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Re: ksh / CVE-2019-14868

I meant to include this test run:

(stretch-amd64-default)root@silverfish:/home/brian# SHLVL='2#11+x[$(/bin/echo DANGER WILL ROBINSON >&2)0]' /usr/bin/ksh 
Segmentation fault

As in no echo command is required.

Below is the full stack trace of the segfault (recompiled without the
strip=0 option).

(stretch-i386-default)root@silverfish:/tmp/brian/tmpf2btue5q/build/i386# ulimit -c unlimited
(stretch-i386-default)root@silverfish:/tmp/brian/tmpf2btue5q/build/i386# SHLVL='2#11+x[$(/bin/echo DANGER WILL ROBINSON >&2)0]' /usr/bin/ksh
Segmentation fault (core dumped)
(stretch-i386-default)root@silverfish:/tmp/brian/tmpf2btue5q/build/i386# ls core
(stretch-i386-default)root@silverfish:/tmp/brian/tmpf2btue5q/build/i386# gdb /usr/bin/ksh core                                              
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Reading symbols from /usr/bin/ksh...Reading symbols from /usr/lib/debug/.build-id/5f/77fa7b40ec10980eebf65a2ba1cebac53d8894.debug...done.

warning: core file may not match specified executable file.
[New LWP 15788]
Core was generated by `/usr/bin/ksh'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  job_alloc () at ./src/cmd/ksh93/sh/jobs.c:1871
1871    ./src/cmd/ksh93/sh/jobs.c: No such file or directory.
(gdb) bt
#0  job_alloc () at ./src/cmd/ksh93/sh/jobs.c:1871
#1  job_post (shp=0x56772fa0 <sh>, pid=15789, join=<optimized out>) at ./src/cmd/ksh93/sh/jobs.c:1353
#2  0x5663b25a in _sh_fork (shp=0x56772fa0 <sh>, parent=15789, flags=0, jobid=0xffa15798) at ./src/cmd/ksh93/sh/xec.c:3142
#3  0x5663bcc2 in sh_ntfork (shp=shp@entry=0x56772fa0 <sh>, argv=0xf7caf334, jobid=0xffa15798, flag=<optimized out>, t=<optimized out>, t=<optimized out>) at ./src/cmd/ksh93/sh/xec.c:4034
#4  0x56640ed7 in sh_exec (t=<optimized out>, flags=<optimized out>) at ./src/cmd/ksh93/sh/xec.c:1686
#5  0x56638291 in sh_subshell (shp=0x56772fa0 <sh>, t=0xf7caf298, flags=<optimized out>, comsub=1) at ./src/cmd/ksh93/sh/subshell.c:625
#6  0x56619654 in comsubst (mp=0xf7cb1880, t=<optimized out>, t@entry=0x0, type=type@entry=1) at ./src/cmd/ksh93/sh/macro.c:2135
#7  0x56615af7 in varsub (mp=mp@entry=0xf7cb1880) at ./src/cmd/ksh93/sh/macro.c:1168
#8  0x56618d14 in copyto (mp=mp@entry=0xf7cb1880, endch=endch@entry=0, newquote=<optimized out>) at ./src/cmd/ksh93/sh/macro.c:633
#9  0x56619108 in sh_mactrim (shp=0x56772fa0 <sh>, str=0xf7cb1034 "[$(/bin/echo DANGER WILL ROBINSON >&2)0]", mode=0) at ./src/cmd/ksh93/sh/macro.c:183
#10 0x565ef8d7 in scope (np=0xf7cbb390, lvalue=lvalue@entry=0xffa16598, assign=assign@entry=0) at ./src/cmd/ksh93/sh/arith.c:161
#11 0x565f0288 in arith (ptr=0xffa16594, lvalue=0xffa16598, type=2, n=<optimized out>) at ./src/cmd/ksh93/sh/arith.c:444
#12 0x56634bd2 in arith_exec (ep=0xf7caf248) at ./src/cmd/ksh93/sh/streval.c:249
#13 0x56636441 in strval (shp=0x56772fa0 <sh>, s=0xf7cb102e "2#11+x[$(/bin/echo DANGER WILL ROBINSON >&2)0]", end=0xffa16778, conv=0x565eff40 <arith>, emode=1) at ./src/cmd/ksh93/sh/streval.c:964
#14 0x565ef560 in sh_strnum (str=0xf7cb102e "2#11+x[$(/bin/echo DANGER WILL ROBINSON >&2)0]", ptr=0x0, mode=1) at ./src/cmd/ksh93/sh/arith.c:525
#15 0x565f0e70 in sh_arith (shp=0x56772fa0 <sh>, str=0xf7cb102e "2#11+x[$(/bin/echo DANGER WILL ROBINSON >&2)0]") at ./src/cmd/ksh93/sh/arith.c:538
#16 0x5661d0ae in nv_putval (np=0xf7cb0bf4, string=0xf7cb102e "2#11+x[$(/bin/echo DANGER WILL ROBINSON >&2)0]", flags=0) at ./src/cmd/ksh93/sh/name.c:1783
#17 0x566015ed in env_init (shp=0x56772fa0 <sh>) at ./src/cmd/ksh93/sh/init.c:2036
#18 sh_init (argc=1, argv=0xffa18a74, userinit=0x0) at ./src/cmd/ksh93/sh/init.c:1380
#19 0x565e4da6 in sh_main ()
#20 0x565e4af9 in main (argc=1, argv=0xffa18a74) at ./src/cmd/ksh93/sh/pmain.c:45

I am guessing this segfault might be because we are attempting to access
job.freejobs[x] before job.freejobs has been initialised because we
weren't expecting to run jobs yet.

Yes, this appears to be the case:

 * initialize the shell
Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)      
    env_init(shp);   /* function that causes the error */
    /* initialize jobs table */
Brian May <bam@debian.org>

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