Use After Free in vim/vim

Description

Greetings,

A Use After Free issue was discovered in Vim.

The POC file is reduced to the absolute minimum to reproduce the problem. Please see sanitizer output and the "trimmed" POC file link below.

System info OS version : Ubuntu 20.04.2 LTS + Clang 12 with ASan Vim Version : master(3c5904d) - Sun Oct 24 14:50:07 2021 +0100

Steps to reproduce:

git clone https://github.com/vim/vim

LD=lld-12 AS=llvm-as-12 AR=llvm-ar-12 RANLIB=llvm-ranlib-12 CC=clang-12 CXX=clang++-12 CFLAGS="-fsanitize=address" CXXFLAGS="-fsanitize=address" LDFLAGS="-ldl -fsanitize=address" ./configure --with-features=huge --enable-gui=none && make

3. Download POC from This URL

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

Sanitizer output:

=================================================================
==126137==ERROR: AddressSanitizer: heap-use-after-free on address 0x602000006dd1 at pc 0x00000121b06b bp 0x7fff1d7d60b0 sp 0x7fff1d7d60a8
READ of size 1 at 0x602000006dd1 thread T0
    #0 0x121b06a in nfa_regmatch /src/fuzzer11/triage_yeni/vim/src/./regexp_nfa.c:5855:13
    #1 0x11f4062 in nfa_regtry /src/fuzzer11/triage_yeni/vim/src/./regexp_nfa.c:7237:14
    #2 0x11f4062 in nfa_regexec_both /src/fuzzer11/triage_yeni/vim/src/./regexp_nfa.c:7432:14
    #3 0x113f156 in vim_regexec_multi /src/fuzzer11/triage_yeni/vim/src/regexp.c:2915:14
    #4 0x9163de in ex_substitute /src/fuzzer11/triage_yeni/vim/src/ex_cmds.c:3999:11
    #5 0x94ff7b in do_one_cmd /src/fuzzer11/triage_yeni/vim/src/ex_docmd.c:2614:2
    #6 0x94ff7b in do_cmdline /src/fuzzer11/triage_yeni/vim/src/ex_docmd.c:1000:17
    #7 0x136cde4 in do_source /src/fuzzer11/triage_yeni/vim/src/scriptfile.c:1406:5
    #8 0x13699e1 in cmd_source /src/fuzzer11/triage_yeni/vim/src/scriptfile.c:971:14
    #9 0x13699e1 in ex_source /src/fuzzer11/triage_yeni/vim/src/scriptfile.c:997:2
    #10 0x94ff7b in do_one_cmd /src/fuzzer11/triage_yeni/vim/src/ex_docmd.c:2614:2
    #11 0x94ff7b in do_cmdline /src/fuzzer11/triage_yeni/vim/src/ex_docmd.c:1000:17
    #12 0x1bcecfc in exe_commands /src/fuzzer11/triage_yeni/vim/src/main.c:3081:2
    #13 0x1bcecfc in vim_main2 /src/fuzzer11/triage_yeni/vim/src/main.c:773:2
    #14 0x1bc5a8f in main /src/fuzzer11/triage_yeni/vim/src/main.c:425:12
    #15 0x7fc9c677c0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #16 0x41f64d in _start (/src/fuzzer11/triage_yeni/vim/src/vim+0x41f64d)

0x602000006dd1 is located 1 bytes inside of 10-byte region [0x602000006dd0,0x602000006dda)
freed by thread T0 here:
    #0 0x49a642 in free (/src/fuzzer11/triage_yeni/vim/src/vim+0x49a642)
    #1 0xd5ee9f in ml_flush_line /src/fuzzer11/triage_yeni/vim/src/memline.c:4059:2

previously allocated by thread T0 here:
    #0 0x49a8ad in malloc (/src/fuzzer11/triage_yeni/vim/src/vim+0x49a8ad)
    #1 0x4cc2cb in lalloc /src/fuzzer11/triage_yeni/vim/src/alloc.c:244:11

SUMMARY: AddressSanitizer: heap-use-after-free /src/fuzzer11/triage_yeni/vim/src/./regexp_nfa.c:5855:13 in nfa_regmatch
Shadow bytes around the buggy address:
  0x0c047fff8d60: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff8d70: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff8d80: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff8d90: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff8da0: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
=>0x0c047fff8db0: fa fa fd fd fa fa fd fd fa fa[fd]fd fa fa 00 05
  0x0c047fff8dc0: fa fa 02 fa fa fa 00 03 fa fa fd fa fa fa fd fa
  0x0c047fff8dd0: fa fa 00 03 fa fa 00 03 fa fa 01 fa fa fa 05 fa
  0x0c047fff8de0: fa fa 05 fa fa fa 05 fa fa fa 01 fa fa fa fa fa
  0x0c047fff8df0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8e00: 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
  Shadow gap:              cc
==126137==ABORTING

References:

CWE-416: Use After Free - https://cwe.mitre.org/data/definitions/416.html

This vulnerability is capable of crashing software, bypass protection mechanism, modify of memory, and successful exploitation may lead to code execution