On Wed, Jan 2, 2019 at 12:32 PM Christian Brauner <christian@xxxxxxxxxx> wrote: > > Since binderfs can be mounted by userns root in non-initial user namespaces > some precautions are in order. First, a way to set a maximum on the number > of binder devices that can be allocated per binderfs instance and second, a > way to reserve a reasonable chunk of binderfs devices for the initial ipc > namespace. > A first approach as seen in [1] used sysctls similiar to devpts but was > shown to be flawed (cf. [2] and [3]) since some aspects were unneeded. This > is an alternative approach which avoids sysctls completely and instead > switches to a single mount option. > > Starting with this commit binderfs instances can be mounted with a limit on > the number of binder devices that can be allocated. The max=<count> mount > option serves as a per-instance limit. If max=<count> is set then only > <count> number of binder devices can be allocated in this binderfs > instance. > > This allows to safely bind-mount binderfs instances into unprivileged user > namespaces since userns root in a non-initial user namespace cannot change > the mount option as long as it does not own the mount namespace the > binderfs mount was created in and hence cannot drain the host of minor > device numbers > > [1]: https://lore.kernel.org/lkml/20181221133909.18794-1-christian@xxxxxxxxxx/ > [2]; https://lore.kernel.org/lkml/20181221163316.GA8517@xxxxxxxxx/ > [3]: https://lore.kernel.org/lkml/CAHRSSEx+gDVW4fKKK8oZNAir9G5icJLyodO8hykv3O0O1jt2FQ@xxxxxxxxxxxxxx/ > [4]: https://lore.kernel.org/lkml/20181221192044.5yvfnuri7gdop4rs@xxxxxxxxxx/ > > Cc: Todd Kjos <tkjos@xxxxxxxxxx> > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx> Fwiw, once this lands I intend to add a binderfs.txt file to Documentation/filesystems so that we have proper documentation too. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/driverdev-devel
