Xen SYSCALL singlestep Handling Privilege Escalation (XSA-204)

#
# (C) Tenable Network Security, Inc.
#

include("compat.inc");

if (description)
{
  script_id(96960);
  script_version("1.7");
  script_set_attribute(attribute:"plugin_modification_date", value:"2020/07/10");

  script_cve_id("CVE-2016-10013");
  script_bugtraq_id(94963);
  script_xref(name:"IAVB", value:"2017-B-0008-S");

  script_name(english:"Xen SYSCALL singlestep Handling Privilege Escalation (XSA-204)");
  script_summary(english:"Checks 'xl info' output for the Xen hypervisor version.");

  script_set_attribute(attribute:"synopsis", value:
"The remote Xen hypervisor installation is missing a security update.");
  script_set_attribute(attribute:"description", value:
"According to its self-reported version number, the Xen hypervisor
installed on the remote host is missing a security update. It is,
therefore, affected by a privilege elevation vulnerability in the
instruction emulator when handling SYSCALL by single-stepping
applications. This is due to incorrect exceptions being raised based
on the flags at the start of an instruction. A local attacker can
exploit this issue to gain elevated privileges on the guest operating
system.

Please note the following items :

  - The vulnerability is only exposed to 64-bit x86 HVM
    guests.

  - On Xen version 4.6 and earlier, the vulnerability is
    exposed to all guest user processes, including
    unprivileged processes in such guests.

  - On Xen version 4.7 and later, the vulnerability is
    exposed only to guest user processes granted a degree of
    privilege (e.g., direct hardware access) by the guest
    administrator, or else to all user processes when the
    VM has been explicitly configured with a non-default CPU
    vendor string (in xm/xl, this would be done with a
    'cpuid=' domain config option).

  - A 64-bit guest kernel which uses an IST for #DB handling
    will most likely mitigate the issue but will have a
    single unexpected #DB exception frame to deal with. In
    practice, this means that Linux is not vulnerable.

  - The vulnerability is not exposed to 32-bit HVM guests.
    This is because the emulation bug also matches real
    hardware behavior, and a 32-bit guest kernel using
    SYSCALL will already have to be using a Task Gate for
    handling #DB to avoid being susceptible to an escalation
    of privilege.

  - The vulnerability is not exposed to PV guests.

  - ARM systems are not vulnerable.

Note that Nessus has not tested for this vulnerability but has instead
relied only on the changeset versions based on the xen.git change log.
Nessus did not check guest hardware configurations or if patches were
applied manually to the source code before a recompile and reinstall.");
  script_set_attribute(attribute:"see_also", value:"https://xenbits.xen.org/xsa/advisory-204.html");
  script_set_attribute(attribute:"see_also", value:"https://xenbits.xen.org/gitweb/?p=xen.git;a=summary");
  script_set_attribute(attribute:"solution", value:
"Apply the appropriate patch according to the vendor advisory.");
  script_set_cvss_base_vector("CVSS2#AV:L/AC:L/Au:N/C:P/I:P/A:P");
  script_set_cvss_temporal_vector("CVSS2#E:U/RL:OF/RC:C");
  script_set_cvss3_base_vector("CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:U/RL:O/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2016-10013");

  script_set_attribute(attribute:"exploitability_ease", value:"No known exploits are available");
  script_set_attribute(attribute:"exploit_available", value:"false");

  script_set_attribute(attribute:"vuln_publication_date", value:"2016/12/19");
  script_set_attribute(attribute:"patch_publication_date", value:"2016/12/19");
  script_set_attribute(attribute:"plugin_publication_date", value:"2017/02/02");

  script_set_attribute(attribute:"potential_vulnerability", value:"true");
  script_set_attribute(attribute:"plugin_type", value:"local");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:xen:xen");
  script_set_attribute(attribute:"stig_severity", value:"I");
  script_end_attributes();

  script_category(ACT_GATHER_INFO);
  script_family(english:"Misc.");

  script_copyright(english:"This script is Copyright (C) 2017-2020 and is owned by Tenable, Inc. or an Affiliate thereof.");

  script_dependencies("xen_server_detect.nbin");
  script_require_keys("installed_sw/Xen Hypervisor", "Settings/ParanoidReport");

  exit(0);
}

include("audit.inc");
include("global_settings.inc");
include("install_func.inc");
include("misc_func.inc");

app_name = "Xen Hypervisor";
install  = get_single_install(app_name:app_name);
if (report_paranoia < 2) audit(AUDIT_PARANOID);

version         = install['version'];
display_version = install['display_version'];
path            = install['path'];
managed_status  = install['Managed status'];
changeset       = install['Changeset'];

if (!empty_or_null(changeset))
  display_version += " (changeset " + changeset + ")";

