xen: more reliable Xen/guest detection + container awareness (#173)

Better detect Xen guest type + add container detection

CVE-2017-5754: when we see Xen but we're inside a container,
/proc/xen/capabilities isn't exposed and dmesg is the host's,
so dom0 vs PV DomU can't be told apart. Don't report VULN in
that case, but UNKNOWN instead, and ask to rerun the script on the host.

src/libs/370_hw_vmm.sh

@@ -1,25 +1,41 @@
 # vim: set ts=4 sw=4 sts=4 et:
-# Check whether the system is running as a Xen paravirtualized guest
+
-# Returns: 0 if Xen PV, 1 otherwise
+# Probe Xen presence and guest type using the most reliable sources available.
-is_xen() {
+# Prefer /sys/hypervisor when avalable, fallback to dmesg otherwise.
-    local ret
+# Caches results in g_xen (1/0) and g_xen_guest_type (PV|PVH|HVM|'').
-    if [ ! -d "$g_procfs/xen" ]; then
+_detect_xen() {
-        return 1
+    [ "${g_xen_cached:-0}" = 1 ] && return
+    g_xen=0
+    g_xen_guest_type=''
+    g_xen_cached=1
+
+    # Most reliable: /sys/hypervisor/type is 'xen' on any Xen domain (dom0
+    # included), and /sys/hypervisor/guest_type reports PV, PVH or HVM.
+    if [ -r /sys/hypervisor/type ] && [ "$(cat /sys/hypervisor/type 2>/dev/null)" = xen ]; then
+        g_xen=1
+        if [ -r /sys/hypervisor/guest_type ]; then
+            g_xen_guest_type=$(cat /sys/hypervisor/guest_type 2>/dev/null)
+        fi
+        return
     fi
 
-    # XXX do we have a better way that relying on dmesg?
+    # Fallback for kernels without /sys/hypervisor: /proc/xen plus a dmesg probe.
-    dmesg_grep 'Booting paravirtualized kernel on Xen$'
+    if [ -d "$g_procfs/xen" ]; then
-    ret=$?
+        dmesg_grep 'Booting paravirtualized kernel on Xen$'
-    if [ "$ret" -eq 2 ]; then
+        case $? in
-        pr_warn "dmesg truncated, Xen detection will be unreliable. Please reboot and relaunch this script"
+            0) g_xen=1 ;;
-        return 1
+            2) pr_warn "dmesg truncated, Xen detection will be unreliable. Please reboot and relaunch this script" ;;
-    elif [ "$ret" -eq 0 ]; then
+        esac
-        return 0
-    else
-        return 1
     fi
 }
 
+# Check whether the system is running on Xen (any domain type, dom0 included).
+# Returns: 0 if Xen, 1 otherwise
+is_xen() {
+    _detect_xen
+    [ "$g_xen" = 1 ]
+}
+
 # Check whether the system is a Xen Dom0 (privileged domain)
 # Returns: 0 if Dom0, 1 otherwise
 is_xen_dom0() {
@@ -34,31 +50,77 @@ is_xen_dom0() {
     fi
 }
 
-# Check whether the system is a Xen DomU (unprivileged PV guest)
+# Check whether the system is running as a Xen PV DomU (the only Xen guest type
-# Returns: 0 if DomU, 1 otherwise
+# affected by Meltdown, which needs Xen-level mitigation).
+# Returns: 0 if PV DomU, 1 otherwise
 is_xen_domU() {
     local ret
     if ! is_xen; then
         return 1
     fi
 
-    # PVHVM guests also print 'Booting paravirtualized kernel', so we need this check.
+    if is_xen_dom0; then
+        return 1
+    fi
+
+    # When the reliable guest type is known, only PV domains (which aren't
+    # dom0, checked above) are the PV DomU case. PVH and HVM guests are not.
+    if [ -n "$g_xen_guest_type" ]; then
+        [ "$g_xen_guest_type" = PV ] && return 0
+        return 1
+    fi
+
+    # Fallback (no /sys/hypervisor/guest_type): PVHVM guests also print the
+    # 'Booting paravirtualized kernel' line, so exclude them via dmesg.
     dmesg_grep 'Xen HVM callback vector for event delivery is enabled$'
     ret=$?
     if [ "$ret" -eq 0 ]; then
         return 1
     fi
 
-    if ! is_xen_dom0; then
+    return 0
-        return 0
-    else
-        return 1
-    fi
 }
 
