On Sun, Nov 18, 2018 at 9:43 AM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > Daniel Colascione <dancol@xxxxxxxxxx> writes: > >> On Sun, Nov 18, 2018 at 9:13 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>> On Sun, Nov 18, 2018 at 8:29 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote: >>>> >>>> On Sun, Nov 18, 2018 at 8:17 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>>> > On Sun, Nov 18, 2018 at 7:53 AM Daniel Colascione <dancol@xxxxxxxxxx> wrote: >>>> >> >>>> >> On Sun, Nov 18, 2018 at 7:38 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: >>>> >> > I fully agree that a more comprehensive, less expensive API for >>>> >> > managing processes would be nice. But I also think that this patch >>>> >> > (using the directory fd and ioctl) is better from a security >>>> >> > perspective than using a new file in /proc. >>>> >> >>>> >> That's an assertion, not an argument. And I'm not opposed to an >>>> >> operation on the directory FD, now that it's clear Linus has banned >>>> >> "write(2)-as-a-command" APIs. I just insist that we implement the API >>>> >> with a system call instead of a less-reliable ioctl due to the >>>> >> inherent namespace collision issues in ioctl command names. >>>> > >>>> > Linus banned it because of bugs iike the ones in the patch. >>>> >>>> Maybe: he didn't provide a reason. What's your point? >>> >>> My point is that an API that involves a file like /proc/PID/kill is >>> very tricky to get right. Here are some considerations: >> >> Moot. write(2) for this interface is off the table anyway. The right >> approach here is a system call that accepts a /proc/pid directory file >> descriptor, a signal number, and a signal information field (as in >> sigqueue(2)). > > If we did not have the permission check challenges and could perform > the permission checks in open, write(2) would be on the table. > Performing write(2) would only be concrend about data. > > Unfortunately we have setresuid and exec which make that infeasible > for the kill operations. > >>> Now if we had an ioctlat() API, maybe it would make sense. But we >>> don't, and I think it would be a bit crazy to add one. >> >> A process is not a driver. Why won't this idea of using an ioctl for >> the kill-process-by-dfd thing just won't die? An ioctl has *zero* >> advantage in this context. > > An ioctl has an advantage in implementation complexity. An ioctl is > very much easier to wire up that a system call. > > I don't know if that outweighs ioctls disadvantages in long term > maintainability. It's not just maintainability. It's safety. We want to expose the new kill interface to userspace via some kill(1) extension, probably. So you should be able to write something like `cd /proc/12345 && kill --by-handle .`. How does kill --by-handle know that it's safe to perform the kill-by-proc-dfd operation on the file descriptor that it opens? If the kill operation is an ioctl, you could pass it /proc/self/fd/whatever of a completely different type; kill would call ioctl on whatever FD it got, and potentially do a completely random thing instead of killing a process. In the same situation, a new system call would fail reliably. Yes, kill could check that the device numbers of the file it opened matched proc's somehow, but that's annoying and error-prone and nobody's going to bother in practice. A new system call, by contrast, fails safe. I really don't want to give up safety and fail-safe behavior forever just because it's annoying, today, to wire up a new system call. (The new table-driven system call stuff, if it ever lands, would make things easier.)
