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SubjectRe: [PATCH 3/3] p9auth: add p9auth driver
Quoting Eric W. Biederman (ebiederm@xmission.com):
> "Serge E. Hallyn" <serue@us.ibm.com> writes:
>
> > Quoting Greg KH (greg@kroah.com):
> >> On Tue, Apr 20, 2010 at 08:29:08PM -0500, Serge E. Hallyn wrote:
> >> > This is a driver that adds Plan 9 style capability device
> >> > implementation. See Documentation/p9auth.txt for a description
> >> > of how to use this.
> >>
> >> Hm, you didn't originally write this driver, so it would be good to get
> >> some original authorship information in here to keep everything correct,
> >> right?
> >
> > That's why I left the MODULE_AUTHOR line in there - not sure what
> > else to do for that. I'll add a comment in p9auth.txt, especially
> > pointing back to Ashwin's original paper.
> >
> >> > Documentation/p9auth.txt | 47 ++++
> >> > drivers/char/Kconfig | 2 +
> >> > drivers/char/Makefile | 2 +
> >> > drivers/char/p9auth/Kconfig | 9 +
> >> > drivers/char/p9auth/Makefile | 1 +
> >> > drivers/char/p9auth/p9auth.c | 517 ++++++++++++++++++++++++++++++++++++++++++
> >>
> >> Is this code really ready for drivers/char/? What has changed in it
> >> that makes it ok to move out of the staging tree?
> >
> > It was dropped from staging :) I don't particularly care to see it
> > go back into staging, as opposed to working out issues out of tree
> > (assuming they are solvable). For one thing, as you note below,
> > there is the question of whether it should be a device driver at
> > all.
> >
> >> And who is going to maintain it? You? Or someone else?
> >
> > If Ashwin doesn't want to maintain it, I'll do it. Either way.
> >
> >> > 6 files changed, 578 insertions(+), 0 deletions(-)
> >> > create mode 100644 Documentation/p9auth.txt
> >> > create mode 100644 drivers/char/p9auth/Kconfig
> >> > create mode 100644 drivers/char/p9auth/Makefile
> >> > create mode 100644 drivers/char/p9auth/p9auth.c
> >> >
> >> > diff --git a/Documentation/p9auth.txt b/Documentation/p9auth.txt
> >> > new file mode 100644
> >> > index 0000000..14a69d8
> >> > --- /dev/null
> >> > +++ b/Documentation/p9auth.txt
> >> > @@ -0,0 +1,47 @@
> >> > +The p9auth device driver implements a plan-9 factotum-like
> >> > +capability API. Tasks which are privileged (authorized by
> >> > +possession of the CAP_GRANT_ID privilege (POSIX capability))
> >> > +can write new capabilities to /dev/caphash. The kernel then
> >> > +stores these until a task uses them by writing to the
> >> > +/dev/capuse device. Each capability represents the ability
> >> > +for a task running as userid X to switch to userid Y and
> >> > +some set of groups. Each capability may be used only once,
> >> > +and unused capabilities are cleared after two minutes.
> >> > +
> >> > +The following examples shows how to use the API. Shell 1
> >> > +contains a privileged root shell. Shell 2 contains an
> >> > +unprivileged shell as user 501 in the same user namespace. If
> >> > +not already done, the privileged shell should create the p9auth
> >> > +devices:
> >> > +
> >> > + majfile=/sys/module/p9auth/parameters/cap_major
> >> > + minfile=/sys/module/p9auth/parameters/cap_minor
> >> > + maj=`cat $majfile`
> >> > + mknod /dev/caphash c $maj 0
> >> > + min=`cat $minfile`
> >> > + mknod /dev/capuse c $maj 1
> >> > + chmod ugo+w /dev/capuse
> >>
> >> That is incorrect, you don't need the cap_major/minor files at all, the
> >> device node should be automatically created for you, right?
> >
> > Hmm, where? Not in /dev on my SLES11 partition...
> >
> >> And do you really want to do all of this control through a device node?
> >> Why?
> >
> > Well...
> >
> > At first I was thinking same as you were. So I was going to switch
> > to a pure syscall-based approach. But it just turned out more
> > complicated. The factotum server would call sys_grantid(), and
> > the target task would end up doing some huge sys_setresugid() or
> > else multiple syscalls using the granted id. It just was uglier.
> > I think there's an experimental patchset sitting somewhere I could
> > point to (if I weren't embarassed :).
> >
> > Another possibility would be to use netlink, but that doesn't
> > appear as amenable to segragation by user namespaces. The pid
> > (presumably/hopefully global pid, as __u32) is available, so it
> > shouldn't be impossible, but a simple device with simple synchronous
> > read/write certainly has its appeal. Firing off a message hoping
> > that at some point our credentials will be changes, less so.
>
> pid in the netlink context is the netlink port-id. It is a very
> different concept from struct pid. These days netlink calls to
> the kernel are synchronous, not that I would encourage netlink
> for anything except networking code.
>
> Can we make this a trivial filesystem? I expect that would match
> up better with whatever plan9 userspace apps already exist,
> remove the inode double translation, and would make it much more
> reasonable to do a user namespace aware version if and when

BTW, this current version is user namespace aware.

> that becomes necessary.

An fs actually seems overkill for two write-only files for
process-related information. Would these actually be candidates
for new /proc files?

/proc/grantcred - replaces /dev/caphash, for privileged
tasks to tell the kernel about new setuid
capabilities
/proc/self/usecred - replaces /dev/capuse for unprivileged
tasks to make use of a setuid capability

-serge


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