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Support user namespaces being disabled

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If max_user_namespaces is set to 0, then don't run the build in a user
namespace.

Fixes #4092.
This commit is contained in:
Eelco Dolstra 2020-10-07 22:02:36 +02:00
parent f66bbd8c7b
commit 6aa64627c8
1 changed files with 40 additions and 14 deletions
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54
src/libstore/build.cc
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@ -831,6 +831,10 @@ private:
paths to the sandbox as a result of recursive Nix calls. */ paths to the sandbox as a result of recursive Nix calls. */
AutoCloseFD sandboxMountNamespace; AutoCloseFD sandboxMountNamespace;
/* On Linux, whether we're doing the build in its own user
namespace. */
bool usingUserNamespace = true;
/* The build hook. */ /* The build hook. */
std::unique_ptr<HookInstance> hook; std::unique_ptr<HookInstance> hook;
@ -920,8 +924,8 @@ private:
result. */ result. */
std::map<Path, ValidPathInfo> prevInfos; std::map<Path, ValidPathInfo> prevInfos;
const uid_t sandboxUid = 1000; uid_t sandboxUid = 1000;
const gid_t sandboxGid = 100; gid_t sandboxGid = 100;
const static Path homeDir; const static Path homeDir;
@ -2629,6 +2633,24 @@ void DerivationGoal::startBuilder()
options.allowVfork = false; options.allowVfork = false;
Path maxUserNamespaces = "/proc/sys/user/max_user_namespaces";
static bool userNamespacesEnabled =
pathExists(maxUserNamespaces)
&& trim(readFile(maxUserNamespaces)) != "0";
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([&]() { Pid helper = startProcess([&]() {
/* Drop additional groups here because we can't do it /* Drop additional groups here because we can't do it
@ -2647,9 +2669,11 @@ void DerivationGoal::startBuilder()
PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0); PROT_WRITE | PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1, 0);
if (stack == MAP_FAILED) throw SysError("allocating stack"); if (stack == MAP_FAILED) throw SysError("allocating stack");
int flags = CLONE_NEWUSER | CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD; int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
if (privateNetwork) if (privateNetwork)
flags |= CLONE_NEWNET; flags |= CLONE_NEWNET;
if (usingUserNamespace)
flags |= CLONE_NEWUSER;
pid_t child = clone(childEntry, stack + stackSize, flags, this); pid_t child = clone(childEntry, stack + stackSize, flags, this);
if (child == -1 && errno == EINVAL) { if (child == -1 && errno == EINVAL) {
@ -2697,19 +2721,21 @@ void DerivationGoal::startBuilder()
if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), tmp)) abort(); if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), tmp)) abort();
pid = tmp; pid = tmp;
/* Set the UID/GID mapping of the builder's user namespace if (usingUserNamespace) {
such that the sandbox user maps to the build user, or to /* Set the UID/GID mapping of the builder's user namespace
the calling user (if build users are disabled). */ such that the sandbox user maps to the build user, or to
uid_t hostUid = buildUser ? buildUser->getUID() : getuid(); the calling user (if build users are disabled). */
uid_t hostGid = buildUser ? buildUser->getGID() : getgid(); uid_t hostUid = buildUser ? buildUser->getUID() : getuid();
uid_t hostGid = buildUser ? buildUser->getGID() : getgid();
writeFile("/proc/" + std::to_string(pid) + "/uid_map", writeFile("/proc/" + std::to_string(pid) + "/uid_map",
(format("%d %d 1") % sandboxUid % hostUid).str()); (format("%d %d 1") % sandboxUid % hostUid).str());
writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny"); writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
writeFile("/proc/" + std::to_string(pid) + "/gid_map", writeFile("/proc/" + std::to_string(pid) + "/gid_map",
(format("%d %d 1") % sandboxGid % hostGid).str()); (format("%d %d 1") % sandboxGid % hostGid).str());
}
/* Save the mount namespace of the child. We have to do this /* Save the mount namespace of the child. We have to do this
*before* the child does a chroot. */ *before* the child does a chroot. */
@ -2745,7 +2771,7 @@ void DerivationGoal::startBuilder()
ex.addTrace({}, "while setting up the build environment"); ex.addTrace({}, "while setting up the build environment");
throw ex; throw ex;
} }
debug(msg); debug("sandbox setup: " + msg);
} }
} }