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

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Message-Id: <20230616224407.863c74a3dc9d4f1427802f91@linux-foundation.org>
Date:   Fri, 16 Jun 2023 22:44:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Liam Ni <zhiguangni01@...il.com>
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 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?