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SubjectRe: [PATCH] idr: fix invalid ptr dereference on item delete
On Fri, May 18, 2018 at 03:31:38PM -0700, Andrew Morton wrote:
> On Fri, 18 May 2018 10:50:25 -0700 Matthew Wilcox <> 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:
> > Debugged-by: Roman Kagan <>
> > Signed-off-by: Matthew Wilcox <>
> 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.

That's my fault, Matthew may not have seen the original discussion among
the KVM folks.

> Looknig at the sysbot report, the impact is at least "privileged user
> can trigger a WARN", but I assume there could be worse,

Unfortunately it is worse: the syzcaller test boils down to opening
/dev/kvm, creating an eventfd, and calling a couple of KVM ioctls. None
of this requires superuser. And the result is dereferencing an
uninitialized pointer which is likely a crash.

> as-yet-undiscovered impacts. So I'm thinking a cc:stable is needed,
> yes?

Well the specific path caught by syzbot is via KVM_HYPERV_EVENTD ioctl
which is new in 4.17. But I guess there are other user-triggerable
paths, so cc:stable is probably justified.


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