Description: In mrb_str_format, the “%<name” branch scans for a “>” but if it’s missing and the name is interned/hashed, it does p++ past end-of-string and then dereferences *p in the switch without checking bounds, causing an OOB read.
Trigger conditions: Invoke sprintf (or format) with a format string containing a named specifier missing its closing “>” and supply a hash mapping that named symbol. For example: `sprintf("%<foo", { :foo => 1 })`.
Harness name: mruby_fuzzer
Crashing input:
sprintf("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA%<foo", {:foo => 1})
Crash output:
+ FUZZER=mruby_fuzzer
+ shift
+ '[' '!' -v TESTCASE ']'
+ TESTCASE=/testcase
+ '[' '!' -f /testcase ']'
+ export RUN_FUZZER_MODE=interactive
+ RUN_FUZZER_MODE=interactive
+ export FUZZING_ENGINE=libfuzzer
+ FUZZING_ENGINE=libfuzzer
+ export SKIP_SEED_CORPUS=1
+ SKIP_SEED_CORPUS=1
+ run_fuzzer mruby_fuzzer /testcase
sysctl: setting key "vm.mmap_rnd_bits", ignoring: Read-only file system
Dictionary: 102 entries
/out/mruby_fuzzer: Running 1 inputs 1 time(s) each.
Running: /testcase
=================================================================
==43==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x52100000d8a6 at pc 0x5558b1097bd5 bp 0x7ffe9461a150 sp 0x7ffe9461a148
READ of size 1 at 0x52100000d8a6 thread T0
SCARINESS: 12 (1-byte-read-heap-buffer-overflow)
Stack Frame #0 in mrb_str_format sprintf.c
Stack Frame #1 in mrb_f_sprintf sprintf.c
Stack Frame #2 in mrb_vm_exec (/out/mruby_fuzzer+0x2e9297)
Stack Frame #3 in mrb_vm_run (/out/mruby_fuzzer+0x2d18e2)
Stack Frame #4 in mrb_top_run (/out/mruby_fuzzer+0x32d589)
Stack Frame #5 in mrb_load_exec (/out/mruby_fuzzer+0x3c61c5)
Stack Frame #6 in mrb_load_nstring_cxt (/out/mruby_fuzzer+0x3c712c)
Stack Frame #7 in mrb_load_string_cxt (/out/mruby_fuzzer+0x3c7260)
Stack Frame #8 in mrb_load_string (/out/mruby_fuzzer+0x3c72d8)
Stack Frame #9 in LLVMFuzzerTestOneInput (/out/mruby_fuzzer+0x2a7c5b)
Stack Frame #10 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:614:13
Stack Frame #11 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:327:6
Stack Frame #12 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:862:9
Stack Frame #13 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
Stack Frame #14 in __libc_start_main /build/glibc-B3wQXB/glibc-2.31/csu/../csu/libc-start.c:308:16
Stack Frame #15 in _start (/out/mruby_fuzzer+0x13fb0d)
DEDUP_TOKEN: mrb_str_format--mrb_f_sprintf--mrb_vm_exec--mrb_vm_run--mrb_top_run
0x52100000d8a6 is located 0 bytes after 4006-byte region [0x52100000c900,0x52100000d8a6)
allocated by thread T0 here:
Stack Frame #0 in realloc /src/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:82:3
Stack Frame #1 in mrb_basic_alloc_func (/out/mruby_fuzzer+0x2a9955)
Stack Frame #2 in mrb_realloc_simple (/out/mruby_fuzzer+0x41ecc0)
Stack Frame #3 in mrb_realloc (/out/mruby_fuzzer+0x41f417)
Stack Frame #4 in mrb_malloc (/out/mruby_fuzzer+0x41f5c8)
Stack Frame #5 in str_init_normal_capa string.c
Stack Frame #6 in str_init_normal string.c
Stack Frame #7 in str_new string.c
Stack Frame #8 in mrb_str_new (/out/mruby_fuzzer+0x32ea34)
Stack Frame #9 in mrb_vm_exec (/out/mruby_fuzzer+0x301c26)
Stack Frame #10 in mrb_vm_run (/out/mruby_fuzzer+0x2d18e2)
Stack Frame #11 in mrb_top_run (/out/mruby_fuzzer+0x32d589)
Stack Frame #12 in mrb_load_exec (/out/mruby_fuzzer+0x3c61c5)
Stack Frame #13 in mrb_load_nstring_cxt (/out/mruby_fuzzer+0x3c712c)
Stack Frame #14 in mrb_load_string_cxt (/out/mruby_fuzzer+0x3c7260)
Stack Frame #15 in mrb_load_string (/out/mruby_fuzzer+0x3c72d8)
Stack Frame #16 in LLVMFuzzerTestOneInput (/out/mruby_fuzzer+0x2a7c5b)
Stack Frame #17 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerLoop.cpp:614:13
Stack Frame #18 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:327:6
Stack Frame #19 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerDriver.cpp:862:9
Stack Frame #20 in main /src/llvm-project/compiler-rt/lib/fuzzer/FuzzerMain.cpp:20:10
Stack Frame #21 in __libc_start_main /build/glibc-B3wQXB/glibc-2.31/csu/../csu/libc-start.c:308:16
DEDUP_TOKEN: __interceptor_realloc--mrb_basic_alloc_func--mrb_realloc_simple--mrb_realloc--mrb_malloc
SUMMARY: AddressSanitizer: heap-buffer-overflow sprintf.c in mrb_str_format
Shadow bytes around the buggy address:
0x52100000d600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x52100000d680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x52100000d700: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x52100000d780: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x52100000d800: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x52100000d880: 00 00 00 00[06]fa fa fa fa fa fa fa fa fa fa fa
0x52100000d900: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x52100000d980: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x52100000da00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x52100000da80: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x52100000db00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
==43==ABORTING
/out/mruby_fuzzer -rss_limit_mb=2560 -timeout=25 /testcase -dict=mruby.dict -only_ascii=1 < /dev/null
Patch:
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -383,6 +383,9 @@
for (; p < end && *p != term; )
p++;
+ if (p >= end) {
+ mrb_raisef(mrb, E_ARGUMENT_ERROR, "malformed format string - unterminated named argument");
+ }
if (id) {
mrb_raisef(mrb, E_ARGUMENT_ERROR, "name%l after <%n>",
start, p - start + 1, id);