[Xen-changelog] [xen-unstable] x86, vtd: [CVE-2011-1898] Protect against malicious MSIs from untrusted devices.

[Xen patchbot-unstable <patchbot@xxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir@xxxxxxx>
# Date 1305214771 -3600
# Node ID cc91832a02c7cb6b09729ca8e9fc497e5cb2ba4d
# Parent  955d4f369d5ac4d4392ce6ee6818c9a57abcc1c0
x86, vtd: [CVE-2011-1898] Protect against malicious MSIs from untrusted devices.

In the absence of VT-d interrupt remapping support, a device can send
arbitrary APIC messages to host CPUs. One class of attack that results
is to confuse the hypervisor by delivering asynchronous interrupts to
vectors that are expected to handle only synchronous
traps/exceptions.

We block this class of attack by:
(1) setting APIC.TPR=0x10, to block all interrupts below vector
0x20. This blocks delivery to all architectural exception vectors.
(2) checking APIC.ISR[vec] for vectors 0x80 (fast syscall) and 0x82
(hypercall). In these cases we BUG if we detect we are handling a
hardware interrupt -- turning a potentially more severe infiltration
into a straightforward system crash (i.e, DoS).

Thanks to Invisible Things Lab <http://www.invisiblethingslab.com>
for discovery and detailed investigation of this attack.

Signed-off-by: Keir Fraser <keir@xxxxxxx>
---


diff -r 955d4f369d5a -r cc91832a02c7 xen/arch/x86/apic.c
--- a/xen/arch/x86/apic.c       Thu May 12 09:33:59 2011 +0100
+++ b/xen/arch/x86/apic.c       Thu May 12 16:39:31 2011 +0100
@@ -560,12 +560,9 @@
     init_apic_ldr();
 
     /*
-     * Set Task Priority to 'accept all'. We never change this
-     * later on.
+     * Set Task Priority to reject any interrupts below FIRST_DYNAMIC_VECTOR.
      */
-    value = apic_read(APIC_TASKPRI);
-    value &= ~APIC_TPRI_MASK;
-    apic_write_around(APIC_TASKPRI, value);
+    apic_write_around(APIC_TASKPRI, (FIRST_DYNAMIC_VECTOR & 0xF0) - 0x10);
 
     /*
      * After a crash, we no longer service the interrupts and a pending
@@ -1439,3 +1436,9 @@
 
     return 0;
 }
+
+void check_for_unexpected_msi(unsigned int vector)
+{
+    unsigned long v = apic_read(APIC_ISR + ((vector & ~0x1f) >> 1));
+    BUG_ON(v & (1 << (vector & 0x1f)));
+}
diff -r 955d4f369d5a -r cc91832a02c7 xen/arch/x86/x86_64/compat/entry.S
--- a/xen/arch/x86/x86_64/compat/entry.S        Thu May 12 09:33:59 2011 +0100
+++ b/xen/arch/x86/x86_64/compat/entry.S        Thu May 12 16:39:31 2011 +0100
@@ -10,12 +10,22 @@
 #include <asm/page.h>
 #include <asm/desc.h>
 #include <public/xen.h>
+#include <irq_vectors.h>
 
         ALIGN
 ENTRY(compat_hypercall)
         pushq $0
         movl  $TRAP_syscall,4(%rsp)
         SAVE_ALL
+
+        cmpb  $0,untrusted_msi(%rip)
+UNLIKELY_START(ne, msi_check)
+        movl  $HYPERCALL_VECTOR,%edi
+        call  check_for_unexpected_msi
+        RESTORE_ALL
+        SAVE_ALL
+UNLIKELY_END(msi_check)
+
         GET_CURRENT(%rbx)
 
         cmpl  $NR_hypercalls,%eax
diff -r 955d4f369d5a -r cc91832a02c7 xen/arch/x86/x86_64/entry.S
--- a/xen/arch/x86/x86_64/entry.S       Thu May 12 09:33:59 2011 +0100
+++ b/xen/arch/x86/x86_64/entry.S       Thu May 12 16:39:31 2011 +0100
@@ -297,6 +297,14 @@
         pushq $0
         SAVE_ALL
 
+        cmpb  $0,untrusted_msi(%rip)
+UNLIKELY_START(ne, msi_check)
+        movl  $0x80,%edi
+        call  check_for_unexpected_msi
+        RESTORE_ALL
+        SAVE_ALL
+UNLIKELY_END(msi_check)
+
         GET_CURRENT(%rbx)
 
         /* Check that the callback is non-null. */
diff -r 955d4f369d5a -r cc91832a02c7 xen/drivers/passthrough/vtd/iommu.c
--- a/xen/drivers/passthrough/vtd/iommu.c       Thu May 12 09:33:59 2011 +0100
+++ b/xen/drivers/passthrough/vtd/iommu.c       Thu May 12 16:39:31 2011 +0100
@@ -45,6 +45,9 @@
 #define nr_ioapics              iosapic_get_nr_iosapics()
 #endif
 
+/* Possible unfiltered LAPIC/MSI messages from untrusted sources? */
+bool_t __read_mostly untrusted_msi;
+
 int nr_iommus;
 
 static void setup_dom0_devices(struct domain *d);
@@ -1579,6 +1582,14 @@
     if (!pdev)
         return -ENODEV;
 
+    /*
+     * Devices assigned to untrusted domains (here assumed to be any domU)
+     * can attempt to send arbitrary LAPIC/MSI messages. We are unprotected
+     * by the root complex unless interrupt remapping is enabled.
+     */
+    if ( (target != dom0) && !iommu_intremap )
+        untrusted_msi = 1;
+
     ret = domain_context_unmap(source, bus, devfn);
     if ( ret )
         return ret;

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