Re: [PATCH] idr: fix invalid ptr dereference on item delete

[Matthew Wilcox]

On Fri, May 18, 2018 at 03:31:38PM -0700, Andrew Morton wrote:
> On Fri, 18 May 2018 10:50:25 -0700 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > If the radix tree underlying the IDR happens to be full and we attempt
> > to remove an id which is larger than any id in the IDR, we will call
> > __radix_tree_delete() with an uninitialised 'slot' pointer, at which
> > point anything could happen.  This was easiest to hit with a single entry
> > at id 0 and attempting to remove a non-0 id, but it could have happened
> > with 64 entries and attempting to remove an id >= 64.
> > 
> > Fixes: 0a835c4f090a ("Reimplement IDR and IDA using the radix tree")
> > Reported-by: syzbot+35666cba7f0a337e2e79@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Debugged-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
> > Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> 
> Neither of the changelogs I'm seeing attempt to describe the end-user
> impact of the bug.  People like to know that so they can decide which
> kernel version(s) need patching, so please always remember it.

The problem is that it could be user-triggerable a dozen different
ways.

> Looknig at the sysbot report, the impact is at least "privileged user
> can trigger a WARN", but I assume there could be worse,
> as-yet-undiscovered impacts.  So I'm thinking a cc:stable is needed,
> yes?

I thought if I used the Fixes: tag it would automatically get picked up.
Did I misunderstand?  I can imagine many different parts of the kernel
that use the IDR could trigger such a warning (although syzbot should
probably have tripped over them before now) so I wouldn't downplay
it to "only privileged users".