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

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Date:   Sun, 18 Jun 2023 10:16:27 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Liam Ni <zhiguangni01@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/sparse:avoid null pointer access in memory_present()

On Sat, Jun 17, 2023 at 04:59:46PM +0800, Liam Ni wrote:
> On Sat, 17 Jun 2023 at 15:01, Mike Rapoport <rppt@...nel.org> wrote:
> >
> > On Sat, Jun 17, 2023 at 02:17:58PM +0800, Liam Ni wrote:
> > > 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?
> >
> > Yes, if __nr_to_section ever returns NULL the system will crash anyway.
> 
> I got it,do we need to print some information by panic()?

Accessing a NULL pointer will cause panic and there will be lots of
information spilled into the log anyway.

-- 
Sincerely yours,
Mike.