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Don't run builds as root in the user namespace

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This largely reverts c68e5913c7. Running
builds as root breaks "cp -p", since when running as root, "cp -p"
assumes that it can succesfully chown() files. But that's not actually
the case since the user namespace doesn't provide a complete uid
mapping. So it barfs with a fatal error message ("cp: failed to
preserve ownership for 'foo': Invalid argument").
This commit is contained in:
Eelco Dolstra 2016-09-08 18:16:23 +02:00
parent e4bdd49022
commit ff0c0b645c
1 changed files with 31 additions and 16 deletions
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47
src/libstore/build.cc
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@ -804,6 +804,9 @@ private:
result. */
ValidPathInfos prevInfos;
const uid_t sandboxUid = 1000;
const gid_t sandboxGid = 100;
public:
DerivationGoal(const Path & drvPath, const StringSet & wantedOutputs,
Worker & worker, BuildMode buildMode = bmNormal);
@ -1941,14 +1944,18 @@ void DerivationGoal::startBuilder()
createDirs(chrootRootDir + "/etc");
writeFile(chrootRootDir + "/etc/passwd",
"root:x:0:0:Nix build user:/:/noshell\n"
"nobody:x:65534:65534:Nobody:/:/noshell\n");
(format(
"root:x:0:0:Nix build user:/:/noshell\n"
"nixbld:x:%1%:%2%:Nix build user:/:/noshell\n"
"nobody:x:65534:65534:Nobody:/:/noshell\n") % sandboxUid % sandboxGid).str());
/* Declare the build user's group so that programs get a consistent
view of the system (e.g., "id -gn"). */
writeFile(chrootRootDir + "/etc/group",
"root:x:0:\n"
"nobody:x:65534:\n");
(format(
"root:x:0:\n"
"nixbld:!:%1%:\n"
"nogroup:x:65534:\n") % sandboxGid).str());
/* Create /etc/hosts with localhost entry. */
if (!fixedOutput)
@ -2130,7 +2137,12 @@ void DerivationGoal::startBuilder()
Pid helper = startProcess([&]() {
/* Drop additional groups here because we can't do it
after we've created the new user namespace. */
after we've created the new user namespace. FIXME:
this means that if we're not root in the parent
namespace, we can't drop additional groups; they will
be mapped to nogroup in the child namespace. There does
not seem to be a workaround for this. (But who can tell
from reading user_namespaces(7)?)*/
if (getuid() == 0 && setgroups(0, 0) == -1)
throw SysError("setgroups failed");
@ -2163,19 +2175,19 @@ void DerivationGoal::startBuilder()
if (!string2Int<pid_t>(readLine(builderOut.readSide.get()), 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();
/* Set the UID/GID mapping of the builder's user namespace
such that the sandbox user maps to the build user, or to
the calling user (if build users are disabled). */
uid_t hostUid = buildUser.enabled() ? buildUser.getUID() : getuid();
uid_t hostGid = buildUser.enabled() ? buildUser.getGID() : getgid();
writeFile("/proc/" + std::to_string(pid) + "/uid_map",
(format("0 %d 1") % targetUid).str());
(format("%d %d 1") % sandboxUid % hostUid).str());
writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
writeFile("/proc/" + std::to_string(pid) + "/gid_map",
(format("0 %d 1") % targetGid).str());
(format("%d %d 1") % sandboxGid % hostGid).str());
/* Signal the builder that we've updated its user
namespace. */
@ -2341,6 +2353,7 @@ 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 0
if (pathExists("/dev/pts/ptmx") &&
!pathExists(chrootRootDir + "/dev/ptmx")
&& dirsInChroot.find("/dev/pts") == dirsInChroot.end())
@ -2353,6 +2366,7 @@ void DerivationGoal::runChild()
Linux versions, it is created with permissions 0. */
chmod_(chrootRootDir + "/dev/pts/ptmx", 0666);
}
#endif
/* Do the chroot(). */
if (chdir(chrootRootDir.c_str()) == -1)
@ -2373,11 +2387,12 @@ 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)
/* 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)
throw SysError("setgid failed");
if (setuid(0) == -1)
if (setuid(sandboxUid) == -1)
throw SysError("setuid failed");
setUser = false;