merge revision(s) f65a6c090c: [Backport #20921]

Fix use-after-free in constant cache

	[Bug #20921]

	When we create a cache entry for a constant, the following sequence of
	events could happen:

	- vm_track_constant_cache is called to insert a constant cache.
	- In vm_track_constant_cache, we first look up the ST table for the ID
	  of the constant. Assume the ST table exists because another iseq also
	  holds a cache entry for this ID.
	- We then insert into this ST table with the iseq_inline_constant_cache.
	- However, while inserting into this ST table, it allocates memory, which
	  could trigger a GC. Assume that it does trigger a GC.
	- The GC frees the one and only other iseq that holds a cache entry for
	  this ID.
	- In remove_from_constant_cache, it will appear that the ST table is now
	  empty because there are no more iseq with cache entries for this ID, so
	  we free the ST table.
	- We complete GC and continue our st_insert. However, this ST table has
	  been freed so we now have a use-after-free.

	This issue is very hard to reproduce, because it requires that the GC runs
	at a very specific time. However, we can make it show up by applying this
	patch which runs GC right before the st_insert to mimic the st_insert
	triggering a GC:

	    diff --git a/vm_insnhelper.c b/vm_insnhelper.c
	    index 3cb23f06f0..a93998136a 100644
	    --- a/vm_insnhelper.c
	    +++ b/vm_insnhelper.c
	    @@ -6338,6 +6338,10 @@ vm_track_constant_cache(ID id, void *ic)
	            rb_id_table_insert(const_cache, id, (VALUE)ics);
	        }

	    +    if (id == rb_intern("MyConstant")) rb_gc();
	    +
	        st_insert(ics, (st_data_t) ic, (st_data_t) Qtrue);
	    }

	And if we run this script:

	    Object.const_set("MyConstant", "Hello!")

	    my_proc = eval("-> { MyConstant }")
	    my_proc.call

	    my_proc = eval("-> { MyConstant }")
	    my_proc.call

	We can see that ASAN outputs a use-after-free error:

	    ==36540==ERROR: AddressSanitizer: heap-use-after-free on address 0x606000049528 at pc 0x000102f3ceac bp 0x00016d607a70 sp 0x00016d607a68
	    READ of size 8 at 0x606000049528 thread T0
	        #0 0x102f3cea8 in do_hash st.c:321
	        #1 0x102f3ddd0 in rb_st_insert st.c:1132
	        #2 0x103140700 in vm_track_constant_cache vm_insnhelper.c:6345
	        #3 0x1030b91d8 in vm_ic_track_const_chain vm_insnhelper.c:6356
	        #4 0x1030b8cf8 in rb_vm_opt_getconstant_path vm_insnhelper.c:6424
	        #5 0x1030bc1e0 in vm_exec_core insns.def:263
	        #6 0x1030b55fc in rb_vm_exec vm.c:2585
	        #7 0x1030fe0ac in rb_iseq_eval_main vm.c:2851
	        #8 0x102a82588 in rb_ec_exec_node eval.c:281
	        #9 0x102a81fe0 in ruby_run_node eval.c:319
	        #10 0x1027f3db4 in rb_main main.c:43
	        #11 0x1027f3bd4 in main main.c:68
	        #12 0x183900270  (<unknown module>)

	    0x606000049528 is located 8 bytes inside of 56-byte region [0x606000049520,0x606000049558)
	    freed by thread T0 here:
	        #0 0x104174d40 in free+0x98 (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x54d40)
	        #1 0x102ada89c in rb_gc_impl_free default.c:8183
	        #2 0x102ada7dc in ruby_sized_xfree gc.c:4507
	        #3 0x102ac4d34 in ruby_xfree gc.c:4518
	        #4 0x102f3cb34 in rb_st_free_table st.c:663
	        #5 0x102bd52d8 in remove_from_constant_cache iseq.c:119
	        #6 0x102bbe2cc in iseq_clear_ic_references iseq.c:153
	        #7 0x102bbd2a0 in rb_iseq_free iseq.c:166
	        #8 0x102b32ed0 in rb_imemo_free imemo.c:564
	        #9 0x102ac4b44 in rb_gc_obj_free gc.c:1407
	        #10 0x102af4290 in gc_sweep_plane default.c:3546
	        #11 0x102af3bdc in gc_sweep_page default.c:3634
	        #12 0x102aeb140 in gc_sweep_step default.c:3906
	        #13 0x102aeadf0 in gc_sweep_rest default.c:3978
	        #14 0x102ae4714 in gc_sweep default.c:4155
	        #15 0x102af8474 in gc_start default.c:6484
	        #16 0x102afbe30 in garbage_collect default.c:6363
	        #17 0x102ad37f0 in rb_gc_impl_start default.c:6816
	        #18 0x102ad3634 in rb_gc gc.c:3624
	        #19 0x1031406ec in vm_track_constant_cache vm_insnhelper.c:6342
	        #20 0x1030b91d8 in vm_ic_track_const_chain vm_insnhelper.c:6356
	        #21 0x1030b8cf8 in rb_vm_opt_getconstant_path vm_insnhelper.c:6424
	        #22 0x1030bc1e0 in vm_exec_core insns.def:263
	        #23 0x1030b55fc in rb_vm_exec vm.c:2585
	        #24 0x1030fe0ac in rb_iseq_eval_main vm.c:2851
	        #25 0x102a82588 in rb_ec_exec_node eval.c:281
	        #26 0x102a81fe0 in ruby_run_node eval.c:319
	        #27 0x1027f3db4 in rb_main main.c:43
	        #28 0x1027f3bd4 in main main.c:68
	        #29 0x183900270  (<unknown module>)

