linux-kernel - Re: [PATCH 00/10] mm/hugetlb: Make huge_pte

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Date:   Tue, 29 Nov 2022 12:49:44 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        James Houghton <jthoughton@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Muchun Song <songmuchun@...edance.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 00/10] mm/hugetlb: Make huge_pte_offset() thread-safe
 for pmd unshare

On Tue, 29 Nov 2022 14:35:16 -0500 Peter Xu <peterx@...hat.com> wrote:

> Based on latest mm-unstable (9ed079378408).
> 
> This can be seen as a follow-up series to Mike's recent hugetlb vma lock
> series for pmd unsharing, but majorly covering safe use of huge_pte_offset.

We're at -rc7 (a -rc8 appears probable this time) and I'm looking to
settle down and stabilize things...

> 
> ...
>
> huge_pte_offset() is always called with mmap lock held with either read or
> write.  It was assumed to be safe but it's actually not.  One race
> condition can easily trigger by: (1) firstly trigger pmd share on a memory
> range, (2) do huge_pte_offset() on the range, then at the meantime, (3)
> another thread unshare the pmd range, and the pgtable page is prone to lost
> if the other shared process wants to free it completely (by either munmap
> or exit mm).

That sounds like a hard-to-hit memory leak, but what we have here is a
user-triggerable use-after-free and an oops.  Ugh.

Could people please prioritize the review and testing of this patchset?