linux-kernel - Re: WARNING: kmalloc bug in input_mt_init

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Date:   Wed, 17 Oct 2018 08:43:30 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Christopher Lameter <cl@...ux.com>
CC:     Matthew Wilcox <willy@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot+87829a10073277282ad1@...kaller.appspotmail.com,
        Pekka Enberg <penberg@...nel.org>,
        "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Henrik Rydberg <rydberg@...math.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: WARNING: kmalloc bug in input_mt_init_slots

On October 17, 2018 8:35:15 AM PDT, Christopher Lameter <cl@...ux.com> wrote:
>On Tue, 16 Oct 2018, Dmitry Torokhov wrote:
>
>> On Thu, Sep 27, 2018 at 07:35:37AM -0700, Matthew Wilcox wrote:
>> > On Mon, Sep 24, 2018 at 11:41:58AM -0700, Dmitry Torokhov wrote:
>> > > > How large is the allocation? AFACIT nRequests larger than
>KMALLOC_MAX_SIZE
>> > > > are larger than the maximum allowed by the page allocator. Thus
>the warning
>> > > > and the NULL return.
>> > >
>> > > The size in this particular case is being derived from a value
>passed
>> > > from userspace. Input core does not care about any limits on size
>of
>> > > memory kmalloc() can support and is perfectly happy with getting
>NULL
>> > > and telling userspace to go away with their silly requests by
>returning
>> > > -ENOMEM.
>> > >
>> > > For the record: I definitely do not want to pre-sanitize size
>neither in
>> > > uinput nor in input core.
>> >
>> > Probably should be using kvzalloc then.
>>
>> No. No sane input device can track so many contacts so we need to use
>> kvzalloc(). Failing to allocate memory is proper response here.
>
>What is a "contact" here? Are we talking about SG segments?

No, we are talking about maximum number of fingers a person can have. Devices don't usually track more than 10 distinct contacts on the touch surface at a time.


Thanks.

-- 
Dmitry