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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        syzbot <syzbot+353be47c9ce21b68b7ed@syzkaller.appspotmail.com>,
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        x86@kernel.org
Subject: Re: WARNING in memtype_reserve
In-Reply-To: <Pine.LNX.4.44L0.2005090939210.4307-100000@netrider.rowland.org>
References: <Pine.LNX.4.44L0.2005090939210.4307-100000@netrider.rowland.org>
Date:   Wed, 13 May 2020 18:21:39 +0200
Message-ID: <8736833iwc.fsf@nanos.tec.linutronix.de>
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Alan Stern <stern@rowland.harvard.edu> writes:
> On Sat, 9 May 2020, Thomas Gleixner wrote:
>
>> Greg KH <gregkh@linuxfoundation.org> writes:
>> > On Sat, May 09, 2020 at 12:20:14AM -0700, syzbot wrote:
>> >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
>> >> WARNING: CPU: 1 PID: 7025 at arch/x86/mm/pat/memtype.c:589 memtype_reserve+0x69f/0x820 arch/x86/mm/pat/memtype.c:589
>> >
>> > So should memtype_reserve() not do a WARN if given invalid parameters as
>> > it can be triggered by userspace requests?
>> >
>> > A normal "invalid request" debug line is probably all that is needed,
>> > right?
>> 
>> I disagree. The callsite espcially if user space triggerable should not
>> attempt to ask for a reservation where start > end:
>> 
>>   >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
>> 
>> The real question is which part of the call chain is responsible for
>> this. That needs to be fixed.
>
> What about all the other ways in which a reservation request could be
> invalid?  The MM core already checks for these; what point is there in
> duplicating these checks in many places higher up the call chain?

Defensive programming rule #1: Check crap early but have the check which
ultimatively catches it at the last possible place as well.