[PATCH v3 1/2] igvm: validate byte_offset before using it in parameter directives

[Luigi Leonardi <leonardi@redhat.com>]

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_get_param_data(), which looks up the parameter area,
checks byte_offset against its size, and returns the offset-adjusted
data pointer directly (NULL on failure), with the remaining space
returned via an output parameter, 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                | 70 +++++++++++++++++++++++++++++++++++-------
 include/system/igvm-internal.h |  6 ++++
 2 files changed, 65 insertions(+), 11 deletions(-)

diff --git a/backends/igvm.c b/backends/igvm.c
index 7b7bdc72b7..a8340d7d55 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, returns the offset-adjusted write location
+ * within the parameter area and sets *param_size to the remaining space
+ * there, so callers never need to touch param->byte_offset themselves.
+ * Returns NULL on failure.
+ */
+uint8_t *
+qigvm_get_param_data(QIgvm *igvm, uint32_t parameter_area_index,
+                     const IGVM_VHS_PARAMETER *param,
+                     uint32_t *param_size,
+                     Error **errp)
+{
+    QIgvmParameterData *param_entry;
+
+    assert(param_size);
+
+    param_entry = qigvm_find_param_entry(igvm, parameter_area_index, errp);
+    if (!param_entry) {
+        return NULL;
+    }
+
+    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 NULL;
+    }
+
+    *param_size = param_entry->size - param->byte_offset;
+    return param_entry->data + param->byte_offset;
+}
+
 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 +715,25 @@ 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 *param_data;
+    uint32_t param_size;
     uint32_t *vp_count;
     CPUState *cpu;
 
-    param_entry = qigvm_find_param_entry(ctx,
-                                         param->parameter_area_index, errp);
-    if (param_entry == NULL) {
+    param_data = qigvm_get_param_data(ctx, param->parameter_area_index,
+                                       param, &param_size, errp);
+    if (!param_data) {
         return -1;
     }
 
-    vp_count = (uint32_t *)(param_entry->data + param->byte_offset);
+    if (sizeof(*vp_count) > param_size) {
+        error_setg(errp,
+                   "IGVM: vp-count parameter exceeds parameter area "
+                   "defined in IGVM file");
+        return -1;
+    }
+
+    vp_count = (uint32_t *)param_data;
     *vp_count = 0;
     CPU_FOREACH(cpu)
     {
@@ -707,17 +748,24 @@ 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 *param_data;
+    uint32_t param_size;
     IgvmEnvironmentInfo *environmental_state;
 
-    param_entry = qigvm_find_param_entry(ctx,
-                                         param->parameter_area_index, errp);
-    if (param_entry == NULL) {
+    param_data = qigvm_get_param_data(ctx, param->parameter_area_index,
+                                       param, &param_size, errp);
+    if (!param_data) {
+        return -1;
+    }
+
+    if (sizeof(*environmental_state) > param_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 *)param_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..f5ee2d5b1b 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);
 
+uint8_t *
+qigvm_get_param_data(QIgvm *igvm, uint32_t parameter_area_index,
+                     const IGVM_VHS_PARAMETER *param,
+                     uint32_t *param_size,
+                     Error **errp);
+
 #endif

-- 
2.55.0