x86/32on64: properly honor add-to-physmap-batch's size

Commit 407a3c00ff ("compat/memory: fix build with old gcc") "fixed" a
build issue by switching to the use of uninitialized data. Due to
- the bounding of the uninitialized data item
- the accessed area being outside of Xen space
- arguments being properly verified by the native hypercall function
this is not a security issue.

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: 144aec4140515c53bb1676df71a469f3e285c557
master date: 2017-04-26 09:48:45 +0200

diff --git a/xen/common/compat/memory.c b/xen/common/compat/memory.c
index a37a9483318df8d4e8c87d921fa51a8a8da8ba59..35bb259808e269b6331d2cfbb3b60c245ff1cb22 100644 (file)
--- a/xen/common/compat/memory.c
+++ b/xen/common/compat/memory.c
@@ -251,7 +251,7 @@ int compat_memory_op(unsigned int cmd, XEN_GUEST_HANDLE_PARAM(void) compat)
             unsigned int limit = (COMPAT_ARG_XLAT_SIZE - sizeof(*nat.atpb))
                                  / (sizeof(nat.atpb->idxs.p) + sizeof(nat.atpb->gpfns.p));
             /* Use an intermediate variable to suppress warnings on old gcc: */
-            unsigned int size = cmp.atpb.size;
+            unsigned int size;
             xen_ulong_t *idxs = (void *)(nat.atpb + 1);
             xen_pfn_t *gpfns = (void *)(idxs + limit);
             /*
@@ -262,8 +262,10 @@ int compat_memory_op(unsigned int cmd, XEN_GUEST_HANDLE_PARAM(void) compat)
             enum XLAT_add_to_physmap_batch_u u =
                 XLAT_add_to_physmap_batch_u_res0;
 
-            if ( copy_from_guest(&cmp.atpb, compat, 1) ||
-                 !compat_handle_okay(cmp.atpb.idxs, size) ||
+            if ( copy_from_guest(&cmp.atpb, compat, 1) )
+                return -EFAULT;
+            size = cmp.atpb.size;
+            if ( !compat_handle_okay(cmp.atpb.idxs, size) ||
                  !compat_handle_okay(cmp.atpb.gpfns, size) ||
                  !compat_handle_okay(cmp.atpb.errs, size) )
                 return -EFAULT;