-# Check whether the system is running as a guest inside a virtual machine.
+# Check whether we're running inside an OS-level container (LXC, Docker,
+# systemd-nspawn, etc.). Containers share the host kernel, so host/hypervisor
+# introspection (e.g. telling a Xen dom0 from a domU) is unreliable from inside
+# one: /proc/xen is exposed but empty, dmesg is the host's, etc. (issue #173)
+# Returns: 0 if in a container, 1 otherwise
+# Sets: g_is_container (1/0), g_container_reason
+is_running_in_container() {
+    local ctype
+    if [ "${g_is_container_cached:-0}" != 1 ]; then
+        g_is_container=0
+        g_container_reason=''
+        # systemd and most runtimes export 'container=' to PID 1's environment
+        if [ -r "$g_procfs/1/environ" ]; then
+            ctype=$(tr '\0' '\n' <"$g_procfs/1/environ" 2>/dev/null | sed -n 's/^container=//p' | head -n1)
+            if [ -n "$ctype" ]; then
+                g_is_container=1
+                g_container_reason="container=$ctype in $g_procfs/1/environ"
+            fi
+        fi
+        # Docker (and some others) drop a marker file at the filesystem root
+        if [ "$g_is_container" = 0 ] && [ -e /.dockerenv ]; then
+            g_is_container=1
+            g_container_reason="/.dockerenv present"
+        fi
+        # cgroup membership often reveals the runtime (lxc, docker, kubepods, ...)
+        if [ "$g_is_container" = 0 ] && [ -r "$g_procfs/1/cgroup" ]; then
+            if grep -qE '(^|[:/])(lxc|docker|kubepods|libpod|containerd|machine\.slice)([/.]|$)' "$g_procfs/1/cgroup" 2>/dev/null; then
+                g_is_container=1
+                g_container_reason="container runtime found in $g_procfs/1/cgroup"
+            fi
+        fi
+        g_is_container_cached=1
+    fi
+    [ "$g_is_container" = 1 ]
+}
+
+# Check whether the system is running as a guest inside a VM.
 # Uses the 'hypervisor' CPUID feature flag exposed in /proc/cpuinfo by KVM,
 # VMware, Hyper-V, VirtualBox, and most other type-1 and type-2 hypervisors.
+# Xen PV/PVH DomUs don't set that flag, so they're detected separately.
 # Returns: 0 if running as a VM guest, 1 otherwise
 # Sets: g_is_guest_vm (1=guest, 0=not a guest), g_is_guest_vm_reason
 is_running_as_guest() {
@@ -69,6 +131,13 @@ is_running_as_guest() {
             g_is_guest_vm=1
             g_is_guest_vm_reason="'hypervisor' flag in $g_procfs/cpuinfo"
         fi
+        # Xen PV/PVH DomUs don't expose the 'hypervisor' CPUID flag. Don't
+        # classify a container on a Xen host as a guest here: we can't tell
+        # dom0 from domU from inside a container (handled separately).
+        if [ "$g_is_guest_vm" = 0 ] && is_xen && ! is_xen_dom0 && ! is_running_in_container; then
+            g_is_guest_vm=1
+            g_is_guest_vm_reason="Xen ${g_xen_guest_type:-PV} DomU"
+        fi
         g_is_guest_vm_cached=1
     fi
     [ "$g_is_guest_vm" = 1 ]

src/vulns/CVE-2017-5754.sh

@@ -45,7 +45,7 @@ check_CVE_2017_5754() {
 }
 
 check_CVE_2017_5754_linux() {
-    local status sys_interface_available msg kpti_support kpti_can_tell kpti_enabled dmesg_grep pti_xen_pv_domU xen_pv_domo xen_pv_domu explain_text
+    local status sys_interface_available msg kpti_support kpti_can_tell kpti_enabled dmesg_grep pti_xen_pv_domU xen_pv_domo xen_pv_domu xen_unknown_container explain_text
     status=UNK
     sys_interface_available=0
     msg=''
@@ -167,14 +167,24 @@ check_CVE_2017_5754_linux() {
     # Test if the current host is a Xen PV Dom0 / DomU
     xen_pv_domo=0
     xen_pv_domu=0
-    is_xen_dom0 && xen_pv_domo=1
+    xen_unknown_container=0
-    is_xen_domU && xen_pv_domu=1
+    if is_xen && ! is_xen_dom0 && is_running_in_container; then
+        # We can see Xen, but we're inside a container so /proc/xen/capabilities
+        # isn't exposed and dmesg is the host's: we can't tell a safe Dom0 from
+        # a vulnerable PV DomU from in here (issue #173).
+        xen_unknown_container=1
+    else
+        is_xen_dom0 && xen_pv_domo=1
+        is_xen_domU && xen_pv_domu=1
+    fi
 
     if [ "$g_mode" = live ]; then
         # checking whether we're running under Xen PV 64 bits. If yes, we are affected by affected_variant3
         # (unless we are a Dom0)
         pr_info_nol "* Running as a Xen PV DomU: "
-        if [ "$xen_pv_domu" = 1 ]; then
+        if [ "$xen_unknown_container" = 1 ]; then
+            pstatus yellow UNKNOWN "running in a container, can't query Xen from here"
+        elif [ "$xen_pv_domu" = 1 ]; then
             pstatus yellow YES
         else
             pstatus blue NO
@@ -187,7 +197,10 @@ check_CVE_2017_5754_linux() {
     elif [ -z "$msg" ]; then
         # if msg is empty, sysfs check didn't fill it, rely on our own test
         if [ "$g_mode" = live ]; then
-            if [ "$kpti_enabled" = 1 ]; then
+            if [ "$xen_unknown_container" = 1 ]; then
+                pvulnstatus "$cve" UNK "running inside a container on a Xen host, can't determine if the underlying domain is a vulnerable PV DomU"
+                explain "This system looks like a container ($g_container_reason) running on a Xen host. Whether the underlying domain is a safe Dom0 or a vulnerable PV DomU can't be reliably determined from inside a container (/proc/xen is exposed but empty, and dmesg belongs to the host). Please re-run this script directly on the host, outside the container, to get an accurate result."
+            elif [ "$kpti_enabled" = 1 ]; then
                 pvulnstatus "$cve" OK "PTI mitigates the vulnerability"
             elif [ "$xen_pv_domo" = 1 ]; then
                 pvulnstatus "$cve" OK "Xen Dom0s are safe and do not require PTI"