On Mon, Jan 13, 2020 at 12:42 PM Christian Brauner <christian.brauner@xxxxxxxxxx> wrote: > > On Mon, Jan 13, 2020 at 11:27:03AM -0800, Daniel Colascione wrote: > > On Mon, Jan 13, 2020 at 11:10 AM Christian Brauner > > <christian.brauner@xxxxxxxxxx> wrote: > > > This does not > > > affect the permission checking you're performing here. > > > > Pidfds-as-capabilities sounds like a good change. Can you clarify what > > you mean here though? Do you mean that in order to perform some > > process-directed operation X on process Y, the pidfd passed to X must > > have been opened with PIDFD_CAP_X *and* the process *using* the pidfds > > must be able to perform operation X on process Y? Or do pidfds in this > > model "carry" permissions in the same way that an ordinary file > > descriptor "carries" the ability to write to a file if it was opened > > with O_WRONLY even if the FD is passed to a process that couldn't > > otherwise write to that file? Right now, pidfds are identity-only and > > always rely on the caller's permissions. I like the capability bit > > model because it makes pidfds more consistent with other file > > descriptors and enabled delegation of capabilities across the system. > > I'm going back and forth on this. My initial implementation has it that > you'd need both, PIDFD_FLAG/CAP_X and the process using the pidfd must > be able to perform the operation X on process Y. The alternative becomes > tricky for e.g. anything that requires ptrace_may_access() permissions > such as getting an fd out from another task based on its pidfd and so > on. I think the alternative is necessary though. What's the point of the pidfd capability bits if they don't grant access? If I have a pidfd for Y that doesn't let me do operation X, but I have ambient authority to do Y anyway, then I can just make my own pidfd for Y and then use that new pidfd to do X. AFAICT, pidfd capabilities only do something when they replace ptrace_may_access and friends for access control. Otherwise, they seem purely advisory. Am I missing something?
