Re: [OATH-Toolkit-help] pam_oath and pam_unix' try_first_pass

[Christian Hesse]

Simon Josefsson <address@hidden> on Thu, 2012/05/31 11:41:
> Christian Hesse <address@hidden> writes:
> 
> > Simon Josefsson <address@hidden> on Thu, 2012/05/31 10:40:
> >> Christian Hesse <address@hidden> writes:
> >> 
> >> > Christian Hesse <address@hidden> on Wed, 2012/05/09 11:55:
> >> >> Hello everybody,
> >> >> 
> >> >> This is from man pam_unix:
> >> >> > try_first_pass
> >> >> >            Before prompting the user for their password, the module
> >> >> > first tries the previous stacked module's password in case that
> >> >> > satisfies this module as well.
> >> >> 
> >> >> I think pam_oath has a similar option.
> >> >> So I tested with these in my pam configuration:
> >> >> 
> >> >> auth sufficient pam_unix.so
> >> >> -auth required pam_oath.so usersfile=/etc/users.oath digits=6
> >> >> try_first_pass
> >> >> 
> >> >> and
> >> >> 
> >> >> -auth sufficient pam_oath.so usersfile=/etc/users.oath digits=6
> >> >> auth required pam_unix.so try_first_pass
> >> >> 
> >> >> My understanding is that I get one prompt that accepts unix and oath
> >> >> password. But obviously this does not work, authentication fails for
> >> >> the line containing try_first_pass.
> >> >> Anything I misunderstood? How is this expected to work?
> >> >
> >> > I did some debugging... Looks like pam_get_item() at line 161 writes a
> >> > memory address to password that is not guaranteed to persist. The
> >> > memory is overwritten or cleared in pam_set_item() at line 275, thus
> >> > the following strncpy() does not copy anything useful.
> >> >
> >> > The attached patch fixes it for me, though I am not sure this is a
> >> > 'clean' way...
> >> 
> >> Thanks for debugging!  Could you analyze a bit further why the patch
> >> solves the problem -- maybe the password returned via
> >> 
> >>       retval = pam_get_item (pamh, PAM_AUTHTOK, (const void **)
> >> &password);
> >> 
> >> stop working later down in that function?  Could you pin-point where?
> >> Maybe it goes away after this call?
> >> 
> >>       retval = pam_set_item (pamh, PAM_AUTHTOK, onlypasswd);
> >>
> >> That could make some sense.  The PAM library might re-allocate or use a
> >> static buffer for the password, and this call overwrites the earlier
> >> value.
> >
> > As said before... Yes, that is the problem. ;) See above, I wrote about
> > line 275, that is the function you mentioned.
> 
> Heh, I guess I read code better than e-mails. :-)

Never mind. ;)

> >> Is there an easy way to reproduce the problem you discovered?
> >
> > This is perfectly reproducible with version 1.12.2, it works with my patch
> > applied.
> >
> > Probably it's enough to just reorder the code so we do not have to
> > allocate memory for password. New patch is attached, works for me as well.
> 
> I like this variant better.  Applied to master, thank you!

Me too. Thanks a lot!
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