+ }
}
--
2.47.1
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Copyright (C) 1999 Darren O. Benham,
1997,2003 nCipher Corporation Ltd,
1994-97 Ian Jackson.