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Subject: Re: [PATCH 2/2] mm/uffd: Sanity check write bit for uffd-wp protected
 ptes
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20221110151702.1478763-3-peterx@redhat.com>
Date:   Thu, 10 Nov 2022 10:43:25 -0800
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Ives van Hoorne <ives@codesandbox.io>
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Message-Id: <58FFF1A3-DC37-4D9F-ABF0-3C5DA519BFFB@gmail.com>
References: <20221110151702.1478763-1-peterx@redhat.com>
 <20221110151702.1478763-3-peterx@redhat.com>
To:     Peter Xu <peterx@redhat.com>
Precedence: bulk

On Nov 10, 2022, at 7:17 AM, Peter Xu <peterx@redhat.com> wrote:

> +#ifdef CONFIG_DEBUG_VM
> +	/*
> +	 * Having write bit for wr-protect-marked present ptes is fatal,
> +	 * because it means the uffd-wp bit will be ignored and write will
> +	 * just go through.
> +	 *
> +	 * Use any chance of pgtable walking to verify this (e.g., when
> +	 * page swapped out or being migrated for all purposes). It means
> +	 * something is already wrong.  Tell the admin even before the
> +	 * process crashes. We also nail it with wrong pgtable setup.
> +	 */
> +	WARN_ON_ONCE(wp && pte_write(pte));

How about VM_WARN_ON_ONCE() and no ifdef?