Fix use-after-free in ep in Proc#dup for ifunc procs

[Bug #20950]

ifunc proc has the ep allocated in the cfunc_proc_t which is the data of
the TypedData object. If an ifunc proc is duplicated, the ep points to
the ep of the source object. If the source object is freed, then the ep
of the duplicated object now points to a freed memory region. If we try
to use the ep we could crash.

For example, the following script crashes:

    p = { a: 1 }.to_proc
    100.times do
      p = p.dup
      GC.start
      p.call
    rescue ArgumentError
    end

This commit changes ifunc proc to also duplicate the ep when it is duplicated.

internal/proc.h

@@ -23,6 +23,7 @@ VALUE rb_block_to_s(VALUE self, const struct rb_block *block, const char *additi
 VALUE rb_callable_receiver(VALUE);
 
 VALUE rb_func_proc_new(rb_block_call_func_t func, VALUE val);
+VALUE rb_func_proc_dup(VALUE src_obj);
 VALUE rb_func_lambda_new(rb_block_call_func_t func, VALUE val, int min_argc, int max_argc);
 VALUE rb_iseq_location(const struct rb_iseq_struct *iseq);
 VALUE rb_sym_to_proc(VALUE sym);

proc.c

@@ -680,6 +680,29 @@ cfunc_proc_new(VALUE klass, VALUE ifunc)
     return procval;
 }
 
+VALUE
+rb_func_proc_dup(VALUE src_obj)
+{
+    RUBY_ASSERT(rb_typeddata_is_instance_of(src_obj, &proc_data_type));
+
+    rb_proc_t *src_proc;
+    GetProcPtr(src_obj, src_proc);
+    RUBY_ASSERT(vm_block_type(&src_proc->block) == block_type_ifunc);
+
+    cfunc_proc_t *proc;
+    VALUE proc_obj = TypedData_Make_Struct(rb_obj_class(src_obj), cfunc_proc_t, &proc_data_type, proc);
+
+    memcpy(&proc->basic, src_proc, sizeof(rb_proc_t));
+
+    VALUE *ep = *(VALUE **)&proc->basic.block.as.captured.ep = proc->env + VM_ENV_DATA_SIZE - 1;
+    ep[VM_ENV_DATA_INDEX_FLAGS]   = src_proc->block.as.captured.ep[VM_ENV_DATA_INDEX_FLAGS];
+    ep[VM_ENV_DATA_INDEX_ME_CREF] = src_proc->block.as.captured.ep[VM_ENV_DATA_INDEX_ME_CREF];
+    ep[VM_ENV_DATA_INDEX_SPECVAL] = src_proc->block.as.captured.ep[VM_ENV_DATA_INDEX_SPECVAL];
+    ep[VM_ENV_DATA_INDEX_ENV]     = src_proc->block.as.captured.ep[VM_ENV_DATA_INDEX_ENV];
+
+    return proc_obj;
+}
+
 static VALUE
 sym_proc_new(VALUE klass, VALUE sym)
 {
@@ -1311,12 +1334,17 @@ proc_eq(VALUE self, VALUE other)
         }
         break;
       case block_type_ifunc:
-        if (self_block->as.captured.ep != \
-                other_block->as.captured.ep ||
-                self_block->as.captured.code.ifunc != \
+        if (self_block->as.captured.code.ifunc != \
                 other_block->as.captured.code.ifunc) {
             return Qfalse;
         }
+
+        if (memcmp(
+                ((cfunc_proc_t *)self_proc)->env,
+                ((cfunc_proc_t *)other_proc)->env,
+                sizeof(((cfunc_proc_t *)self_proc)->env))) {
+            return Qfalse;
+        }
         break;
       case block_type_proc:
         if (self_block->as.proc != other_block->as.proc) {
@@ -1445,6 +1473,7 @@ rb_hash_proc(st_index_t hash, VALUE prc)
         break;
       case block_type_ifunc:
         hash = rb_st_hash_uint(hash, (st_index_t)proc->block.as.captured.code.ifunc->func);
+        hash = rb_st_hash_uint(hash, (st_index_t)proc->block.as.captured.code.ifunc->data);
         break;
       case block_type_symbol:
         hash = rb_st_hash_uint(hash, rb_any_hash(proc->block.as.symbol));
@@ -1456,7 +1485,14 @@ rb_hash_proc(st_index_t hash, VALUE prc)
         rb_bug("rb_hash_proc: unknown block type %d", vm_block_type(&proc->block));
     }
 
-    return rb_hash_uint(hash, (st_index_t)proc->block.as.captured.ep);
+    /* ifunc procs have their own allocated ep. If an ifunc is duplicated, they
+     * will point to different ep but they should return the same hash code, so
+     * we cannot include the ep in the hash. */
+    if (vm_block_type(&proc->block) != block_type_ifunc) {
+        hash = rb_hash_uint(hash, (st_index_t)proc->block.as.captured.ep);
+    }
+
+    return hash;
 }
 
 

test/ruby/test_proc.rb

@@ -469,6 +469,18 @@ class TestProc < Test::Unit::TestCase
     assert_throw(:initialize_dup) {c1.new{}.dup}
   end
 
+  def test_dup_ifunc_proc_bug_20950
+    assert_normal_exit(<<~RUBY, "[Bug #20950]")
+      p = { a: 1 }.to_proc
+      100.times do
+        p = p.dup
+        GC.start
+        p.call
+      rescue ArgumentError
+      end
+    RUBY
+  end
+
   def test_clone_subclass
     c1 = Class.new(Proc)
     assert_equal c1, c1.new{}.clone.class, '[Bug #17545]'

vm.c

@@ -1198,7 +1198,16 @@ rb_proc_dup(VALUE self)
     rb_proc_t *src;
 
     GetProcPtr(self, src);
-    procval = proc_create(rb_obj_class(self), &src->block, src->is_from_method, src->is_lambda);
+
+    switch (vm_block_type(&src->block)) {
+      case block_type_ifunc:
+        procval = rb_func_proc_dup(self);
+        break;
+      default:
+        procval = proc_create(rb_obj_class(self), &src->block, src->is_from_method, src->is_lambda);
+        break;
+    }
+
     if (RB_OBJ_SHAREABLE_P(self)) FL_SET_RAW(procval, RUBY_FL_SHAREABLE);
     RB_GC_GUARD(self); /* for: body = rb_proc_dup(body) */
     return procval;