linux-kernel - Re: [PATCH] powerpc/mm: Don't see NULL pointer dereference as a KUAP fault

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Message-ID: <16dd1637-cfaf-bfb3-07c6-7e71d33640b8@csgroup.eu>
Date:   Fri, 4 Dec 2020 07:30:52 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/mm: Don't see NULL pointer dereference as a KUAP
 fault



Le 03/12/2020 à 12:55, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>> Sometimes, NULL pointer dereferences are expected. Even when they
>> are accidental they are unlikely an exploit attempt because the
>> first page is never mapped.
> 
> The first page can be mapped if mmap_min_addr is 0.
> 
> Blocking all faults to the first page would potentially break any
> program that does that.
> 
> Also if there is something mapped at 0 it's a good chance it is an
> exploit attempt :)

Ok, I see.

In fact, we hit this warning because we don't provide copy_from_kernel_nofault_allowed()

I'll cook a patch for that.

Christophe