linux-kernel - Re: [PATCH] mm: fix kernel NULL pointer dereference in page_vma_mapped

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Date:   Thu, 7 Apr 2022 15:17:27 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     zhenwei pi <pizhenwei@...edance.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix kernel NULL pointer dereference in
 page_vma_mapped_walk

On Thu, Apr 07, 2022 at 02:54:42PM +0100, Matthew Wilcox wrote:
> On Thu, Apr 07, 2022 at 02:40:08PM +0800, zhenwei pi wrote:
> > size_to_hstate(4K) returns NULL pointer, this leads kernel BUG in
> > function page_vma_mapped_walk.
> 
> Yes, I think this is the right fix.  It's not immediately obvious from
> the bug and the patch, but what's going on is:
> 
> page_mapped_in_vma() sets nr_pages to 1.  This is correct because we
> usually only want to know about the precise page, and not about the
> folio containing it.  But hugetlbfs is special (... in so many ways ...)
> and actually wants to work on the entire folio.  We could set nr_pages
> specially for hugetlb pages, but it's better to ignore it in
> page_vma_mapped_walk() for the hugetlb case.
> 
> I'll fix up the changelog and add it to my pile of fixes that I'm
> sending tomorrow.
> https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/for-next

I looked over the patch in context, and realised we now don't need to
call size_to_hstate since we're getting the hstate from the VMA.

https://git.infradead.org/users/willy/pagecache.git/commitdiff/98ea02597b9967c0817d29fee2f96d21b9e59ca5