cups userspace -- trusted programs?
Hey all,
I'm wondering if the intent of the cups userspace tools are to be trusted programs? Specifically I'm curious about cupsaccept, cupsreject, cupsenable and cupsdisable. The reason I ask is because if they are supposed to be trusted programs, they don't generate unique audit messages like other programs.
Personally, I think these tools should generate messages since they are a source for leaking information, and therefore should be restricted to administrators.
Thanks,
Mike
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Hi Mike,
Matt is away this week so he'll probably have a more detailed response but in the meantime, I have a few comments/questions.
Quoted 5 lines
I'm wondering if the intent of the cups userspace tools are to be trusted programs? Specifically I'm curious about cupsaccept, cupsreject, cupsenable and cupsdisable. The reason I ask is because if they are supposed to be trusted programs, they don't generate unique audit messages like other programs.
I don't think these programs are trusted programs because all they do is talk to the cupsd, which is a trusted program. The cupsd makes all the decisions and takes all the actions. These programs (really just 'accept' as the rest I believe are symlinks to it) are not setuid and do not make any access or other decisions, at least that's my understanding.
Quoted 3 lines
Personally, I think these tools should generate messages since they are a source for leaking information, and therefore should be restricted to administrators.
I think the real question is which actions should be audited. Should enabling/disabling a printer queue be audited? I don't believe its required to be and if its not security relevant, do we want it in the audit logs? Cups has a comprehensive logging facility so there is all kinds of information about happening with the print subsystem that I don't think we want to replicate in the audit logs, but perhaps there are more actions that would make sense to audit than we currently are auditing.
Do you have specific examples of actions that you think should be audited aside from what's required for LSPP?
-- ljk
Quoted 4 lines
Thanks,
Mike
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Quoted 8 lines · Linda Knippers
Hi Mike,
Matt is away this week so he'll probably have a more detailed response but in the meantime, I have a few comments/questions.
Quoted 2 lines
I'm wondering if the intent of the cups userspace tools are to be trusted programs? Specifically I'm curious about cupsaccept, cupsreject, cupsenable and cupsdisable. The reason I ask is because if they are supposed to be trusted programs, they don't generate unique audit messages like other programs.
Quoted 6 lines
I don't think these programs are trusted programs because all they do is talk to the cupsd, which is a trusted program. The cupsd makes all the decisions and takes all the actions. These programs (really just 'accept' as the rest I believe are symlinks to it) are not setuid and do not make any access or other decisions, at least that's my understanding.
You are correct. accept, reject, cupsenable and cupsdisable are all done through the accept binary, and it does not responsible for decisions, it only facilitate actions. I learned this after reading some code :p
Quoted 10 lines
Quoted 1 line
Personally, I think these tools should generate messages since they are a source for leaking information, and therefore should be restricted to administrators.
I think the real question is which actions should be audited. Should enabling/disabling a printer queue be audited? I don't believe its required to be and if its not security relevant, do we want it in the audit logs? Cups has a comprehensive logging facility so there is all kinds of information about happening with the print subsystem that I don't think we want to replicate in the audit logs, but perhaps there are more actions that would make sense to audit than we currently are auditing.
According to Klaus, this is not strictly speaking required for LSPP. Your point about cups logging such actions is well taken (and over looked by me initially).
Quoted 2 lines
Do you have specific examples of actions that you think should be audited aside from what's required for LSPP?
Aside from what is *required*, I thought it would be a good thing to log the queue/printer enable/disable. However, if cups is logging that, I'm not sure it is worth being redundant in our logs.
Mike
Quoted 1 line · Michael C Thompson
Quoted 1 line
Quoted 1 line
Personally, I think these tools should generate messages since they are a source for leaking information, and therefore should be restricted to administrators.
I don't think they should be considered a source for leaking information. The only thing I see isn't a leak so much as a (extremely low bandwidth) covert channel of "is the printer enabled or disabled?" Since the use of these programs is restricted, we're covered under no-evil-admin.
Quoted 1 line
Aside from what is *required*, I thought it would be a good thing to log the queue/printer enable/disable. However, if cups is logging that, I'm not sure it is worth being redundant in our logs.
As long as LogLevel is set to info or higher you'll get a message in /var/log/cups/error_log like:
[Timestamp] Printer 'foo' stopped by 'root'.
I think I agree with you that its probably not worth being redundant, but if for someone finds a requirement for this to go to the audit log I don't see any issues around adding that.
-matt
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Quoted 4 lines · Matt Anderson
Quoted 1 line · Michael C Thompson
Quoted 1 line
Quoted 1 line
Personally, I think these tools should generate messages since they are a source for leaking information, and therefore should be restricted to administrators.
I don't think they should be considered a source for leaking information. The only thing I see isn't a leak so much as a (extremely low bandwidth) covert channel of "is the printer enabled or disabled?" Since the use of these programs is restricted, we're covered under no-evil-admin.
How are these restricted? Or rather, how are they supposed to be restricted? I am able to cupsenable, cupsdisable, accept and reject my printer as a non-root user under both permissive and enforcing modes.
Quoted 9 lines
Quoted 1 line
Aside from what is *required*, I thought it would be a good thing to log the queue/printer enable/disable. However, if cups is logging that, I'm not sure it is worth being redundant in our logs.
As long as LogLevel is set to info or higher you'll get a message in
/var/log/cups/error_loglike:[Timestamp] Printer 'foo' stopped by 'root'.
I think I agree with you that its probably not worth being redundant, but if for someone finds a requirement for this to go to the audit log I don't see any issues around adding that.
-matt
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Quoted 10 lines
Quoted 5 lines
I don't think they should be considered a source for leaking information. The only thing I see isn't a leak so much as a (extremely low bandwidth) covert channel of "is the printer enabled or disabled?" Since the use of these programs is restricted, we're covered under no-evil-admin.
How are these restricted? Or rather, how are they supposed to be restricted? I am able to cupsenable, cupsdisable, accept and reject my printer as a non-root user under both permissive and enforcing modes.
To which groups does your user account belong? By default, cups
will allow anyone in group sys to perform administrative functions but this is configurable in cupsd.conf. We'll have to decide whether allowing sys group members is ok or we'll have to modify the cupsd.conf for the evaluated config. I suspect we'll modify cupsd.conf.
-- ljk
Quoted 6 lines · Linda Knippers
Quoted 6 lines
Quoted 5 lines
I don't think they should be considered a source for leaking information. The only thing I see isn't a leak so much as a (extremely low bandwidth) covert channel of "is the printer enabled or disabled?" Since the use of these programs is restricted, we're covered under no-evil-admin.
Quoted 6 lines
Quoted 4 lines
How are these restricted? Or rather, how are they supposed to be restricted? I am able to cupsenable, cupsdisable, accept and reject my printer as a non-root user under both permissive and enforcing modes.
To which groups does your user account belong?
uid=500(mcthomps) gid=500(mcthomps) groups=500(mcthomps) context=user_u:user_r:user_t:SystemLow
Quoted 6 lines
By default, cups
will allow anyone in group sys to perform administrative functions but this is configurable in cupsd.conf. We'll have to decide whether allowing sys group members is ok or we'll have to modify the cupsd.conf for the evaluated config. I suspect we'll modify cupsd.conf.
I've butchered my cupsd.conf pretty badly, so it could be a result of that. I've not tried doing this with a fresh install, but if it works on your end, I'll assume it's my config mangling.
Mike