[Bug 279762] libedit's cv_prev_word() can read before start of buffer

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279762

            Bug ID: 279762
           Summary: libedit's cv_prev_word() can read before start of
                    buffer
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: rtm@lcs.mit.edu

This code in contrib/libedit/chared.c cv_prev_word():

        while (n--) {
                ...;
                test = (*wtest)(*p);
                while ((p >= low) && (*wtest)(*p) == test)
                        p--;

The ">=" can cause p to be decremented even when it is equal to low,
so that the next iteration of the outer loop dereferences an illegal p.

If I run valgrind /bin/sh and type
$ set -o vi
then:
ESC
option-shift-s
option-shift-3
2
b

valgrind says:

  Invalid read of size 4
     at 0x4873DA1: cv_prev_word (rtm/freebsd/contrib/libedit/chared.c:319)
     by 0x489C6D0: vi_prev_word (rtm/freebsd/contrib/libedit/vi.c:189)
     by 0x4889747: el_wgets (rtm/freebsd/contrib/libedit/read.c:540)
     by 0x4879B3C: el_gets (rtm/freebsd/contrib/libedit/eln.c:75)
     by 0x123D6E: preadfd (rtm/freebsd/bin/sh/input.c:138)
     by 0x12387A: preadbuffer (rtm/freebsd/bin/sh/input.c:210)
     by 0x1326C5: xxreadtoken (rtm/freebsd/bin/sh/parser.c:910)
     by 0x12E4FC: readtoken (rtm/freebsd/bin/sh/parser.c:827)
     by 0x12E379: parsecmd (rtm/freebsd/bin/sh/parser.c:222)
     by 0x129786: cmdloop (rtm/freebsd/bin/sh/main.c:206)
     by 0x1295D2: main (rtm/freebsd/bin/sh/main.c:167)
   Address 0x552186c is 4 bytes before a block of size 4,096 alloc'd
     at 0x4851735: calloc (vg_replace_malloc.c:1599)
     by 0x48740AA: ch_init (rtm/freebsd/contrib/libedit/chared.c:399)
     by 0x4877714: el_init_internal (rtm/freebsd/contrib/libedit/el.c:107)
     by 0x48775B0: el_init_fd (rtm/freebsd/contrib/libedit/el.c:124)
     by 0x487754A: el_init (rtm/freebsd/contrib/libedit/el.c:66)
     by 0x1215BC: histedit (rtm/freebsd/bin/sh/histedit.c:183)
     by 0x12C1ED: optschanged (rtm/freebsd/bin/sh/options.c:130)
     by 0x12BEA1: procargs (rtm/freebsd/bin/sh/options.c:121)
     by 0x1293EC: main (rtm/freebsd/bin/sh/main.c:131)

FreeBSD stock14 15.0-CURRENT FreeBSD 15.0-CURRENT #22
main-n270479-ecd1d1f1dac4-dirty: Thu May 30 18:42:11 AST 2024    
root@stock14:/usr/obj/usr/src/amd64.amd64/sys/GENERIC amd64

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