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Dynamically disable user namespaces if CLONE_NEWUSER fails

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This makes builds work inside nixos-enter.

Fixes #3145.
This commit is contained in:
Eelco Dolstra 2020-10-07 22:46:01 +02:00
parent 6aa64627c8
commit 97ffc1e013
1 changed files with 21 additions and 24 deletions
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45
src/libstore/build.cc
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@ -924,8 +924,8 @@ private:
result. */
std::map<Path, ValidPathInfo> prevInfos;
uid_t sandboxUid = 1000;
gid_t sandboxGid = 100;
uid_t sandboxUid() { return usingUserNamespace ? 1000 : buildUser->getUID(); }
gid_t sandboxGid() { return usingUserNamespace ? 100 : buildUser->getGID(); }
const static Path homeDir;
@ -2428,15 +2428,14 @@ void DerivationGoal::startBuilder()
"root:x:0:0:Nix build user:%3%:/noshell\n"
"nixbld:x:%1%:%2%:Nix build user:%3%:/noshell\n"
"nobody:x:65534:65534:Nobody:/:/noshell\n",
sandboxUid, sandboxGid, settings.sandboxBuildDir));
sandboxUid(), sandboxGid(), settings.sandboxBuildDir));
/* 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"
fmt("root:x:0:\n"
"nixbld:!:%1%:\n"
"nogroup:x:65534:\n") % sandboxGid).str());
"nogroup:x:65534:\n", sandboxGid()));
/* Create /etc/hosts with localhost entry. */
if (!(derivationIsImpure(derivationType)))
@ -2640,17 +2639,6 @@ void DerivationGoal::startBuilder()
usingUserNamespace = userNamespacesEnabled;
if (usingUserNamespace) {
sandboxUid = 1000;
sandboxGid = 100;
} else {
debug("note: not using a user namespace");
if (!buildUser)
throw Error("cannot perform a sandboxed build because user namespaces are not enabled; check /proc/sys/user/max_user_namespaces");
sandboxUid = buildUser->getUID();
sandboxGid = buildUser->getGID();
}
Pid helper = startProcess([&]() {
/* Drop additional groups here because we can't do it
@ -2682,11 +2670,12 @@ void DerivationGoal::startBuilder()
flags &= ~CLONE_NEWPID;
child = clone(childEntry, stack + stackSize, flags, this);
}
if (child == -1 && (errno == EPERM || errno == EINVAL)) {
if (usingUserNamespace && child == -1 && (errno == EPERM || errno == EINVAL)) {
/* Some distros patch Linux to not allow unprivileged
* user namespaces. If we get EPERM or EINVAL, try
* without CLONE_NEWUSER and see if that works.
*/
usingUserNamespace = false;
flags &= ~CLONE_NEWUSER;
child = clone(childEntry, stack + stackSize, flags, this);
}
@ -2697,7 +2686,8 @@ void DerivationGoal::startBuilder()
_exit(1);
if (child == -1) throw SysError("cloning builder process");
writeFull(builderOut.writeSide.get(), std::to_string(child) + "\n");
writeFull(builderOut.writeSide.get(),
fmt("%d %d\n", usingUserNamespace, child));
_exit(0);
}, options);
@ -2718,7 +2708,10 @@ void DerivationGoal::startBuilder()
});
pid_t tmp;
if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), tmp)) abort();
auto ss = tokenizeString<std::vector<std::string>>(readLine(builderOut.readSide.get()));
assert(ss.size() == 2);
usingUserNamespace = ss[0] == "1";
if (!string2Int<pid_t>(ss[1], tmp)) abort();
pid = tmp;
if (usingUserNamespace) {
@ -2729,12 +2722,16 @@ void DerivationGoal::startBuilder()
uid_t hostGid = buildUser ? buildUser->getGID() : getgid();
writeFile("/proc/" + std::to_string(pid) + "/uid_map",
(format("%d %d 1") % sandboxUid % hostUid).str());
fmt("%d %d 1", sandboxUid(), hostUid));
writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
writeFile("/proc/" + std::to_string(pid) + "/gid_map",
(format("%d %d 1") % sandboxGid % hostGid).str());
fmt("%d %d 1", sandboxGid(), hostGid));
} else {
debug("note: not using a user namespace");
if (!buildUser)
throw Error("cannot perform a sandboxed build because user namespaces are not enabled; check /proc/sys/user/max_user_namespaces");
}
/* Save the mount namespace of the child. We have to do this
@ -3595,9 +3592,9 @@ void DerivationGoal::runChild()
/* 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)
if (setgid(sandboxGid()) == -1)
throw SysError("setgid failed");
if (setuid(sandboxUid) == -1)
if (setuid(sandboxUid()) == -1)
throw SysError("setuid failed");
setUser = false;