linux-kernel - Re: [PATCH 2/2] mm: Fix a few rare cases of using swapin error pte marker

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Message-ID: <2eb1cc61-01ec-10c2-e81f-f4325ca7b03f@redhat.com>
Date:   Fri, 16 Dec 2022 17:04:06 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH 2/2] mm: Fix a few rare cases of using swapin error pte
 marker

On 14.12.22 21:04, Peter Xu wrote:
> This patch should harden commit 15520a3f0469 ("mm: use pte markers for swap
> errors") on using pte markers for swapin errors on a few corner cases.
> 
> 1. Propagate swapin errors across fork()s: if there're swapin errors in
>     the parent mm, after fork()s the child should sigbus too when an error
>     page is accessed.
> 
> 2. Fix a rare condition race in pte_marker_clear() where a uffd-wp pte
>     marker can be quickly switched to a swapin error.
> 
> 3. Explicitly ignore swapin error pte markers in change_protection().
> 
> I mostly don't worry on (2) or (3) at all, but we should still have them.
> Case (1) is special because it can potentially cause silent data corrupt on
> child when parent has swapin error triggered with swapoff, but since swapin
> error is rare itself already it's probably not easy to trigger either.
> 
> Currently there is a priority difference between the uffd-wp bit and the
> swapin error entry, in which the swapin error always has higher
> priority (e.g. we don't need to wr-protect a swapin error pte marker).
> 
> If there will be a 3rd bit introduced, we'll probably need to consider a
> more involved approach so we may need to start operate on the bits.  Let's
> leave that for later.
> 
> This patch is tested with case (1) explicitly where we'll get corrupted
> data before in the child if there's existing swapin error pte markers, and
> after patch applied the child can be rightfully killed.
> 
> We don't need to copy stable for this one since 15520a3f0469 just landed as
> part of v6.2-rc1, only "Fixes" applied.
> 
> Fixes: 15520a3f0469 ("mm: use pte markers for swap errors")
> Signed-off-by: Peter Xu <peterx@...hat.com>

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb