linux-kernel - Re: [GIT PULL] Driver core fix for 4.5-rc4

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Message-ID: <CA+55aFw-7fYXMTs10ggXqp4E3Nf+WMY7dotczSMGsAgFiYjxtA@mail.gmail.com>
Date:	Sun, 14 Feb 2016 12:57:37 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Greg KH <gregkh@...uxfoundation.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] Driver core fix for 4.5-rc4

On Sun, Feb 14, 2016 at 11:02 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
>
> Here is one driver core, well klist, fix for 4.5-rc4.  It fixes a
> problem found in the scsi device list traversal that probably also could
> be triggered by other subsystems.

So I pulled this, but quite frankly, the fix smells bad to me.

If the n_ref kref can go down to zero at any time, how is that "struct
klist_node *n" safe to ever even touch in the caller?

IOW, what is it that protects that klist_node from not having entirely
been released, and any access to the kref might be a use-after-free
(and the use of "kref_get_unless_zero()" just hides the problem).

So it smells to me like if the kref can go down to zero, the caller is
basically passing in a random pointer.

Please make me feel better about my pull. I need a virtual hug.

(Also, rather than assigning i_dur twice like this:

+       i->i_cur = NULL;
+       if (n && kref_get_unless_zero(&n->n_ref))
+               i->i_cur = n;

I think it would have been cleaner to [in]validate "n" first (perhaps
with a comment about _why_ that is needed yet safe):

+       if (n && !kref_get_unless_zero(&n->n_ref))
+               n = NULL;

and then just do a simple:

+       i->i_cur = n;

afterwards).

But I care less about that small syntactic issue than I care about
understanding why it's safe to pass around a klist_node that might not
exist any more.

               Linus