Deny file mode changes outside of specified paths in sandbox

Library/Homebrew/sandbox.rb

@@ -37,6 +37,7 @@ def add_rule(allow:, operation:, filter: nil, modifier: nil)
   def allow_write(path:, type: :literal)
     add_rule allow: true, operation: "file-write*", filter: path_filter(path, type)
     add_rule allow: true, operation: "file-write-setugid", filter: path_filter(path, type)
+    add_rule allow: true, operation: "file-write-mode", filter: path_filter(path, type)
   end
 
   sig { params(path: T.any(String, Pathname), type: Symbol).void }
@@ -289,6 +290,7 @@ class SandboxProfile
           (regex #"^/dev/tty[a-z0-9]*$")
           )
       (deny file-write*) ; deny non-allowlist file write operations
+      (deny file-write-mode) ; deny non-allowlist file write mode operations
       (allow process-exec
           (literal "/bin/ps")
           (with no-sandbox)

Library/Homebrew/test/sandbox_spec.rb

@@ -57,4 +57,18 @@
         .and output(a_string_matching(/foo/).and(matching(/bar/).and(not_matching(/Python/)))).to_stdout
     end
   end
+
+  describe "#disallow chmod on some directory" do
+    it "formula does a chmod to opt" do
+      expect { sandbox.exec "chmod", "ug-w", HOMEBREW_PREFIX}.to raise_error(ErrorDuringExecution)
+    end
+
+    it "allows chmod on a path allowed to write" do
+      mktmpdir do |path|
+        FileUtils.touch path/"foo"
+        sandbox.allow_write_path(path)
+        expect { sandbox.exec "chmod", "ug-w", path/"foo"}.not_to raise_error(ErrorDuringExecution)
+      end
+    end
+  end
 end