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ssh/tailssh: fix SSH on busybox systems

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This involved the following:

1. Pass the su command path as first of args in call to unix.Exec to make sure that busybox sees the correct program name.
   Busybox is a single executable userspace that implements various core userspace commands in a single binary. You'll
   see it used via symlinking, so that for example /bin/su symlinks to /bin/busybox. Busybox knows that you're trying
   to execute /bin/su because argv[0] is '/bin/su'. When we called unix.Exec, we weren't including the program name for
   argv[0], which caused busybox to fail with 'applet not found', meaning that it didn't know which command it was
   supposed to run.
2. Tell su to whitelist the SSH_AUTH_SOCK environment variable in order to support ssh agent forwarding.
3. Run integration tests on alpine, which uses busybox.
4. Increment CurrentCapabilityVersion to allow turning on SSH V2 behavior from control.

Fixes #12849

Signed-off-by: Percy Wegmann <percy@tailscale.com>
pull/13048/head
Percy Wegmann 4 months ago committed by Percy Wegmann
parent 7675c3ebf2
commit 7d83056a1b
5 changed files with 59 additions and 38 deletions
Show Stats Download Patch File Download Diff File

3
Makefile
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@ -117,7 +117,8 @@ sshintegrationtest: ## Run the SSH integration tests in various Docker container
echo "Testing on ubuntu:focal" && docker build --build-arg="BASE=ubuntu:focal" -t ssh-ubuntu-focal ssh/tailssh/testcontainers && \
echo "Testing on ubuntu:jammy" && docker build --build-arg="BASE=ubuntu:jammy" -t ssh-ubuntu-jammy ssh/tailssh/testcontainers && \
echo "Testing on ubuntu:mantic" && docker build --build-arg="BASE=ubuntu:mantic" -t ssh-ubuntu-mantic ssh/tailssh/testcontainers && \
echo "Testing on ubuntu:noble" && docker build --build-arg="BASE=ubuntu:noble" -t ssh-ubuntu-noble ssh/tailssh/testcontainers
echo "Testing on ubuntu:noble" && docker build --build-arg="BASE=ubuntu:noble" -t ssh-ubuntu-noble ssh/tailssh/testcontainers && \
echo "Testing on alpine:latest" && docker build --build-arg="BASE=alpine:latest" -t ssh-alpine-latest ssh/tailssh/testcontainers
help: ## Show this help
@echo "\nSpecify a command. The choices are:\n"

23
ssh/tailssh/incubator.go
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@ -399,7 +399,7 @@ func tryExecLogin(dlogf logger.Logf, ia incubatorArgs) error {
return nil
}
loginArgs := ia.loginArgs(loginCmdPath)
dlogf("logging in with %s %+v", loginCmdPath, loginArgs)
dlogf("logging in with %+v", loginArgs)
// If Exec works, the Go code will not proceed past this:
err = unix.Exec(loginCmdPath, loginArgs, os.Environ())
@ -435,13 +435,18 @@ func trySU(dlogf logger.Logf, ia incubatorArgs) (handled bool, err error) {
defer sessionCloser()
}
loginArgs := []string{"-l", ia.localUser}
loginArgs := []string{
su,
"-w", "SSH_AUTH_SOCK", // pass through SSH_AUTH_SOCK environment variable to support ssh agent forwarding
"-l",
ia.localUser,
}
if ia.cmd != "" {
// Note - unlike the login command, su allows using both -l and -c.
loginArgs = append(loginArgs, "-c", ia.cmd)
}
dlogf("logging in with %s %q", su, loginArgs)
dlogf("logging in with %+v", loginArgs)
// If Exec works, the Go code will not proceed past this:
err = unix.Exec(su, loginArgs, os.Environ())
@ -473,9 +478,15 @@ func findSU(dlogf logger.Logf, ia incubatorArgs) string {
return ""
}
// First try to execute su -l <user> -c true to make sure su supports the
// necessary arguments.
err = exec.Command(su, "-l", ia.localUser, "-c", "true").Run()
// First try to execute su -w SSH_AUTH_SOCK -l <user> -c true
// to make sure su supports the necessary arguments.
err = exec.Command(
su,
"-w", "SSH_AUTH_SOCK",
"-l",
ia.localUser,
"-c", "true",
).Run()
if err != nil {
dlogf("su check failed: %s", err)
return ""

4
ssh/tailssh/tailssh_integration_test.go
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@ -349,7 +349,7 @@ func TestSSHAgentForwarding(t *testing.T) {
// Run tailscale SSH server and connect to it
username := "testuser"
tailscaleAddr := testServer(t, username, false) // TODO: make this false to use V2 behavior
tailscaleAddr := testServer(t, username, false)
tcl, err := ssh.Dial("tcp", tailscaleAddr, &ssh.ClientConfig{
HostKeyCallback: ssh.InsecureIgnoreHostKey(),
})
@ -387,7 +387,7 @@ func TestSSHAgentForwarding(t *testing.T) {
o, err := s.CombinedOutput(fmt.Sprintf(`ssh -T -o StrictHostKeyChecking=no -p %s upstreamuser@%s "true"`, upstreamPort, upstreamHost))
if err != nil {
t.Fatalf("unable to call true command: %s\n%s", err, o)
t.Fatalf("unable to call true command: %s\n%s\n-------------------------", err, o)
}
}

