[Xen-devel] [PATCH v4 2/2] xsm: hide detailed Xen version from unprivileged guests

[Sergey Dyasli <sergey.dyasli@citrix.com>]

Hide the following information that can help identify the running Xen
binary version: XENVER_[extraversion|compile_info|changeset]
This makes harder for malicious guests to fingerprint Xen to identify
exploitable systems.

Introduce xsm_filter_denied() to hvmloader to remove "<denied>" string
from guest's DMI tables that otherwise would be shown in tools like
dmidecode.

While at it, add explicit cases for XENVER_[commandline|build_id]
for better code readability. Add a default case with an ASSERT to make
sure that every case is explicitly listed as well.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
---
v3 --> v4:
- Updated commit message
- Re-add hvmloader filtering

v2 --> v3:
- Remove hvmloader filtering
- Add ASSERT_UNREACHABLE

v1 --> v2:
- Added xsm_filter_denied() to hvmloader instead of modifying xen_deny()
- Made behaviour the same for both Release and Debug builds
- XENVER_capabilities is no longer hided

---
 tools/firmware/hvmloader/hvmloader.c |  1 +
 tools/firmware/hvmloader/smbios.c    |  1 +
 tools/firmware/hvmloader/util.c      | 11 +++++++++++
 tools/firmware/hvmloader/util.h      |  2 ++
 xen/include/xsm/dummy.h              | 15 +++++++++++----
 5 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/tools/firmware/hvmloader/hvmloader.c b/tools/firmware/hvmloader/hvmloader.c
index 598a226278..b35899f2fb 100644
--- a/tools/firmware/hvmloader/hvmloader.c
+++ b/tools/firmware/hvmloader/hvmloader.c
@@ -147,6 +147,7 @@ static void init_hypercalls(void)
     /* Print version information. */
     cpuid(base + 1, &eax, &ebx, &ecx, &edx);
     hypercall_xen_version(XENVER_extraversion, extraversion);
+    xsm_filter_denied(extraversion);
     printf("Detected Xen v%u.%u%s\n", eax >> 16, eax & 0xffff, extraversion);
 }
 
diff --git a/tools/firmware/hvmloader/smbios.c b/tools/firmware/hvmloader/smbios.c
index 97a054e9e3..a71bfe8392 100644
--- a/tools/firmware/hvmloader/smbios.c
+++ b/tools/firmware/hvmloader/smbios.c
@@ -275,6 +275,7 @@ hvm_write_smbios_tables(
     xen_minor_version = (uint16_t) xen_version;
 
     hypercall_xen_version(XENVER_extraversion, xen_extra_version);
+    xsm_filter_denied(xen_extra_version);
 
     /* build up human-readable Xen version string */
     p = xen_version_str;
diff --git a/tools/firmware/hvmloader/util.c b/tools/firmware/hvmloader/util.c
index 0c3f2d24cd..49b4b321e3 100644
--- a/tools/firmware/hvmloader/util.c
+++ b/tools/firmware/hvmloader/util.c
@@ -28,6 +28,7 @@
 #include <xen/xen.h>
 #include <xen/memory.h>
 #include <xen/sched.h>
+#include <xen/version.h>
 #include <xen/hvm/hvm_xs_strings.h>
 #include <xen/hvm/params.h>
 
@@ -995,6 +996,16 @@ void hvmloader_acpi_build_tables(struct acpi_config *config,
     hvm_param_set(HVM_PARAM_VM_GENERATION_ID_ADDR, config->vm_gid_addr);
 }
 
+void xsm_filter_denied(char *str)
+{
+    xen_denied_string_t deny_str = "";
+
+    hypercall_xen_version(XENVER_denied_string, deny_str);
+
+    if ( strcmp(str, deny_str) == 0 )
+        *str = '\0';
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/tools/firmware/hvmloader/util.h b/tools/firmware/hvmloader/util.h
index 7bca6418d2..e4fd26de9d 100644
--- a/tools/firmware/hvmloader/util.h
+++ b/tools/firmware/hvmloader/util.h
@@ -286,6 +286,8 @@ struct acpi_config;
 void hvmloader_acpi_build_tables(struct acpi_config *config,
                                  unsigned int physical);
 
+void xsm_filter_denied(char *str);
+
 #endif /* __HVMLOADER_UTIL_H__ */
 
 /*
diff --git a/xen/include/xsm/dummy.h b/xen/include/xsm/dummy.h
index 72a101b106..2567ccaa0a 100644
--- a/xen/include/xsm/dummy.h
+++ b/xen/include/xsm/dummy.h
@@ -751,16 +751,23 @@ static XSM_INLINE int xsm_xen_version (XSM_DEFAULT_ARG uint32_t op)
     case XENVER_denied_string:
         /* These sub-ops ignore the permission checks and return data. */
         return 0;
-    case XENVER_extraversion:
-    case XENVER_compile_info:
+
     case XENVER_capabilities:
-    case XENVER_changeset:
     case XENVER_pagesize:
     case XENVER_guest_handle:
         /* These MUST always be accessible to any guest by default. */
         return xsm_default_action(XSM_HOOK, current->domain, NULL);
-    default:
+
+    case XENVER_extraversion:
+    case XENVER_compile_info:
+    case XENVER_changeset:
+    case XENVER_commandline:
+    case XENVER_build_id:
         return xsm_default_action(XSM_PRIV, current->domain, NULL);
+
+    default:
+        ASSERT_UNREACHABLE();
+        return -EPERM;
     }
 }
 
-- 
2.17.1


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