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Prohibit /proc and /sys to be symlinks

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Commit 3291d66b98 introduced a check for /proc and /sys, making sure
the destination (dest) is a directory (and not e.g. a symlink).

Later, a hunk from commit 0ca91f44f switched from using filepath.Join
to SecureJoin for dest. As SecureJoin follows and resolves symlinks,
the check whether dest is a symlink no longer works.

To fix, do the check without/before using SecureJoin.

Add integration tests to make sure we won't regress.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
This commit is contained in:
Kir Kolyshkin
2023-03-16 14:35:50 -07:00
parent 206008ab9c
commit 0d72adf96d
2 changed files with 48 additions and 18 deletions
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47
libcontainer/rootfs_linux.go
View File

@@ -375,6 +375,35 @@ func doTmpfsCopyUp(m *configs.Mount, rootfs, mountLabel string) (Err error) {
func mountToRootfs(m *configs.Mount, c *mountConfig) error {
rootfs := c.root
// procfs and sysfs are special because we need to ensure they are actually
// mounted on a specific path in a container without any funny business.
switch m.Device {
case "proc", "sysfs":
// If the destination already exists and is not a directory, we bail
// out. This is to avoid mounting through a symlink or similar -- which
// has been a "fun" attack scenario in the past.
// TODO: This won't be necessary once we switch to libpathrs and we can
// stop all of these symlink-exchange attacks.
dest := filepath.Clean(m.Destination)
if !strings.HasPrefix(dest, rootfs) {
// Do not use securejoin as it resolves symlinks.
dest = filepath.Join(rootfs, dest)
}
if fi, err := os.Lstat(dest); err != nil {
if !os.IsNotExist(err) {
return err
}
} else if !fi.IsDir() {
return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device)
}
if err := os.MkdirAll(dest, 0o755); err != nil {
return err
}
// Selinux kernels do not support labeling of /proc or /sys.
return mountPropagate(m, rootfs, "", nil)
}
mountLabel := c.label
mountFd := c.fd
dest, err := securejoin.SecureJoin(rootfs, m.Destination)
@@ -383,24 +412,6 @@ func mountToRootfs(m *configs.Mount, c *mountConfig) error {
}
switch m.Device {
case "proc", "sysfs":
// If the destination already exists and is not a directory, we bail
// out This is to avoid mounting through a symlink or similar -- which
// has been a "fun" attack scenario in the past.
// TODO: This won't be necessary once we switch to libpathrs and we can
// stop all of these symlink-exchange attacks.
if fi, err := os.Lstat(dest); err != nil {
if !os.IsNotExist(err) {
return err
}
} else if fi.Mode()&os.ModeDir == 0 {
return fmt.Errorf("filesystem %q must be mounted on ordinary directory", m.Device)
}
if err := os.MkdirAll(dest, 0o755); err != nil {
return err
}
// Selinux kernels do not support labeling of /proc or /sys
return mountPropagate(m, rootfs, "", nil)
case "mqueue":
if err := os.MkdirAll(dest, 0o755); err != nil {
return err

19
tests/integration/mask.bats
View File

@@ -56,3 +56,22 @@ function teardown() {
[ "$status" -eq 1 ]
[[ "${output}" == *"Operation not permitted"* ]]
}
@test "mask paths [prohibit symlink /proc]" {
ln -s /symlink rootfs/proc
runc run -d --console-socket "$CONSOLE_SOCKET" test_busybox
[ "$status" -eq 1 ]
[[ "${output}" == *"must be mounted on ordinary directory"* ]]
}
@test "mask paths [prohibit symlink /sys]" {
# In rootless containers, /sys is a bind mount not a real sysfs.
requires root
ln -s /symlink rootfs/sys
runc run -d --console-socket "$CONSOLE_SOCKET" test_busybox
[ "$status" -eq 1 ]
# On cgroup v1, this may fail before checking if /sys is a symlink,
# so we merely check that it fails, and do not check the exact error
# message like for /proc above.
}