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Date: Thu, 19 Oct 2017 17:10:13 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: posix_spawnp stack overflow/corruption by child when PATH
 is large?

On Thu, Oct 19, 2017 at 04:05:19PM -0500, Will Dietz wrote:
> (soft ping)

Oops, I probably should have gotten this into the release. At least it
makes a good motivation to pick up the release pace and make another
release soon.

Rich



> On Mon, Sep 18, 2017 at 2:31 PM, Will Dietz <w@...z.org> wrote:
> > Thanks for taking a look and for the confirmation!
> >
> > I agree that 1024+PATH_MAX would be a reasonable value here, good call.
> >
> > I had similar thought about making the extra stack usage conditional,
> > but would rather keep it simple and clear-- as weighed against my possibly
> > wrong "expectation" that the difference won't be significant for folks.
> > I don't feel strongly about it and of course defer to your judgement :).
> >
> > Patch making the discussed change is attached.
> >
> > ~Will
> >
> >
> > On Fri, Sep 15, 2017 at 9:17 AM, Rich Felker <dalias@...c.org> wrote:
> >> On Thu, Sep 14, 2017 at 03:39:35PM -0500, Will Dietz wrote:
> >>> Hi,
> >>>
> >>> I believe there is a bug in posix_spawn/execvpe, please take a look and confirm
> >>> or kindly let me know if I'm mistaken and accept my apologies :).
> >>>
> >>> It looks like __posix_spawnx calls clone() with a 1024-byte stack buffer
> >>> (allocated from its own stack), which is insufficient to handle stack
> >>> allocations performed
> >>> in execvpe which are something around a few bytes more than NAME_MAX+PATH_MAX.
> >>>
> >>> This path is taken when using posix_spawnp, and the problem exists on
> >>> 1.1.16 and latest git.
> >>>
> >>> For what it's worth I tracked this down from a crash in 'bison' when
> >>> invoking m4,
> >>> but I've had success reproducing it with the following demo program
> >>> and driver script:
> >>>
> >>> -------------------------------------------
> >>> #include <spawn.h>
> >>> #include <stdio.h>
> >>> #include <stdlib.h>
> >>> #include <sys/types.h>
> >>> #include <sys/wait.h>
> >>>
> >>> extern char **environ;
> >>>
> >>> int main() {
> >>>
> >>>   pid_t p;
> >>>   char *argv[] = {"sh", "-c", "echo Hello", NULL};
> >>>   int s, status;
> >>>   s = posix_spawnp(&p, "sh", NULL, NULL, argv, environ);
> >>>   if (s) {
> >>>     perror("posix_spawn");
> >>>     exit(1);
> >>>   }
> >>>
> >>>   s = waitpid(p, &status, 0);
> >>>
> >>>   printf("pid: %d, s: %d, status: %d\n", p, s, status);
> >>>
> >>>   return 0;
> >>> }
> >>> --------------
> >>>
> >>> And little shell script to create a suitably large PATH (mostly to
> >>> demonstrate what I mean, not for unmodified use):
> >>> ---------------
> >>> #!/bin/sh
> >>>
> >>> SLASH_100_As="/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"
> >>> SUFFIX="/123456789012345678901234567" #1234567890" #1234567890"
> >>>
> >>> VAR="/bin:$SUFFIX"
> >>> for x in `seq 10`; do
> >>>   VAR="${SLASH_100_As}:$VAR"
> >>> done
> >>>
> >>> echo $VAR
> >>> echo $VAR|wc -c
> >>>
> >>> # Works fine with normal PATH
> >>> ~/cur/musl-spawn/test
> >>> ~/cur/musl-spawn/test
> >>>
> >>> # Crashes when PATH is ~1050 characters
> >>> PATH=$VAR \
> >>> ~/cur/musl-spawn/test
> >>> --------------
> >>>
> >>> Where "~/cur/musl-spawn/test" is the test program compiled against musl.
> >>>
> >>> I cannot speak regarding any security implications, but since this may
> >>> grant some measure of stack-scribbling-powers it seems to warrant
> >>> being given brief attention in this context.
> >>>
> >>> An easy fix is to bump the size of the 'char stack[1024]' in
> >>> src/process/posix_spawn.c to a suitable value-- 8096 is overkill but
> >>> does the trick, for example.
> >>>
> >>> Please let me know if I'm missing something or if details are not clear.
> >>
> >> It's very clear, and this seems pretty serious. 1024+PATH_MAX would
> >> probably be a safe limit. If we care about minimal stack usage when
> >> plain posix_spawn (not spawnp) is called, it could be something like
> >> "exec==execve ? 1024 : 1024+PATH_MAX", perhaps.
> >>
> >> Rich

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