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Date: Sat, 21 Oct 2017 10:54:52 -0500
From: Will Dietz <w@...z.org>
To: musl@...ts.openwall.com
Subject: Re: posix_spawnp stack overflow/corruption by child when PATH
 is large?

Sounds like a plan! Don't mean to bug, just want to make sure it's not
lost in the bustle of the release :).

~Will

On Thu, Oct 19, 2017 at 4:10 PM, Rich Felker <dalias@...c.org> wrote:
> 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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