# Installations that are vendor-managed are handled by OS-specific local package checks
if (managed_status == "managed")
  audit(AUDIT_INST_PATH_NOT_VULN, app_name, display_version, path);

# XSA-204 4.4
fixes['4.4']['fixed_ver']           = '4.4.4';
fixes['4.4']['fixed_ver_display']   = '4.4.4 (changeset 5a343e4)';
fixes['4.4']['affected_ver_regex']  = '^4\\.4\\.';
fixes['4.4']['affected_changesets'] = make_list("619db7d", "149c34a",
  "1c1bfc1", "6639a20", "14fa85a", "1827d52", "e8a46a2", "ed77368",
  "488b7d2", "dfddbf3", "2f3e08d", "fbe0fb4", "0fe7d69", "36a5a87",
  "27f0143", "ec5925c", "aea2669", "8ce712f", "45c1210", "a0b99ab",
  "9b2061d", "35fe0d6", "7e42cb6", "c1f9a26", "0eb5ef2", "98d7429",
  "2dbe363", "5f07492", "3534322", "6428217", "17d7046", "7bd50f1",
  "6f76ac2", "5519488", "04e831a", "9b7d6d2", "76a62af", "0006b20",
  "08a1d2b", "0b5c527", "6e86c87", "cbbb4d1", "e9c81e9", "01311b9",
  "09f9f79", "ab6f899", "6717d99", "5cf1b52", "24ebffa", "c2f8ab3",
  "0ae1e71", "83c5e46", "02426e9", "46b8f78", "ff87c9a", "a611ed5");

# XSA-204 4.5
fixes['4.5']['fixed_ver']           = '4.5.5';
fixes['4.5']['fixed_ver_display']   = '4.5.5 (changeset e3426e2)';
fixes['4.5']['affected_ver_regex']  = '^4\\.5\\.';
fixes['4.5']['affected_changesets'] = make_list("37281bc", "27be856",
  "bdf3ef1", "cc325c0", "8e7b84d", "387b8ae", "34fbae7", "1530da2",
  "274a1f6", "b679cfa", "877b760", "cfe165d", "84e4e56", "e4ae4b0");

# XSA-204 4.6
fixes['4.6']['fixed_ver']           = '4.6.5';
fixes['4.6']['fixed_ver_display']   = '4.6.5-pre (changeset aa281a1)';
fixes['4.6']['affected_ver_regex']  = '^4\\.6\\.';
fixes['4.6']['affected_changesets'] = make_list("ac699ed", "57e3ac3",
  "7789292", "62add85", "22f70a3", "0ba9562", "7902dba", "5f85ab0",
  "7bd27ba", "514173d", "a4902ca", "c03035b", "e0fbb85", "fcab9d3",
  "46529a1", "ffda122", "805bb93");

# XSA-204 4.7
fixes['4.7']['fixed_ver']           = '4.7.2';
fixes['4.7']['fixed_ver_display']   = '4.7.2-pre (changeset c5feb91)';
fixes['4.7']['affected_ver_regex']  = '^4\\.7\\.';
fixes['4.7']['affected_changesets'] = make_list("7a71cea", "e0ea04d",
  "4be57d3", "e144f21", "0726cb5", "32282af", "cf21f0c", "a2d232d",
  "206fc70", "a6b0650", "98eaf9c", "1b65a34", "8ce2238", "2cd9fa0",
  "42bc34b", "e98e17e", "0561a33");

# XSA-204 4.8
fixes['4.8']['fixed_ver']           = '4.8.1';
fixes['4.8']['fixed_ver_display']   = '4.8.1-pre (changeset b996efb)';
fixes['4.8']['affected_ver_regex']  = '^4\\.8\\.';
fixes['4.8']['affected_changesets'] = make_list("7967daf", "1f4ea16");

fix = NULL;
foreach ver_branch (keys(fixes))
{
  if (version =~ fixes[ver_branch]['affected_ver_regex'])
  {
    ret = ver_compare(ver:version, fix:fixes[ver_branch]['fixed_ver']);
    if (ret < 0)
      fix = fixes[ver_branch]['fixed_ver_display'];
    else if (ret == 0)
    {
      if (empty_or_null(changeset))
        fix = fixes[ver_branch]['fixed_ver_display'];
      else
        foreach affected_changeset (fixes[ver_branch]['affected_changesets'])
          if (changeset == affected_changeset)
            fix = fixes[ver_branch]['fixed_ver_display'];
    }
  }
}

if (empty_or_null(fix))
  audit(AUDIT_INST_PATH_NOT_VULN, app_name, display_version, path);

items  = make_array("Installed version", display_version,
                    "Fixed version", fix,
                    "Path", path);
order  = make_list("Path", "Installed version", "Fixed version");
report = report_items_str(report_items:items, ordered_fields:order) + '\n';

security_report_v4(port:0, extra:report, severity:SECURITY_WARNING);