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Date:   Sat, 19 May 2018 09:26:36 +0300
From:   Roman Kagan <rkagan@virtuozzo.com>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Matthew Wilcox <willy@infradead.org>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] idr: fix invalid ptr dereference on item delete
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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@infradead.org> 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@syzkaller.appspotmail.com
> > Debugged-by: Roman Kagan <rkagan@virtuozzo.com>
> > Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
> 
> 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.

Thanks,
Roman.