[xen staging-4.18] xen/dmop: check number of input buffers

[patchbot@xxxxxxx]

commit 4de4c5cde70a8fc16e80aa500a6f8a4ae202c814
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Jul 14 13:13:12 2026 +0200
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jul 28 13:10:03 2026 +0100

    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: e3b93b3c5954 ("dmop: add xendevicemodel_modified_memory_bulk()")
    Fixes: 85cb15dfe4d1 ("x86/hvm/dmop: only copy what is needed to/from the 
guest")
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit aef948a300da3f10ca5d033177dbd794afc45e2b)
---
 xen/arch/x86/hvm/dm.c | 9 +++++++++
 xen/common/dm.c       | 3 +++
 2 files changed, 12 insertions(+)

diff --git a/xen/arch/x86/hvm/dm.c b/xen/arch/x86/hvm/dm.c
index 462691f91d..58915ad0dc 100644
--- a/xen/arch/x86/hvm/dm.c
+++ b/xen/arch/x86/hvm/dm.c
@@ -493,6 +493,12 @@ 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;
@@ -650,6 +656,9 @@ int compat_dm_op(
     unsigned int i;
     int rc;
 
+    if ( !nr_bufs )
+        return -ENODATA;
+
     if ( nr_bufs > ARRAY_SIZE(args.buf) )
         return -E2BIG;
 
diff --git a/xen/common/dm.c b/xen/common/dm.c
index 201b652deb..8689728ab7 100644
--- a/xen/common/dm.c
+++ b/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;
 
--
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