[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 3cb23f0..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>

Rename the fork branch 'mmtk' #18

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