pam_exec: fix segfault when existing auth token is null on `pam_sm_set_cred`

[nyan_myuji.xyz]

Summary

According to pam_exec(8), the expose_authtok option should be ignored when
the service function is pam_sm_setcred. Currently pam_exec only prevent
prompt for anth token when expose_authtok is set on pam_sm_setcred. This
subsequently led to segfault when there isn't an existing auth token available.

Bug reported on this: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263893

After reading https://reviews.freebsd.org/rS349556 I am not sure if the default
behaviour supposed to be simply not prompt for authentication token, or is it
to ignore the option entirely as stated in the man page.

This patch is therefore only adding an additional NULL check on the item
pam_get_item provide, and exit with PAM_SYSTEM_ERR when such item is NULL.

[nyan_myuji.xyz]

also check rc

check value of rc to only catch the cases when item is null but rc is success

[nyan_myuji.xyz]

Fix the submitted diff

[zirias]

Is it ever expected there *is* an authtok available when setcred is called? If not, wouldn't this still fail (and just avoid the crash)?

[des]

In D35169#798299, @felix_palmen-it.de wrote:

Is it ever expected there *is* an authtok available when setcred is called?

Yes, if an earlier module (e.g. passwdqc) already prompted for a new password. And the old one will be in PAM_OLD_AUTHTOK if memory serves.

[zirias]

So, does this patch look fine?

Cause if so, I'd apply it locally, so I can move forward writing/testing a "unix selfauth" helper for use with pam_exec :)

[des]

lib/libpam/modules/pam_exec/pam_exec.c

267

269–271

There's no harm in setting authtok to NULL, so you can just leave the assignment outside the if.

[nyan_myuji.xyz]

• Rephrase error message and formatting

[nyan_myuji.xyz]

@des updated

[des]

Hang on, I think the change I requested may have been incorrect...

[des]

Ah, sorry, I forgot that OUT() is a macro that wraps return, so everything is fine.

lib/libpam/modules/pam_exec/pam_exec.c

264

extraneous blank line

[nyan_myuji.xyz]

• Rephrase error message and formatting
• Removed blank line

[zirias]

Would someone mind to commit this, please? It enables correct operation of a suid-root helper that will probably be necessary to support newer versions of xscreensaver.
I also wonder whether this might warrant an EN for the next round of patches?

[khng]

I will commit this.