fix(sandbox): make the job's /tmp sticky

commit: 5ac9bb1494e30a14c2d7120c1e6d86376e7fbd17
author: brice · 8d ago · 2026-10-14 15:40 UTC
committer: bnema · 8d ago · 2026-10-14 15:40 UTC
parent: 2e816bd
signature: SSH UnverifiedSHA256:VOCA5lYARHcbKDZ7GhTV45VmcjWnJ6ZQIBazj17LivA The key is unknown

4 files · +39 -2

M internal/guest/vmsetup.go +1 -1


@@ -140,7 +140,7 @@ func setupVM(log zerowrap.Logger, args map[string]string) error {
140	140		log.Warn().Err(err).Str("target", m.dst).Msg("init: mount")
141	141		}
142	142		}
143		-	_ = os.Chmod("/tmp", 0o1777)
	143	+	_ = os.Chmod("/tmp", sandbox.TmpMode)
144	144
145	145		gw := args[sandbox.CmdlineGateway]
146	146		if err := writeResolvConf(gw); err != nil {


@@ -140,7 +140,7 @@ func setupVM(log zerowrap.Logger, args map[string]string) error {
140		log.Warn().Err(err).Str("target", m.dst).Msg("init: mount")	140		log.Warn().Err(err).Str("target", m.dst).Msg("init: mount")
141		}	141		}
142		}	142		}
143	-	_ = os.Chmod("/tmp", 0o1777)	143	+	_ = os.Chmod("/tmp", sandbox.TmpMode)
144			144
145		gw := args[sandbox.CmdlineGateway]	145		gw := args[sandbox.CmdlineGateway]
146		if err := writeResolvConf(gw); err != nil {	146		if err := writeResolvConf(gw); err != nil {

M internal/sandbox/container/driver.go +1 -1


@@ -414,7 +414,7 @@ func (s *containerSandbox) runScratch(ctx context.Context, spec sandbox.JobSpec)
414	414		return err
415	415		}
416	416		}
417		-	return os.Chmod(filepath.Join(mnt, "tmp"), 0o1777)
	417	+	return os.Chmod(filepath.Join(mnt, "tmp"), sandbox.TmpMode)
418	418		}
419	419
420	420		// runNetwork creates the job's bridge and installs the firewall policy.


@@ -414,7 +414,7 @@ func (s *containerSandbox) runScratch(ctx context.Context, spec sandbox.JobSpec)
414		return err	414		return err
415		}	415		}
416		}	416		}
417	-	return os.Chmod(filepath.Join(mnt, "tmp"), 0o1777)	417	+	return os.Chmod(filepath.Join(mnt, "tmp"), sandbox.TmpMode)
418		}	418		}
419			419
420		// runNetwork creates the job's bridge and installs the firewall policy.	420		// runNetwork creates the job's bridge and installs the firewall policy.

M internal/sandbox/contract.go +8 -0


@@ -2,6 +2,8 @@
2	2
3	3		package sandbox
4	4
	5	+	import "os"
	6	+
5	7		// The host/guest contract shared by the drivers and the guest side
6	8		// (internal/guest). Changing a value here changes both sides at once because
7	9		// the host always attaches its own executable to the sandbox.

@@ -37,3 +39,9 @@ const (
37	39		// CmdlineExternals=1: the externals disk is attached as the fourth drive.
38	40		CmdlineExternals = "ermite.externals"
39	41		)
	42	+
	43	+	// TmpMode is the mode of the job's /tmp, as on GitHub's runners: everyone may
	44	+	// write, only the owner may remove or rename (sticky). os.FileMode has its own
	45	+	// sticky flag: the octal 0o1777 would give a plain 0777, which programs that
	46	+	// check their temp directory refuse as replaceable.
	47	+	const TmpMode = 0o777 \| os.ModeSticky


@@ -2,6 +2,8 @@
2		2
3	package sandbox	3		package sandbox
4		4
		5	+	import "os"
		6	+
5	// The host/guest contract shared by the drivers and the guest side	7		// The host/guest contract shared by the drivers and the guest side
6	// (internal/guest). Changing a value here changes both sides at once because	8		// (internal/guest). Changing a value here changes both sides at once because
7	// the host always attaches its own executable to the sandbox.	9		// the host always attaches its own executable to the sandbox.

@@ -37,3 +39,9 @@ const (
37	// CmdlineExternals=1: the externals disk is attached as the fourth drive.	39		// CmdlineExternals=1: the externals disk is attached as the fourth drive.
38	CmdlineExternals = "ermite.externals"	40		CmdlineExternals = "ermite.externals"
39	)	41		)
		42	+
		43	+	// TmpMode is the mode of the job's /tmp, as on GitHub's runners: everyone may
		44	+	// write, only the owner may remove or rename (sticky). os.FileMode has its own
		45	+	// sticky flag: the octal 0o1777 would give a plain 0777, which programs that
		46	+	// check their temp directory refuse as replaceable.
		47	+	const TmpMode = 0o777 \| os.ModeSticky

A internal/sandbox/tmpmode_test.go +29 -0

@@ -0,0 +1,29 @@
	1	+	// SPDX-License-Identifier: MIT
	2	+
	3	+	package sandbox
	4	+
	5	+	import (
	6	+	"os"
	7	+	"path/filepath"
	8	+	"testing"
	9	+	)
	10	+
	11	+	// The job's /tmp is world-writable and sticky, as on GitHub's runners: a
	12	+	// program that refuses a replaceable temp directory (a writable ancestor
	13	+	// without the sticky bit) must accept it.
	14	+	func TestTmpModeIsStickyAndWorldWritable(t *testing.T) {
	15	+	dir := filepath.Join(t.TempDir(), "tmp")
	16	+	if err := os.Mkdir(dir, 0o755); err != nil {
	17	+	t.Fatal(err)
	18	+	}
	19	+	if err := os.Chmod(dir, TmpMode); err != nil {
	20	+	t.Fatal(err)
	21	+	}
	22	+	fi, err := os.Stat(dir)
	23	+	if err != nil {
	24	+	t.Fatal(err)
	25	+	}
	26	+	if fi.Mode()&os.ModeSticky == 0 \|\| fi.Mode().Perm() != 0o777 {
	27	+	t.Fatalf("mode %v, want sticky and 0777", fi.Mode())
	28	+	}
	29	+	}

@@ -0,0 +1,29 @@
	1	+	// SPDX-License-Identifier: MIT
	2	+
	3	+	package sandbox
	4	+
	5	+	import (
	6	+	"os"
	7	+	"path/filepath"
	8	+	"testing"
	9	+	)
	10	+
	11	+	// The job's /tmp is world-writable and sticky, as on GitHub's runners: a
	12	+	// program that refuses a replaceable temp directory (a writable ancestor
	13	+	// without the sticky bit) must accept it.
	14	+	func TestTmpModeIsStickyAndWorldWritable(t *testing.T) {
	15	+	dir := filepath.Join(t.TempDir(), "tmp")
	16	+	if err := os.Mkdir(dir, 0o755); err != nil {
	17	+	t.Fatal(err)
	18	+	}
	19	+	if err := os.Chmod(dir, TmpMode); err != nil {
	20	+	t.Fatal(err)
	21	+	}
	22	+	fi, err := os.Stat(dir)
	23	+	if err != nil {
	24	+	t.Fatal(err)
	25	+	}
	26	+	if fi.Mode()&os.ModeSticky == 0 \|\| fi.Mode().Perm() != 0o777 {
	27	+	t.Fatalf("mode %v, want sticky and 0777", fi.Mode())
	28	+	}
	29	+	}