Messages in this thread |  | | Date | Wed, 5 Oct 2016 13:02:34 +0300 | From | Jarkko Sakkinen <> | Subject | Re: [PATCH] tpm: don't destroy chip device prematurely |
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On Tue, Oct 04, 2016 at 10:47:38AM -0600, Jason Gunthorpe wrote: > On Tue, Oct 04, 2016 at 08:19:46AM +0300, Jarkko Sakkinen wrote: > > > > Make the driver uncallable first. The worst race that can happen is that > > > open("/dev/tpm0", ...) returns -EPIPE. I do not consider this fatal at > > > all. > > > > No responses for this reasonable proposal so I'll show what I mean: > > How is this any better than what Thomas proposed? It seems much worse > to me since now we have even more stuff in the wrong order.
It moves a logical block to the front instead of moving one thing from one logical block to another place.
I'll repeat my question: what worse can happen than returning -EPIPE? I though the whole rw lock scheme was introduced just for this purpose.
Why there's even that branch in tpm-dev.c if it's so bad to let it happen?
/Jarkko
> There are three purposes to the ordering as it stands today > 1) To guarantee that tpm2_shutdown is the last command delivered to > the TPM. When it is issued all other ways to access the device > are hard fenced off. > 2) To hard fence the tpm subsystem for the 'platform' driver. Once > tpm_del_char_device completes no callback into the driver > is possible *at all*. The driver can destroy everything > (iounmap, dereg irq, etc) and the driver module can be unloaded. > 3) To prevent oopsing with the sysfs code. Recall this comment > > /* The sysfs routines rely on an implicit tpm_try_get_ops, device_del > * is called before ops is null'd and the sysfs core synchronizes this > * removal so that no callbacks are running or can run again > */ > > device_del is what eliminates the sysfs access path, so > ordering device_del after ops = null is just unconditionally > wrong. > > I still haven't heard an explanation why Thomas's other patches need > this, or why trying to change this ordering makes any sense at > all considering how the subsystem is constructed. > > Further, if tpm_crb now needs a registered device, how on earth do all > the chip ops we call work *before* registration? Or is that another > bug? > > Why can't tpm_crb return to the pre-registration operating state > in the driver remove function before calling unregister? > > None of this makes any sense to me. > > This whole thing was very carefully constructed to work *correctly* > during unregister. Many other subsystems have races and bugs during > remove (eg see the securityfs discussion). TPM has a hard requirement > to support safe unregister due to the vtpm stuff, so we don't get to > screw it up just to support one driver. > > Jason
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