linux-kernel - Re: [Linux Kernel Bug][mm/gup] 3 Warning Crashes: kmalloc bug in gup_test_ioctl, is_valid_gup_args, pin_user_pages

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Date: Fri, 26 Jan 2024 11:08:46 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Matthew Wilcox <willy@...radead.org>, Chenyuan Yang <chenyuan0y@...il.com>
CC: <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, <syzkaller@...glegroups.com>, Zijie Zhao
	<zzjas98@...il.com>
Subject: Re: [Linux Kernel Bug][mm/gup] 3 Warning Crashes: kmalloc bug in
 gup_test_ioctl, is_valid_gup_args, pin_user_pages_fast

On 1/26/24 10:34, Matthew Wilcox wrote:
> On Fri, Jan 26, 2024 at 11:31:26AM -0600, Chenyuan Yang wrote:
>> In this context, I would like to seek your valued opinion. Do you
>> believe it would be more prudent to avoid fuzz testing the
>> `gup_test_ioctl`, or are the warnings in `gup_test_ioctl` an
>> anticipated outcome?
> 
> It seems like a waste of time.  Debian certainly disables it, so
> unless you can find a distro who enables it, I wouldn't bother.

+1000! The purpose of gup_test_ioctl is to provide a way to invoke,
from user space, direct testing of some kernel interfaces that are
not actually exposed to user space for production systems.

Fuzzing this interface is exactly what you should never do. :)

> 
>> It seems that `gup_test_ioctl` can indeed be exposed in the kernel by
>> accessing /sys/kernel/debug/gup_test.

That's a debug interface.

> 
> If someone wants to fix these things, they can, but it just doesn't
> seem worth doing.  Part of the art of fuzz testing is finding things
> that are worth testing.

I'll go just slightly further, even: some conceivable "fixes" could end
up hurting test coverage. Without providing any real benefit.


thanks,
-- 
John Hubbard
NVIDIA