On Fri, Aug 09, 2024 at 04:00:41PM GMT, Jann Horn wrote: > On Fri, Aug 9, 2024 at 3:18 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > Talking about f_modown() and security_file_set_fowner(), it looks like > > there are some issues: > > > > On Fri, Aug 09, 2024 at 02:44:06PM +0200, Jann Horn wrote: > > > On Fri, Aug 9, 2024 at 12:59 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > [...] > > > > > > BTW, I don't understand why neither SELinux nor Smack use (explicit) > > > > atomic operations nor lock. > > > > > > Yeah, I think they're sloppy and kinda wrong - but it sorta works in > > > practice mostly because they don't have to do any refcounting around > > > this? > > > > > > > And it looks weird that > > > > security_file_set_fowner() isn't called by f_modown() with the same > > > > locking to avoid races. > > > > > > True. I imagine maybe the thought behind this design could have been > > > that LSMs should have their own locking, and that calling an LSM hook > > > with IRQs off is a little weird? But the way the LSMs actually use the > > > hook now, it might make sense to call the LSM with the lock held and > > > IRQs off... > > > > > > > Would it be OK (for VFS, SELinux, and Smack maintainers) to move the > > security_file_set_fowner() call into f_modown(), especially where > > UID/EUID are populated. That would only call security_file_set_fowner() > > when the fown is actually set, which I think could also fix a bug for > > SELinux and Smack. > > > > Could we replace the uid and euid fields with a pointer to the current > > credentials? This would enables LSMs to not copy the same kind of > > credential informations and save some memory, simplify credential > > management, and improve consistency. > > To clarify: These two paragraphs are supposed to be two alternative > options, right? One option is to call security_file_set_fowner() with > the lock held, the other option is to completely rip out the > security_file_set_fowner() hook and instead let the VFS provide LSMs > with the creds they need for the file_send_sigiotask hook? I think it would be fine to stash the credentials in struct fown_struct same as we do for struct file itself. Calling security_file_set_fowner() with the irq lock held seems suboptimal to me. Plus, this also means one less LSM hook and that seems like a win too.
