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Run builds in a user namespace

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This way, all builds appear to have a uid/gid of 0 inside the
chroot. In the future, this may allow using programs like
systemd-nspawn inside builds, but that will require assigning a larger
UID/GID map to the build.

Issue #625.
This commit is contained in:
Eelco Dolstra 2016-06-09 18:27:39 +02:00
parent 202683a4fc
commit c68e5913c7
1 changed files with 65 additions and 14 deletions
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79
src/libstore/build.cc
View file

@ -746,6 +746,9 @@ private:
/* Pipe for the builder's standard output/error. */
Pipe builderOut;
/* Pipe for synchronising updates to the builder user namespace. */
Pipe userNamespaceSync;
/* The build hook. */
std::shared_ptr<HookInstance> hook;
@ -1930,17 +1933,14 @@ void DerivationGoal::startBuilder()
createDirs(chrootRootDir + "/etc");
writeFile(chrootRootDir + "/etc/passwd",
(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());
"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("nixbld:!:%1%:\n")
% (buildUser.enabled() ? buildUser.getGID() : getgid())).str());
"root:x:0:\n"
"nobody:x:65534:\n");
/* Create /etc/hosts with localhost entry. */
if (!fixedOutput)
@ -2114,32 +2114,66 @@ void DerivationGoal::startBuilder()
if (!fixedOutput)
privateNetwork = true;
userNamespaceSync.create();
ProcessOptions options;
options.allowVfork = false;
Pid helper = startProcess([&]() {
/* Drop additional groups here because we can't do it
after we've created the new user namespace. */
if (getuid() == 0 && setgroups(0, 0) == -1)
throw SysError("setgroups failed");
size_t stackSize = 1 * 1024 * 1024;
char * stack = (char *) mmap(0, stackSize,
PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
if (stack == MAP_FAILED) throw SysError("allocating stack");
int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
if (getuid() != 0)
flags |= CLONE_NEWUSER;
int flags = CLONE_NEWUSER | CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
if (privateNetwork)
flags |= CLONE_NEWNET;
pid_t child = clone(childEntry, stack + stackSize, flags, this);
if (child == -1 && errno == EINVAL)
/* Fallback for Linux < 2.13 where CLONE_NEWPID and
CLONE_PARENT are not allowed together. */
child = clone(childEntry, stack + stackSize, flags & ~CLONE_NEWPID, this);
if (child == -1) throw SysError("cloning builder process");
writeFull(builderOut.writeSide, std::to_string(child) + "\n");
_exit(0);
}, options);
if (helper.wait(true) != 0)
throw Error("unable to start build process");
userNamespaceSync.readSide.close();
pid_t tmp;
if (!string2Int<pid_t>(readLine(builderOut.readSide), tmp)) abort();
pid = tmp;
/* 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("0 %d 1") % targetUid).str());
writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
writeFile("/proc/" + std::to_string(pid) + "/gid_map",
(format("0 %d 1") % targetGid).str());
/* Signal the builder that we've updated its user
namespace. */
writeFull(userNamespaceSync.writeSide, "1");
userNamespaceSync.writeSide.close();
} else
#endif
{
@ -2176,9 +2210,18 @@ void DerivationGoal::runChild()
commonChildInit(builderOut);
bool setUser = true;
#if __linux__
if (useChroot) {
userNamespaceSync.writeSide.close();
if (drainFD(userNamespaceSync.readSide) != "1")
throw Error("user namespace initialisation failed");
userNamespaceSync.readSide.close();
if (privateNetwork) {
/* Initialise the loopback interface. */
@ -2292,8 +2335,7 @@ void DerivationGoal::runChild()
requires the kernel to be compiled with
CONFIG_DEVPTS_MULTIPLE_INSTANCES=y (which is the case
if /dev/ptx/ptmx exists). */
if (getuid() == 0 &&
pathExists("/dev/pts/ptmx") &&
if (pathExists("/dev/pts/ptmx") &&
!pathExists(chrootRootDir + "/dev/ptmx")
&& dirsInChroot.find("/dev/pts") == dirsInChroot.end())
{
@ -2324,6 +2366,15 @@ void DerivationGoal::runChild()
if (rmdir("real-root") == -1)
throw SysError("cannot remove real-root directory");
/* 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(0) == -1)
throw SysError("setuid failed");
setUser = false;
}
#endif
@ -2375,7 +2426,7 @@ void DerivationGoal::runChild()
descriptors except std*, so that's safe. Also note that
setuid() when run as root sets the real, effective and
saved UIDs. */
if (buildUser.enabled()) {
if (setUser && buildUser.enabled()) {
/* Preserve supplementary groups of the build user, to allow
admins to specify groups such as "kvm". */
if (!buildUser.getSupplementaryGIDs().empty() &&
@ -2819,7 +2870,7 @@ void DerivationGoal::registerOutputs()
format("output ‘%1%’ of ‘%2%’ differs from previous round")
% i->path % drvPath);
}
assert(false); // shouldn't happen
abort(); // shouldn't happen
}
if (settings.keepFailed) {