[v8][pku] Add histogram for CFI page mismatch

We assume a maximum OS page size at compile time and need to bail out
of memory protections if there's a mismatch to the size observed at
runtime.
This histogram will be used to see if this is happening in practice or
if we can turn it into a CHECK instead.

Change-Id: I6c76f4c8e45a32be610d5a91fe92699e75fcc54b
Bug: v8:13355
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4530647
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Stephen Röttger <sroettger@google.com>
Cr-Commit-Position: refs/heads/main@{#1143971}

gin/thread_isolation.cc

@@ -12,6 +12,8 @@
 #include "base/allocator/partition_allocator/thread_isolation/alignment.h"
 #include "base/check.h"
 #include "base/check_op.h"
+#include "base/memory/page_size.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/no_destructor.h"
 
 namespace {
@@ -50,6 +52,15 @@ void PkeyDisableWriteAccess(int pkey) {
 namespace gin {
 
 void ThreadIsolationData::InitializeBeforeThreadCreation() {
+  bool page_size_mismatch = PA_THREAD_ISOLATED_ALIGN_SZ < base::GetPageSize();
+  base::UmaHistogramBoolean("V8.CFIPageSizeMismatch", page_size_mismatch);
+  if (page_size_mismatch) {
+    // We write-protect global variables and need to align and pad them to (a
+    // multiple of) the OS page size. But since page size is not a compile time
+    // constant, check at runtime that our value was large enough.
+    return;
+  }
+
   pkey = PkeyAlloc(0);
   if (pkey == -1) {
     return;

tools/metrics/histograms/metadata/v8/histograms.xml

@@ -56,6 +56,19 @@ chromium-metrics-reviews@google.com.
   </summary>
 </histogram>
 
+<histogram name="V8.CFIPageSizeMismatch" units="BooleanOccurred"
+    expires_after="2024-06-01">
+  <owner>sroettger@google.com</owner>
+  <owner>v8-security@google.com</owner>
+  <summary>
+    Whether there's a mismatch between the maximum page size we assume at
+    compile time and the page size we assume at runtime.
+
+    This metric is recorded once at renderer startup when the
+    ThreadIsolatedAllocator is initialized.
+  </summary>
+</histogram>
+
 <histogram name="V8.CodeCacheRejectReason" enum="V8CodeCacheRejectReason"
     expires_after="M77">
   <owner>yangguo@chromium.org</owner>