capabilities carried over execve()

capabilities carried over execve()

[Eric Buddington]

I am attempting to write an init replacement that is capability-smart.
Though I'm pleased that prctl() lets me keep capabilities across a
setreuid(), maintaining caps over execve() seems impossible to do right.

I currently see a few options:
	- use the CLOEXEC-pipe hack that execcap uses (parent notices
	  when pipe closes then rushes to set caps on child before
	  child notices they're gone). This looks like a race to me.
	- tweak linux/fs/exec.c (prepare_binprm) to pretend that all
	  files have cap_inheritable and cap_effective fully set.
	  This seems a more elegant solution, but requires a kernel
	  patch.
	- exec the child in a stopped state, mess with caps, then
	  send it SIGCONT. AFAIK, there is no way to do
	  execve_and_stop.

Is there a better solution available, or one in the works?
I think capabilites may be a key to achieving Pretty Good (tm) security
- but then again, so is running bind as non-root, and nobody even
bothers to do that...

-Eric

Re: capabilities carried over execve()

[Pavel Machek]

Hi!

> I am attempting to write an init replacement that is capability-smart.
> Though I'm pleased that prctl() lets me keep capabilities across a
> setreuid(), maintaining caps over execve() seems impossible to do right.
> 
> I currently see a few options:
> 	- use the CLOEXEC-pipe hack that execcap uses (parent notices
> 	  when pipe closes then rushes to set caps on child before
> 	  child notices they're gone). This looks like a race to me.
> 	- tweak linux/fs/exec.c (prepare_binprm) to pretend that all
> 	  files have cap_inheritable and cap_effective fully set.
> 	  This seems a more elegant solution, but requires a kernel
> 	  patch.
> 	- exec the child in a stopped state, mess with caps, then
> 	  send it SIGCONT. AFAIK, there is no way to do
> 	  execve_and_stop.

What about ptrace? It should be able to do this kind of stuff... but
it is going to be messy.
								Pavel
-- 
I'm pavel@ucw.cz. "In my country we have almost anarchy and I don't care."
Panos Katsaloulis describing me w.r.t. patents at discuss@linmodems.org

Re: capabilities carried over execve()

[willy tarreau]

I personnaly use this simple patch which allows me
to keep caps over execve(). It allows me to give a
few more rights to some trusted users, such as 
kill, insmod... without risking unlink, chown or 
so. I couldn't find any other way to achieve this.

If needed, I can send you the complete prog which
sets the requested capabilities upon login, 
eventually asking for a password and limited in 
time of day.

Regards,
Willy

--- linux-2.2.18-wt11/fs/exec.c Fri Feb 16 23:11:52
2001
+++ linux-2.2.18-wt11+caps/fs/exec.c    Thu Feb 22
20:45:33 2001
@@ -702,7 +702,10 @@
        cap_clear(bprm->cap_inheritable);
        cap_clear(bprm->cap_permitted);
        cap_clear(bprm->cap_effective);
-
+/*** FIXME: just a test : keep permitted and
effective ******/
+bprm->cap_permitted =
cap_intersect(current->cap_inheritable,current->cap_permitted);
+bprm->cap_effective =
cap_intersect(current->cap_inheritable,current->cap_effective);
+/*** /FIXME ****/
        /*  To support inheritance of root-permissions
and suid-root
          *  executables under compatibility mode, we
raise all three
          *  capability sets for the file.



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