58
ssh/tailssh/testcontainers/Dockerfile
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@ -1,19 +1,24 @@
ARG BASE
FROM ${BASE}
ARG BASE
RUN echo "Install openssh, needed for scp."
RUN apt-get update -y && apt-get install -y openssh-client
RUN if echo "$BASE" | grep "ubuntu:"; then apt-get update -y && apt-get install -y openssh-client; fi
RUN if echo "$BASE" | grep "alpine:"; then apk add openssh; fi
RUN groupadd -g 10000 groupone
RUN groupadd -g 10001 grouptwo
# Note - we do not create the user's home directory, pam_mkhomedir will do that
# Note - on Ubuntu, we do not create the user's home directory, pam_mkhomedir will do that
# for us, and we want to test that PAM gets triggered by Tailscale SSH.
RUN useradd -g 10000 -G 10001 -u 10002 testuser
RUN if echo "$BASE" | grep "ubuntu:"; then groupadd -g 10000 groupone && groupadd -g 10001 grouptwo && useradd -g 10000 -G 10001 -u 10002 testuser; fi
# On Alpine, we can't configure pam_mkhomdir, so go ahead and create home directory.
RUN if echo "$BASE" | grep "alpine:"; then addgroup -g 10000 groupone && addgroup -g 10001 grouptwo && adduser -u 10002 -D testuser && addgroup testuser groupone && addgroup testuser grouptwo; fi
RUN echo "Set up pam_mkhomedir."
RUN sed -i -e 's/Default: no/Default: yes/g' /usr/share/pam-configs/mkhomedir || echo "might not be ubuntu"
RUN cat /usr/share/pam-configs/mkhomedir
RUN pam-auth-update --enable mkhomedir
RUN if echo "$BASE" | grep "ubuntu:"; then \
echo "Set up pam_mkhomedir." && \
sed -i -e 's/Default: no/Default: yes/g' /usr/share/pam-configs/mkhomedir && \
cat /usr/share/pam-configs/mkhomedir && \
pam-auth-update --enable mkhomedir \
; fi
COPY tailscaled .
COPY tailssh.test .
@ -22,11 +27,11 @@ RUN chmod 755 tailscaled
# RUN echo "First run tests normally."
RUN eval `ssh-agent -s` && TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestSSHAgentForwarding
RUN rm -Rf /home/testuser
RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSFTP
RUN rm -Rf /home/testuser
RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSCP
RUN rm -Rf /home/testuser
RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSSH
RUN echo "Then run tests as non-root user testuser and make sure tests still pass."
@ -35,28 +40,31 @@ RUN TAILSCALED_PATH=`pwd`tailscaled eval `su -m testuser -c ssh-agent -s` && su
RUN TAILSCALED_PATH=`pwd`tailscaled su -m testuser -c "./tailssh.test -test.v -test.run TestIntegration TestDoDropPrivileges"
RUN chown root:root /tmp/tailscalessh.log
RUN echo "Then run tests in a system that's pretending to be SELinux in enforcing mode"
RUN mv /usr/bin/login /tmp/login_orig
RUN if echo "$BASE" | grep "ubuntu:"; then \
echo "Then run tests in a system that's pretending to be SELinux in enforcing mode" && \
# Remove execute permissions for /usr/bin/login so that it fails.
mv /usr/bin/login /tmp/login_orig && \
# Use nonsense for /usr/bin/login so that it fails.
# It's not the same failure mode as in SELinux, but failure is good enough for test.
RUN echo "adsfasdfasdf" > /usr/bin/login
RUN chmod 755 /usr/bin/login
echo "adsfasdfasdf" > /usr/bin/login && \
chmod 755 /usr/bin/login && \
# Simulate getenforce command
RUN printf "#!/bin/bash\necho 'Enforcing'" > /usr/bin/getenforce
RUN chmod 755 /usr/bin/getenforce
RUN eval `ssh-agent -s` && TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestSSHAgentForwarding
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegration
RUN mv /tmp/login_orig /usr/bin/login
RUN rm /usr/bin/getenforce
printf "#!/bin/bash\necho 'Enforcing'" > /usr/bin/getenforce && \
chmod 755 /usr/bin/getenforce && \
eval `ssh-agent -s` && TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestSSHAgentForwarding && \
TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegration && \
mv /tmp/login_orig /usr/bin/login && \
rm /usr/bin/getenforce \
; fi
RUN echo "Then remove the login command and make sure tests still pass."
RUN rm `which login`
RUN eval `ssh-agent -s` && TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestSSHAgentForwarding
RUN rm -Rf /home/testuser
RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSFTP
RUN rm -Rf /home/testuser
RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSCP
RUN rm -Rf /home/testuser
RUN if echo "$BASE" | grep "ubuntu:"; then rm -Rf /home/testuser; fi
RUN TAILSCALED_PATH=`pwd`tailscaled ./tailssh.test -test.v -test.run TestIntegrationSSH
RUN echo "Then remove the su command and make sure tests still pass."

3
tailcfg/tailcfg.go
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@ -147,7 +147,8 @@ type CapabilityVersion int
// - 102: 2024-07-12: NodeAttrDisableMagicSockCryptoRouting support
// - 103: 2024-07-24: Client supports NodeAttrDisableCaptivePortalDetection
// - 104: 2024-08-03: SelfNodeV6MasqAddrForThisPeer now works
const CurrentCapabilityVersion CapabilityVersion = 104
// - 105: 2024-08-05: Fixed SSH behavior on systems that use busybox (issue #12849)
const CurrentCapabilityVersion CapabilityVersion = 105
type StableID string

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