	    previously allocated by thread T0 here:
	        #0 0x104174c04 in malloc+0x94 (libclang_rt.asan_osx_dynamic.dylib:arm64e+0x54c04)
	        #1 0x102ada0ec in rb_gc_impl_malloc default.c:8198
	        #2 0x102acee44 in ruby_xmalloc gc.c:4438
	        #3 0x102f3c85c in rb_st_init_table_with_size st.c:571
	        #4 0x102f3c900 in rb_st_init_table st.c:600
	        #5 0x102f3c920 in rb_st_init_numtable st.c:608
	        #6 0x103140698 in vm_track_constant_cache vm_insnhelper.c:6337
	        #7 0x1030b91d8 in vm_ic_track_const_chain vm_insnhelper.c:6356
	        #8 0x1030b8cf8 in rb_vm_opt_getconstant_path vm_insnhelper.c:6424
	        #9 0x1030bc1e0 in vm_exec_core insns.def:263
	        #10 0x1030b55fc in rb_vm_exec vm.c:2585
	        #11 0x1030fe0ac in rb_iseq_eval_main vm.c:2851
	        #12 0x102a82588 in rb_ec_exec_node eval.c:281
	        #13 0x102a81fe0 in ruby_run_node eval.c:319
	        #14 0x1027f3db4 in rb_main main.c:43
	        #15 0x1027f3bd4 in main main.c:68
	        #16 0x183900270  (<unknown module>)

	This commit fixes this bug by adding a inserting_constant_cache_id field
	to the VM, which stores the ID that is currently being inserted and, in
	remove_from_constant_cache, we don't free the ST table for ID equal to
	this one.

	Co-Authored-By: Alan Wu <alanwu@ruby-lang.org>

iseq.c

@@ -113,7 +113,9 @@ remove_from_constant_cache(ID id, IC ic)
         st_table *ics = (st_table *)lookup_result;
         st_delete(ics, &ic_data, NULL);
 
-        if (ics->num_entries == 0) {
+        if (ics->num_entries == 0 &&
+                // See comment in vm_track_constant_cache on why we need this check
+                id != vm->inserting_constant_cache_id) {
             rb_id_table_delete(vm->constant_cache, id);
             st_free_table(ics);
         }

version.h

@@ -11,7 +11,7 @@
 # define RUBY_VERSION_MINOR RUBY_API_VERSION_MINOR
 #define RUBY_VERSION_TEENY 6
 #define RUBY_RELEASE_DATE RUBY_RELEASE_YEAR_STR"-"RUBY_RELEASE_MONTH_STR"-"RUBY_RELEASE_DAY_STR
-#define RUBY_PATCHLEVEL 241
+#define RUBY_PATCHLEVEL 242
 
 #include "ruby/version.h"
 #include "ruby/internal/abi.h"

vm_core.h

@@ -724,6 +724,7 @@ typedef struct rb_vm_struct {
     // and Qtrue as values. It is used when inline constant caches need to be
     // invalidated or ISEQs are being freed.
     struct rb_id_table *constant_cache;
+    ID inserting_constant_cache_id;
 
 #ifndef VM_GLOBAL_CC_CACHE_TABLE_SIZE
 #define VM_GLOBAL_CC_CACHE_TABLE_SIZE 1023

vm_insnhelper.c

@@ -5289,7 +5289,8 @@ rb_vm_opt_newarray_min(rb_execution_context_t *ec, rb_num_t num, const VALUE *pt
 static void
 vm_track_constant_cache(ID id, void *ic)
 {
-    struct rb_id_table *const_cache = GET_VM()->constant_cache;
+    rb_vm_t *vm = GET_VM();
+    struct rb_id_table *const_cache = vm->constant_cache;
     VALUE lookup_result;
     st_table *ics;
 
@@ -5301,7 +5302,23 @@ vm_track_constant_cache(ID id, void *ic)
         rb_id_table_insert(const_cache, id, (VALUE)ics);
     }
 
+    /* The call below to st_insert could allocate which could trigger a GC.
+     * If it triggers a GC, it may free an iseq that also holds a cache to this
+     * constant. If that iseq is the last iseq with a cache to this constant, then
+     * it will free this ST table, which would cause an use-after-free during this
+     * st_insert.
+     *
+     * So to fix this issue, we store the ID that is currently being inserted
+     * and, in remove_from_constant_cache, we don't free the ST table for ID
+     * equal to this one.
+     *
+     * See [Bug #20921].
+     */
+    vm->inserting_constant_cache_id = id;
+
     st_insert(ics, (st_data_t) ic, (st_data_t) Qtrue);
+
+    vm->inserting_constant_cache_id = (ID)0;
 }
 
 static void