linux-kernel - Re: [PATCH] mm/sparse:avoid null pointer access in memory

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Message-ID: <CACZJ9cXM9VkJ5=euHphwM5TtX3aZqZ_QynOq10FtrqCxZfZsQA@mail.gmail.com>
Date:   Sat, 17 Jun 2023 14:17:58 +0800
From:   Liam Ni <zhiguangni01@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
        hpa@...or.com, rppt@...nel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm/sparse:avoid null pointer access in memory_present()

On Sat, 17 Jun 2023 at 13:44, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Sat, 17 Jun 2023 14:40:36 +1000 Liam Ni <zhiguangni01@...il.com> wrote:
>
> > __nr_to_section() may return a null pointer,
> > before accessing the member variable section_mem_map,
> > we should first determine whether it is a null pointer.
> >
> > ...
> >
> > --- a/mm/sparse.c
> > +++ b/mm/sparse.c
> > @@ -258,7 +258,7 @@ static void __init memory_present(int nid, unsigned long start, unsigned long en
> >               set_section_nid(section, nid);
> >
> >               ms = __nr_to_section(section);
> > -             if (!ms->section_mem_map) {
> > +             if (ms && !ms->section_mem_map) {
> >                       ms->section_mem_map = sparse_encode_early_nid(nid) |
> >                                                       SECTION_IS_ONLINE;
> >                       __section_mark_present(ms, section);
>
> I'm suspecting that if __nr_to_section() returns NULL here, we should
> just panic.  But a null-deref gives the same information, so why change
> things?

Do you mean if ms is a null pointer，ms->section_mem_map will cause
system panic,so we needn't change?

>