On Wed, Aug 17, 2022 at 11:50 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote: > On 8/17/2022 7:52 AM, Paul Moore wrote: > > On Wed, Aug 17, 2022 at 7:53 AM Francis Laniel > > <flaniel@xxxxxxxxxxxxxxxxxxx> wrote: > >> Le mardi 16 août 2022, 23:59:41 CEST Paul Moore a écrit : > >>> On Mon, Jul 25, 2022 at 8:42 AM Francis Laniel > >>> > >>> <flaniel@xxxxxxxxxxxxxxxxxxx> wrote: > >>>> Hi. > >>>> > >>>> First, I hope you are fine and the same for your relatives. > >>> Hi Francis :) > >>> > >>>> A solution to this problem could be to add a way for the userspace to ask > >>>> the kernel about the capabilities it offers. > >>>> So, in this series, I added a new file to securityfs: > >>>> /sys/kernel/security/capabilities. > >>>> The goal of this file is to be used by "container world" software to know > >>>> kernel capabilities at run time instead of compile time. > >>> ... > >>> > >>>> The kernel already exposes the last capability number under: > >>>> /proc/sys/kernel/cap_last_cap > >>> I'm not clear on why this patchset is needed, why can't the > >>> application simply read from "cap_last_cap" to determine what > >>> capabilities the kernel supports? > >> When you capabilities with, for example, docker, you will fill capabilities > >> like this: > >> docker run --rm --cap-add SYS_ADMIN debian:latest echo foo > >> As a consequence, the "echo foo" will be run with CAP_SYS_ADMIN set. > >> > >> Sadly, each time a new capability is added to the kernel, it means "container > >> stack" software should add a new string corresponding to the number of the > >> capabilities [1]. > > Thanks for clarifying things, I thought you were more concerned about > > detecting what capabilities the running kernel supported, I didn't > > realize it was getting a string literal for each supported capability. > > Unless there is a significant show of support for this > > I believe this could be a significant help in encouraging the use of > capabilities. An application that has to know the list of capabilities > at compile time but is expected to run unmodified for decades isn't > going to be satisfied with cap_last_cap. The best it can do with that > is abort, not being able to ask an admin what to do in the presence of > a capability that wasn't around before because the name isn't known. An application isn't going to be able to deduce the semantic value of a capability based solely on a string value, an integer is just as meaningful in that regard. What might be useful is if the application simply accepts a set of capabilities from the user and then checks those against the maximum supported by the kernel, but once again that doesn't require a string value, it just requires the application taking a set of integers and passing those into the kernel when a capability set is required. I still don't see how adding the capability string names to the kernel is useful here. -- paul-moore.com
