On Tue, Jun 08, 2021 at 10:42:41AM -0500, Jeremy Linton wrote: > On 6/8/21 10:19 AM, Dave Martin wrote: > >On Tue, Jun 08, 2021 at 12:33:18PM +0100, Mark Brown via Libc-alpha wrote: > >>On Mon, Jun 07, 2021 at 07:12:13PM +0100, Catalin Marinas wrote: > >> > >>>I don't think we can document all the filters that can be added on top > >>>various syscalls, so I'd leave it undocumented (or part of the systemd > >>>documentation). It was a user space program (systemd) breaking another > >>>user space program (well, anything with a new enough glibc). The kernel > >>>ABI was still valid when /sbin/init started ;). > >> > >>Indeed. I think from a kernel point of view the main thing is to look > >>at why userspace feels the need to do things like this and see if > >>there's anything we can improve or do better with in future APIs, part > >>of the original discussion here was figuring out that there's not really > >>any other reasonable options for userspace to implement this check at > >>the minute. > > > >Ack, that would be my policy -- just wanted to make it explicit. > >It would be good if there were better dialogue between the systemd > >and kernel folks on this kind of thing. > > > >SECCOMP makes it rather easy to (attempt to) paper over kernel/user API > >design problems, which probably reduces the chance of the API ever being > >fixed properly, if we're not careful... > > Well IMHO the problem is larger than just BTI here, what systemd is trying > to do by fixing the exec state of a service is admirable but its a 90% > solution without the entire linker/loader being in a more privileged > context. While BTI makes finding a generic gadget that can call mprotect > harder, it still seems like it might just be a little too easy. The secomp > filter is providing a nice bonus by removing the ability to disable BTI via > mprotect without also disabling X. So without moving more of the linker into > the kernel its hard to see how one can really lock down X only pages. > > Anyway, i'm testing this on rawhide now. > > Thanks! Well, I agree that there are larger issues here. But we need to be realistic and try not to do too much damage to future maintainability. Note, your "bonus" is really a feature-like bug. This is what we should be trying to avoid IMHO: if it's important, it needs to be designed and guaranteed. Something that works by accident is likely to get broken again by accident in the future. Cheers ---Dave
