Re: [PATCH v2 1/2] igvm: validate byte_offset before using it in parameter directives

[Stefano Garzarella <sgarzare@redhat.com>]

On Fri, Sep 04, 2026 at 01:42:55PM +0200, Luigi Leonardi wrote:
>None of the directive handlers that place data at a byte_offset within
>a parameter area validated that byte_offset actually falls within the
>parameter area's size. A malformed IGVM file with byte_offset > size would
>underflow the "size - byte_offset" computation used to determine
>remaining space, wrapping to a huge value and defeating the size
>check, then write out of bounds through param_entry->data +
>byte_offset.
>
>Add qigvm_find_param_validate(), which looks up the parameter area,
>checks byte_offset against its size, and returns the offset-adjusted
>data pointer and remaining size via output parameters instead of the
>raw QIgvmParameterData entry. This keeps the byte_offset arithmetic in
>one place instead of repeating param_entry->data + byte_offset and
>param_entry->size - byte_offset at every call site.
>
>Use it in the vp-count and environment-info handlers, adding an
>explicit check that the fixed-size write fits in the remaining space.
>
>Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
>---
> backends/igvm.c                | 68 +++++++++++++++++++++++++++++++++++-------
> include/system/igvm-internal.h |  6 ++++
> 2 files changed, 63 insertions(+), 11 deletions(-)
>
>diff --git a/backends/igvm.c b/backends/igvm.c
>index 7b7bdc72b7..8c07f2ce5e 100644
>--- a/backends/igvm.c
>+++ b/backends/igvm.c
>@@ -101,6 +101,39 @@ qigvm_find_param_entry(QIgvm *igvm, uint32_t parameter_area_index,
>     return NULL;
> }
>
>+/*
>+ * Look up a parameter area and check that param->byte_offset falls within
>+ * it, in one step. On success, *data and *size are set to the
>+ * offset-adjusted write location and remaining space within the parameter
>+ * area, so callers never need to touch param->byte_offset themselves.
>+ */
>+bool
>+qigvm_find_param_validate(QIgvm *igvm, uint32_t parameter_area_index,

I'm not sure about the name, but I don't have anything better to
suggest, maybe qigvm_get_param_data() ?

>+                          const IGVM_VHS_PARAMETER *param,
>+                          uint8_t **data, uint32_t *size,
>+                          Error **errp)
>+{
>+    QIgvmParameterData *param_entry;
>+
>+    assert(data && size);
>+
>+    param_entry = qigvm_find_param_entry(igvm, parameter_area_index, errp);
>+    if (!param_entry) {
>+        return false;
>+    }

All others qigvm_* functions returns 0 on success and != 0 on failure,
should we be consistent ?

Or maybe we can return the data pointer (with NULL as error).

>+
>+    if (param->byte_offset > param_entry->size) {
>+        error_setg(errp,
>+                   "IGVM: byte_offset 0x%x exceeds parameter area size 0x%x",
>+                   param->byte_offset, param_entry->size);
>+        return false;
>+    }
>+
>+    *data = param_entry->data + param->byte_offset;
>+    *size = param_entry->size - param->byte_offset;
>+    return true;
>+}
>+
> static int qigvm_directive_page_data(QIgvm *ctx, const uint8_t *header_data,
>                                      Error **errp);
> static int qigvm_directive_vp_context(QIgvm *ctx, const uint8_t *header_data,
>@@ -682,17 +715,24 @@ static int qigvm_directive_vp_count(QIgvm *ctx, const uint8_t *header_data,
>                                     Error **errp)
> {
>     const IGVM_VHS_PARAMETER *param = (const IGVM_VHS_PARAMETER *)header_data;
>-    QIgvmParameterData *param_entry;
>+    uint8_t *data;
>+    uint32_t size;

nit: following the previous param_entry, maybe we can call them
param_data and param_size all over the place.

>     uint32_t *vp_count;
>     CPUState *cpu;
>
>-    param_entry = qigvm_find_param_entry(ctx,
>-                                         param->parameter_area_index, errp);
>-    if (param_entry == NULL) {
>+    if (!qigvm_find_param_validate(ctx, param->parameter_area_index, param,
>+                                   &data, &size, errp)) {
>+        return -1;
>+    }
>+
>+    if (sizeof(*vp_count) > size) {
>+        error_setg(errp,
>+                   "IGVM: vp-count parameter exceeds parameter area "
>+                   "defined in IGVM file");
>         return -1;
>     }
>
>-    vp_count = (uint32_t *)(param_entry->data + param->byte_offset);
>+    vp_count = (uint32_t *)data;
>     *vp_count = 0;
>     CPU_FOREACH(cpu)
>     {
>@@ -707,17 +747,23 @@ static int qigvm_directive_environment_info(QIgvm *ctx,
>                                             Error **errp)
> {
>     const IGVM_VHS_PARAMETER *param = (const IGVM_VHS_PARAMETER *)header_data;
>-    QIgvmParameterData *param_entry;
>+    uint8_t *data;
>+    uint32_t size;
>     IgvmEnvironmentInfo *environmental_state;
>
>-    param_entry = qigvm_find_param_entry(ctx,
>-                                         param->parameter_area_index, errp);
>-    if (param_entry == NULL) {
>+    if (!qigvm_find_param_validate(ctx, param->parameter_area_index, param,
>+                                   &data, &size, errp)) {
>+        return -1;
>+    }
>+
>+    if (sizeof(*environmental_state) > size) {
>+        error_setg(errp,
>+                   "IGVM: environment-info parameter exceeds parameter area "
>+                   "defined in IGVM file");
>         return -1;
>     }
>
>-    environmental_state =
>-        (IgvmEnvironmentInfo *)(param_entry->data + param->byte_offset);
>+    environmental_state = (IgvmEnvironmentInfo *)data;
>     environmental_state->memory_is_shared = 1;
>
>     return 0;
>diff --git a/include/system/igvm-internal.h b/include/system/igvm-internal.h
>index 9e9fa1d9af..6526b5055c 100644
>--- a/include/system/igvm-internal.h
>+++ b/include/system/igvm-internal.h
>@@ -81,4 +81,10 @@ QIgvmParameterData*
> qigvm_find_param_entry(QIgvm *igvm, uint32_t parameter_area_index,
>                        Error **errp);
>
>+bool
>+qigvm_find_param_validate(QIgvm *igvm, uint32_t parameter_area_index,
>+                          const IGVM_VHS_PARAMETER *param,
>+                          uint8_t **data, uint32_t *size,
>+                          Error **errp);
>+
> #endif
>
>-- 
>2.55.0
>