From 97ffc1e0139e124b7e36b5d1a62b90300f231118 Mon Sep 17 00:00:00 2001 From: Eelco Dolstra Date: Wed, 7 Oct 2020 22:46:01 +0200 Subject: [PATCH] Dynamically disable user namespaces if CLONE_NEWUSER fails This makes builds work inside nixos-enter. Fixes #3145. --- src/libstore/build.cc | 45 ++++++++++++++++++++----------------------- 1 file changed, 21 insertions(+), 24 deletions(-) diff --git a/src/libstore/build.cc b/src/libstore/build.cc index a645c429d..05a9ef088 100644 --- a/src/libstore/build.cc +++ b/src/libstore/build.cc @@ -924,8 +924,8 @@ private: result. */ std::map 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(readLine(builderOut.readSide.get()), tmp)) abort(); + auto ss = tokenizeString>(readLine(builderOut.readSide.get())); + assert(ss.size() == 2); + usingUserNamespace = ss[0] == "1"; + if (!string2Int(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;