dash test -w oddities

dash test -w oddities

[Remi Broemeling]

Today I encountered some oddities using `test -w` as well as `[ -w`.  
The machine in question is an NFS-root (exported and mounted read-only) 
server running Ubuntu 8.04 LTS.

------
root@nexhost:~# mount
rootfs on / type rootfs (rw)
<snip non-relevant mounts>
192.168.10.60:/var/lib/tftpboot/netboot/default.r14599-amd64 on / type 
nfs 
(ro,relatime,vers=3,rsize=524288,wsize=524288,hard,nointr,nolock,proto=tcp,timeo=7,retrans=3,sec=sys,addr=192.168.10.60)
<snip non-relevant mounts>
------

If I try using -w in bash, things work as expected:
------
root@nexhost:~# bash
root@nexhost:~# [ -w / ] && echo Writable Root
root@nexhost:~# test -w / && echo Writable Root
root@nexhost:~# /usr/bin/[ -w / ] && echo Writable Root
root@nexhost:~# /usr/bin/test -w / && echo Writable Root
------

However, if I try using -w in dash, the builtin does not seem to work as 
I would expect:
------
# [ -w / ] && echo Writable Root
Writable Root
# test -w / && echo Writable Root
Writable Root
# /usr/bin/[ -w / ] && echo Writable Root
# /usr/bin/test -w / && echo Writable Root
------

I'm not sure if I am misunderstanding what is supposed to be happening 
with test/[, or if this is a bug in the dash built-ins, but I thought 
that I should at least mention it in case it is a bug.

Thanks.
-- 
Remi Broemeling

Sr System Administrator
Nexopia.com Inc.
P: 780.444.1250 X435
F: 780.487.0376
E: remi@nexopia.com
N: rbroemeling


There are two major products that come out of Berkeley: LSD and UNIX. We 
don't believe this to be a coincidence.
Jeremy S. Anderson

Re: dash test -w oddities

[Oleg Verych]

> Today I encountered some oddities using `test -w` as well as `[ -w`.  
> The machine in question is an NFS-root (exported and mounted read-only) 
> server running Ubuntu 8.04 LTS.

important: you *are* root, right?

Some time ago i wanted to rewrite `test`, but due to "`dash` is in
Ubuntu upstream" answer. i've stopped.

So, here it is, naive replacement of access() syscall (with crutches from
GPLed `bash`). I've asked why it's needed and why euidaccess() from glibc
wasn't used, especially in case of selinux stuff. But it seems, like
`dash` fork of `ash` is just in bug-fixing mode.

#/bin/cc
/*
 * Similar to what access(2) does, but uses the effective uid and gid.
 * Doesn't make the mistake of telling root that any file is executable.
 * Returns non-zero if the file is accessible.
 */
static int
test_st_mode(const struct stat64 *st, int mode)
{
        int euid = geteuid();

        if (euid == 0) {
	/* Root can read or write any file. */
                if (mode != X_OK)
                        return 1;
[....]
#cc_end

In case you have no `man` page:
olecom@flower:~$ grep euidaccess </usr/include/unistd.h
extern int euidaccess (__const char *__name, int __type)
olecom@flower:~$

[]
> There are two major products that come out of Berkeley: LSD and UNIX. We 
> don't believe this to be a coincidence.
> Jeremy S. Anderson

LSD && BSD actually :)
____

euidaccess() vs non suid /bin/sh (Re: dash test -w oddities)

[Oleg Verych]

> I've asked why it's needed and why euidaccess() from glibc
> wasn't used, especially in case of selinux stuff.

I wonder even more why access() syscall isn't a solution, because
/bin/sh isn't set-uid by definition.

#/bin/cc glibc/sysdeps/posix/euidaccess.c
[...]
    /* If we are not set-uid or set-gid, access does the same.  */
    return access (path, mode);
[...]
#end_cc
____

Re: euidaccess() vs non suid /bin/sh (Re: dash test -w oddities)

[H. Peter Anvin]

Oleg Verych wrote:
>> I've asked why it's needed and why euidaccess() from glibc
>> wasn't used, especially in case of selinux stuff.
> 
> I wonder even more why access() syscall isn't a solution, because
> /bin/sh isn't set-uid by definition.
> 
> #/bin/cc glibc/sysdeps/posix/euidaccess.c
> [...]
>     /* If we are not set-uid or set-gid, access does the same.  */
>     return access (path, mode);
> [...]
> #end_cc

It might be run from a setuid program.

	-hpa

Re: euidaccess() vs non suid /bin/sh (Re: dash test -w oddities)

[Oleg Verych]

> >I wonder even more why access() syscall isn't a solution, because
> >/bin/sh isn't set-uid by definition.
> >
> >#/bin/cc glibc/sysdeps/posix/euidaccess.c
> >[...]
> >    /* If we are not set-uid or set-gid, access does the same.  */
> >    return access (path, mode);
> >[...]
> >#end_cc
> 
> It might be run from a setuid program.

Scripts with '#!/bin/sh' cannot be set-uid,

#man system (debian):

       Do  not  use  system()  from a program with set-user-ID or set-group-ID
       privileges, because strange values for some environment variables might
       be  used  to subvert system integrity.  Use the exec(3) family of func-
       tions instead, but not execlp(3) or execvp(3).  system() will  not,  in
       fact,  work  properly  from  programs  with set-user-ID or set-group-ID
       privileges on systems on which /bin/sh is bash version 2, since bash  2
       drops  privileges  on startup.  (Debian uses a modified bash which does
       not do this when invoked as sh.)

#end

Programs being designed to be set-uid should have proper setup for
proper running any external command. No?

The call euidaccess() makes sense now, just asking to make clear this
mess a bit.
____

Re: euidaccess() vs non suid /bin/sh (Re: dash test -w oddities)

[H. Peter Anvin]

Oleg Verych wrote:
> 
> Scripts with '#!/bin/sh' cannot be set-uid,
> 
> #man system (debian):
> 
>        Do  not  use  system()  from a program with set-user-ID or set-group-ID
>        privileges, because strange values for some environment variables might
>        be  used  to subvert system integrity.  Use the exec(3) family of func-
>        tions instead, but not execlp(3) or execvp(3).  system() will  not,  in
>        fact,  work  properly  from  programs  with set-user-ID or set-group-ID
>        privileges on systems on which /bin/sh is bash version 2, since bash  2
>        drops  privileges  on startup.  (Debian uses a modified bash which does
>        not do this when invoked as sh.)

This is largely considered a bug in bash, at least when invoked as /bin/sh.

	-hpa

Re: euidaccess() vs non suid /bin/sh (Re: dash test -w oddities)

[Oleg Verych]

> I wonder even more why access() syscall isn't a solution, because
> /bin/sh isn't set-uid by definition.
> 
> #/bin/cc glibc/sysdeps/posix/euidaccess.c

Also why there's no such syscall, i wouldn't require all uid shuffling,
isn't it?

#fs/open.c:sys_faccessat()

        old_fsuid = current->fsuid;
        old_fsgid = current->fsgid;
        old_cap = current->cap_effective;

        current->fsuid = current->uid;
        current->fsgid = current->gid;

#end
____