On Wed, Apr 20, 2016 at 5:55 PM, Eric W. Biederman <ebied...@xmission.com> wrote: > Linus Torvalds <torva...@linux-foundation.org> writes: > >> On Tue, Apr 19, 2016 at 9:36 PM, Konstantin Khlebnikov <koc...@gmail.com> >> wrote: >>> On Wed, Apr 20, 2016 at 6:04 AM, Eric W. Biederman >>>> >>>> The kernel.pty.reserve sysctl is neutered with no way currently >>>> implemented to be able to use the reserved ptys. >>> >>> I think we could convert this into reserve for init user namespace, >>> ssh in host will work even if containers eaten all ptys. >> >> Yes. That's basically how it effectively worked before (ie everything >> but the initial non-newinstance devpts mount would be limited to the >> non-reserved numbers). >> >> We required the non-init namespaces to do a newinstance mount, so the >> whole test for "newinstance" was effectively the same thing as just >> checking for the init namespace from a security standpoint. >> >> And in fact, rewriting it in that form (ie checking for init_ns) would >> just make it much more obvious what the intent it. > > How does this sound. > > When mounting a devpts filesystem. We look at the caller (aka current) > and if we are in the initial mount namespace set a flag in fsi that > allows that instance of devpts to draw into the reserve pool.
Maybe just check current user namespace when task opens /dev/ptmx? IIRR now check looks like: count < limit - (newinstance ? reserved : 0). So, it will be count < limit - (current_in_init_userns ? 0 : newinstance). > > That will still allow crazy pieces of code like xen-create-instance run > by root that mount a devpts filesystem in a chroot environment to draw > into the reserved pool, but any sane users that set up their own mount > namespace won't be able to user the reserve pool. > > I believe that will give an almost identical policy to what we have > today, and it certainly makes a good default test for a container. Just > for cleanliness containers (of anyone's definition) almost always use > mount namespaces instead of chroots. > > Sigh one last past through all of the distros, to confirm that this > works in practice. > > Eric
