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Date: Fri, 15 Sep 2017 10:17:42 -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, 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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