✍️ Description

When fuzzing vim commit fc78a0369 (works with latest build and latest commit 202b4bd3a per this time of this report) with clang 13 and ASan, I discovered a buffer overflow.

Proof of Concept

Here is the poc

https://drive.google.com/file/d/1QFs9OysjzlRQP4hmfnMdfohaEg8lIZ8y/view?usp=sharing

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_mb_copy_char_min -c :qa!

ASan stack trace:

aldo@vps:~/vimbaru/src$ ASAN_OPTIONS=symbolize=1 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer ./vim -u NONE -X -Z -e -s -S poc_mb_copy_char_min -c :qa!
=================================================================
==829948==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x619000000e81 at pc 0x0000006fc616 bp 0x7fffffff8d10 sp 0x7fffffff8d08
WRITE of size 1 at 0x619000000e81 thread T0
    #0 0x6fc615 in append_command /home/aldo/vimtes/src/ex_docmd.c:3449:8
    #1 0x6d705d in do_one_cmd /home/aldo/vimtes/src/ex_docmd.c:2618:6
    #2 0x6ca342 in do_cmdline /home/aldo/vimtes/src/ex_docmd.c:992:17
    #3 0xafd4a5 in do_source_ext /home/aldo/vimtes/src/scriptfile.c:1674:5
    #4 0xafaef0 in do_source /home/aldo/vimtes/src/scriptfile.c:1801:12
    #5 0xafaa29 in cmd_source /home/aldo/vimtes/src/scriptfile.c:1174:14
    #6 0xafa50d in ex_source /home/aldo/vimtes/src/scriptfile.c:1200:2
    #7 0x6d6612 in do_one_cmd /home/aldo/vimtes/src/ex_docmd.c:2567:2
    #8 0x6ca342 in do_cmdline /home/aldo/vimtes/src/ex_docmd.c:992:17
    #9 0x6cd5d0 in do_cmdline_cmd /home/aldo/vimtes/src/ex_docmd.c:586:12
    #10 0xed6bb4 in exe_commands /home/aldo/vimtes/src/main.c:3108:2
    #11 0xed48e9 in vim_main2 /home/aldo/vimtes/src/main.c:780:2
    #12 0xece1d0 in main /home/aldo/vimtes/src/main.c:432:12
    #13 0x7ffff78240b2 in __libc_start_main /build/glibc-sMfBJT/glibc-2.31/csu/../csu/libc-start.c:308:16
    #14 0x41edcd in _start (/home/aldo/vimtes/src/vim+0x41edcd)

0x619000000e81 is located 0 bytes to the right of 1025-byte region [0x619000000a80,0x619000000e81)
allocated by thread T0 here:
    #0 0x499c8d in malloc (/home/aldo/vimtes/src/vim+0x499c8d)
    #1 0x4cb0e0 in lalloc /home/aldo/vimtes/src/alloc.c:246:11
    #2 0x4cb039 in alloc /home/aldo/vimtes/src/alloc.c:151:12
    #3 0xece209 in common_init /home/aldo/vimtes/src/main.c:914:19
    #4 0xecdd84 in main /home/aldo/vimtes/src/main.c:185:5
    #5 0x7ffff78240b2 in __libc_start_main /build/glibc-sMfBJT/glibc-2.31/csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/aldo/vimtes/src/ex_docmd.c:3449:8 in append_command
Shadow bytes around the buggy address:
  0x0c327fff8180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff8190: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff81a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff81b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff81c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c327fff81d0:[01]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff81e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff81f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff8200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff8210: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff8220: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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
==829948==ABORTING

Impact

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