linux-kernel - Re: kobject_init_and

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Message-ID: <1591210928.13983.24.camel@HansenPartnership.com>
Date:   Wed, 03 Jun 2020 12:02:08 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Wang Hai <wanghai38@...wei.com>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, khlebnikov@...dex-team.ru,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: kobject_init_and_add is easy to misuse

On Wed, 2020-06-03 at 15:36 -0300, Jason Gunthorpe wrote:
> On Wed, Jun 03, 2020 at 11:04:35AM -0700, James Bottomley wrote:
> > On Tue, 2020-06-02 at 21:22 -0300, Jason Gunthorpe wrote:
> > > On Tue, Jun 02, 2020 at 02:51:10PM -0700, James Bottomley wrote:
> > > 
> > > > My first thought was "what?  I got suckered into creating a
> > > > patch", thanks ;-)  But now I look, all the error paths do
> > > > unwind back to the initial state, so kfree() on error looks to
> > > > be completely correct. 
> > > 
> > > It doesn't fully unwind if the kobject is put into a kset, then
> > > another thread can get the kref during kset_find_obj() and
> > > kfree() won't wait for the kref to go to 0. It must use put.
> > 
> > That does seem a bit contrived: the only failure
> > kobject_add_internal() can get after kobj_kset_join() is from
> > directory creation.  If directory creation fails, no name appears
> > in sysfs and no event for the name is sent, how did another thread
> > get the name to pass in to kset_find_obj()?
> 
> The other thread just guesses in a hostile way? 
> 
> Eg it looks like the iommu stuff just feeds in user data to
> kobj_kset_join().

Well, if we have to go down the rabbit hole this far, it turns out to
be fixable because of the state_in_sysfs flag:

@@ -899,7 +903,8 @@ struct kobject *kset_find_obj(struct kset *kset, const char *name)
 	spin_lock(&kset->list_lock);
 
 	list_for_each_entry(k, &kset->list, entry) {
-		if (kobject_name(k) && !strcmp(kobject_name(k), name)) {
+		if (kobject_name(k) && k->state_in_sysfs &&
+		    !strcmp(kobject_name(k), name)) {
 			ret = kobject_get_unless_zero(k);
 			break;
 		}

That would ensure the name can't be found until the sysfs directory
creation has succeeded, which would be the point from which
kobject_init_and_add() can't fail.

James