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Message-ID: <20170915141742.GW1627@brightrain.aerifal.cx> 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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