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daemon: Prevent privilege escalation with '--keep-failed' [security].

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Fixes <https://bugs.gnu.org/47229>.
Reported by Nathan Nye of WhiteBeam Security.

* nix/libstore/build.cc (DerivationGoal::startBuilder): When 'useChroot'
is true, add "/top" to 'tmpDir'.
(DerivationGoal::deleteTmpDir): Adjust accordingly.  When
'settings.keepFailed' is true, chown in two steps: first the "/top"
sub-directory, and then rename "/top" to its parent.
This commit is contained in:
Ludovic Courtès 2021-03-18 11:39:39 +01:00
parent 898489f48e
commit ec7fb66994
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GPG key ID: 090B11993D9AEBB5
1 changed files with 41 additions and 2 deletions
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43
nix/libstore/build.cc
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@ -1621,6 +1621,24 @@ void DerivationGoal::startBuilder()
auto drvName = storePathToName(drvPath);
tmpDir = createTempDir("", "guix-build-" + drvName, false, false, 0700);
if (useChroot) {
/* Make the build directory seen by the build process a sub-directory.
That way, "/tmp/guix-build-foo.drv-0" is root-owned, and thus its
permissions cannot be changed by the build process, while
"/tmp/guix-build-foo.drv-0/top" is owned by the build user. This
cannot be done when !useChroot because then $NIX_BUILD_TOP would
be inaccessible to the build user by its full file name.
If the build user could make the build directory world-writable,
then an attacker could create in it a hardlink to a root-owned file
such as /etc/shadow. If 'keepFailed' is true, the daemon would
then chown that hardlink to the user, giving them write access to
that file. */
tmpDir += "/top";
if (mkdir(tmpDir.c_str(), 0700) == 1)
throw SysError("creating top-level build directory");
}
/* In a sandbox, for determinism, always use the same temporary
directory. */
tmpDirInSandbox = useChroot ? canonPath("/tmp", true) + "/guix-build-" + drvName + "-0" : tmpDir;
@ -2626,20 +2644,41 @@ static void _chown(const Path & path, uid_t uid, gid_t gid)
void DerivationGoal::deleteTmpDir(bool force)
{
if (tmpDir != "") {
// When useChroot is true, tmpDir looks like
// "/tmp/guix-build-foo.drv-0/top". Its parent is root-owned.
string top;
if (useChroot) {
if (baseNameOf(tmpDir) != "top") abort();
top = dirOf(tmpDir);
} else top = tmpDir;
if (settings.keepFailed && !force) {
printMsg(lvlError,
format("note: keeping build directory `%2%'")
% drvPath % tmpDir);
% drvPath % top);
chmod(tmpDir.c_str(), 0755);
// Change the ownership if clientUid is set. Never change the
// ownership or the group to "root" for security reasons.
if (settings.clientUid != (uid_t) -1 && settings.clientUid != 0) {
_chown(tmpDir, settings.clientUid,
settings.clientGid != 0 ? settings.clientGid : -1);
if (top != tmpDir) {
// Rename tmpDir to its parent, with an intermediate step.
string pivot = top + ".pivot";
if (rename(top.c_str(), pivot.c_str()) == -1)
throw SysError("pivoting failed build tree");
if (rename((pivot + "/top").c_str(), top.c_str()) == -1)
throw SysError("renaming failed build tree");
rmdir(pivot.c_str());
}
}
}
else
else {
deletePath(tmpDir);
if (top != tmpDir) rmdir(dirOf(tmpDir).c_str());
}
tmpDir = "";
}
}