[Xen-devel] [PATCH 5/5] xen/ARM: Restrict access to most HVM_PARAM's

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

ARM currently has no restrictions on toolstack and guest access to the entire
HVM_PARAM block.  As the paging/monitor/sharing features aren't under security
support, this doesn't need an XSA.

The CALLBACK_IRQ and {STORE,CONSOLE}_{PFN,EVTCHN} details exposed read-only to
the guest, while the *_RING_PFN details are restricted to only toolstack
access.  No other parameters are used.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Paul Durrant <paul.durrant@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Julien Grall <julien.grall@xxxxxxx>

This is only compile tested, and based on my reading of the source.  There
might be other PARAMS needing including.
---
 xen/arch/arm/hvm.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 59 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/hvm.c b/xen/arch/arm/hvm.c
index 76b27c9..3581ba2 100644
--- a/xen/arch/arm/hvm.c
+++ b/xen/arch/arm/hvm.c
@@ -31,6 +31,57 @@
 
 #include <asm/hypercall.h>
 
+static int hvm_allow_set_param(const struct domain *d, unsigned int param)
+{
+    switch ( param )
+    {
+        /*
+         * The following parameters are intended for toolstack usage only.
+         * They may not be set by the domain.
+         */
+    case HVM_PARAM_CALLBACK_IRQ:
+    case HVM_PARAM_STORE_PFN:
+    case HVM_PARAM_STORE_EVTCHN:
+    case HVM_PARAM_CONSOLE_PFN:
+    case HVM_PARAM_CONSOLE_EVTCHN:
+    case HVM_PARAM_PAGING_RING_PFN:
+    case HVM_PARAM_MONITOR_RING_PFN:
+    case HVM_PARAM_SHARING_RING_PFN:
+        return d == current->domain ? -EPERM : 0;
+
+        /* Writeable only by Xen, hole, deprecated, or out-of-range. */
+    default:
+        return -EINVAL;
+    }
+}
+
+static int hvm_allow_get_param(const struct domain *d, unsigned int param)
+{
+    switch ( param )
+    {
+        /* The following parameters can be read by the guest and toolstack. */
+    case HVM_PARAM_CALLBACK_IRQ:
+    case HVM_PARAM_STORE_PFN:
+    case HVM_PARAM_STORE_EVTCHN:
+    case HVM_PARAM_CONSOLE_PFN:
+    case HVM_PARAM_CONSOLE_EVTCHN:
+        return 0;
+
+        /*
+         * The following parameters are intended for toolstack usage only.
+         * They may not be read by the domain.
+         */
+    case HVM_PARAM_PAGING_RING_PFN:
+    case HVM_PARAM_MONITOR_RING_PFN:
+    case HVM_PARAM_SHARING_RING_PFN:
+        return d == current->domain ? -EPERM : 0;
+
+        /* Hole, deprecated, or out-of-range. */
+    default:
+        return -EINVAL;
+    }
+}
+
 long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     long rc = 0;
@@ -46,9 +97,6 @@ long do_hvm_op(unsigned long op, XEN_GUEST_HANDLE_PARAM(void) 
arg)
         if ( copy_from_guest(&a, arg, 1) )
             return -EFAULT;
 
-        if ( a.index >= HVM_NR_PARAMS )
-            return -EINVAL;
-
         d = rcu_lock_domain_by_any_id(a.domid);
         if ( d == NULL )
             return -ESRCH;
@@ -59,10 +107,18 @@ long do_hvm_op(unsigned long op, 
XEN_GUEST_HANDLE_PARAM(void) arg)
 
         if ( op == HVMOP_set_param )
         {
+            rc = hvm_allow_set_param(d, a.index);
+            if ( rc )
+                goto param_fail;
+
             d->arch.hvm.params[a.index] = a.value;
         }
         else
         {
+            rc = hvm_allow_get_param(d, a.index);
+            if ( rc )
+                goto param_fail;
+
             a.value = d->arch.hvm.params[a.index];
             rc = copy_to_guest(arg, &a, 1) ? -EFAULT : 0;
         }
-- 
2.1.4


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