CVE: Fix Receiver malicious tenant (thanos-io#5969)

If running as root or with enough privileges, receiver can create a
directory outside of the configured TenantHeader.

This commit fixes it up by sanitizing the user input and explicity not
allowing such behavior.

Signed-off-by: Daniel Mellado <dmellado@redhat.com>

pkg/receive/handler.go

@@ -12,6 +12,7 @@ import (
 	stdlog "log"
 	"net"
 	"net/http"
+	"path"
 	"sort"
 	"strconv"
 	"sync"
@@ -403,6 +404,13 @@ func (h *Handler) handleRequest(ctx context.Context, rep uint64, tenant string,
 	return h.forward(ctx, tenant, r, wreq)
 }
 
+func (h *Handler) isTenantValid(tenant string) error {
+	if tenant != path.Base(tenant) {
+		return errors.New("Tenant name not valid")
+	}
+	return nil
+}
+
 func (h *Handler) receiveHTTP(w http.ResponseWriter, r *http.Request) {
 	var err error
 	span, ctx := tracing.StartSpan(r.Context(), "receive_http")
@@ -422,6 +430,13 @@ func (h *Handler) receiveHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	err = h.isTenantValid(tenant)
+	if err != nil {
+		level.Error(h.logger).Log("msg", "tenant name not valid", "tenant", tenant)
+		http.Error(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
 	tLogger := log.With(h.logger, "tenant", tenant)
 
 	writeGate := h.Limiter.WriteGate()

pkg/receive/handler_test.go

@@ -1090,6 +1090,54 @@ func Heap(dir string) (err error) {
 	return pprof.WriteHeapProfile(f)
 }
 
+func TestIsTenantValid(t *testing.T) {
+	for _, tcase := range []struct {
+		name   string
+		tenant string
+
+		expectedErr error
+	}{
+		{
+			name:        "test malicious tenant",
+			tenant:      "/etc/foo",
+			expectedErr: errors.New("Tenant name not valid"),
+		},
+		{
+			name:        "test malicious tenant going out of receiver directory",
+			tenant:      "./../../hacker_dir",
+			expectedErr: errors.New("Tenant name not valid"),
+		},
+		{
+			name:        "test slash-only tenant",
+			tenant:      "///",
+			expectedErr: errors.New("Tenant name not valid"),
+		},
+		{
+			name:   "test default tenant",
+			tenant: "default-tenant",
+		},
+		{
+			name:   "test tenant with uuid",
+			tenant: "528d0490-8720-4478-aa29-819d90fc9a9f",
+		},
+		{
+			name:   "test valid tenant",
+			tenant: "foo",
+		},
+	} {
+		t.Run(tcase.name, func(t *testing.T) {
+			h := NewHandler(nil, &Options{})
+			err := h.isTenantValid(tcase.tenant)
+			if tcase.expectedErr != nil {
+				testutil.NotOk(t, err)
+				testutil.Equals(t, tcase.expectedErr.Error(), err.Error())
+				return
+			}
+			testutil.Ok(t, err)
+		})
+	}
+}
+
 func TestRelabel(t *testing.T) {
 	for _, tcase := range []struct {
 		name                 string