xen/dmop: check number of input buffers

The hypercall requires at least one input buffer, as both arch-specific
implementations of dm_op() unconditionally assume ->buf[0] to be valid (and
not stack rubble).

Additionally, XEN_DMOP_modified_memory requires two input buffers, yet the
code was assuming the second buffer to always be provided by the user when
checking for the number of extents.  In case the caller sets nr_bufs to 1,
the code in modified_memory() will read stack garbage as the size of the
buffer, thus allowing the caller some degree of insight on the contents of
the stack by probing whether the hypercall returns -EINVAL or -EFAULT as a
result of such bogus call.

This is XSA-506 / CVE-2026-62433.

Fixes: e3b93b3c59 ("dmop: add xendevicemodel_modified_memory_bulk()")
Fixes: 85cb15dfe4 ("x86/hvm/dmop: only copy what is needed to/from the guest")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit aef948a300)

xen/arch/x86/hvm/dm.c

@@ -494,6 +494,12 @@ int dm_op(const struct dmop_args *op_args)
         struct xen_dm_op_modified_memory *data =
             &op.u.modified_memory;
 
+        if ( op_args->nr_bufs != 2 )
+        {
+            rc = -EINVAL;
+            break;
+        }
+
         rc = modified_memory(d, op_args, data);
         const_op = !rc;
         break;
@@ -655,6 +661,9 @@ int compat_dm_op(
     unsigned int i;
     int rc;
 
+    if ( !nr_bufs )
+        return -ENODATA;
+
     if ( nr_bufs > ARRAY_SIZE(args.buf) )
         return -E2BIG;
 

xen/common/dm.c

@@ -26,6 +26,9 @@ long do_dm_op(
     struct dmop_args args;
     int rc;
 
+    if ( !nr_bufs )
+        return -ENODATA;
+
     if ( nr_bufs > ARRAY_SIZE(args.buf) )
         return -E2BIG;