Messages in this thread | | | Date | Mon, 21 Aug 2023 17:13:36 +0200 | From | Aleksander Mazur <> | Subject | Re: [PATCH] /proc/modules: honor kptr_restrict even without CONFIG_KALLSYMS |
| |
Dnia 2023-05-27, o godz. 11:44:35 "Arnd Bergmann" <arnd@arndb.de> napisał(a):
> > Dnia 2022-06-05, o godz. 22:43:47 > > Aleksander Mazur <deweloper@wp.pl> napisał(a): > > > >> Commit e4a8ca3baa55 fixed building without CONFIG_KALLSYMS by providing > >> dummy kallsyms_show_value(). Unfortunately -- due to hard-coded "false" > >> being returned -- access to addresses in /proc/modules became permanently > >> disabled. > >> > >> My proposal is to change this unconditional "false" to !kptr_restrict. > >> This re-enables addresses in /proc/modules even without CONFIG_KALLSYSMS > >> unless restricted by means of sysctl (kernel.kptr_restrict). > > I just looked at the original 516fb7f2e73dc ("/proc/module: use the > same logic as /proc/kallsyms for address exposure") commit again, > the intention here was to use the same logic for /proc/modules > and /proc/kallsyms. > > I agree that this means my patch went too far, but I'm not sure > about yours either. Maybe we can just move kallsyms_show_value() > into a different location that is always built and rename it > accordingly. Then it can be used by both kallsyms and /proc/modules > regardless of which combination of these two is enabled in the > kernel. >
I don't see any reason why should I enable both KALLSYMS and PERF_EVENTS just to see addressess inside /proc/modules. But this is currently required by the kernel, what in my opinion violates the description of kptr_restrict in Documentation/admin-guide/sysctl/kernel.rst.
My patch restores exposing kernel addresses via /proc/modules in the case of kptr_restrict==0 and it doesn't change current behaviour in other cases, especially for kptr_restrict==1; this means that even with my patch applied, the code is still more restrictive than the description of kptr_restrict (because it doesn't make an exception for processes with CAP_SYSLOG). Therefore I cannot agree that my patch goes "too far"; it rather goes just one step in the direction towards compliance with kernel documentation.
Moving kallsyms_show_value() somewhere else and re-using it won't help in my case because that function honors kptr_restrict==0 only if profiling is compiled in and perf_event_paranoid<=1. BTW. I think this is wrong since according to the docs, perf_event_paranoid "controls use of the performance events system by unprivileged users", while placing restrictions on exposing kernel addresses via /proc should be controlled by kptr_restrict. Or maybe I'm wrong and /proc/modules silently became a part of "the performance events system"?...
-- Aleksander Mazur
| |