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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-6d88125a191sm67790396d6.114.2024.12.04.08.13.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 04 Dec 2024 08:13:08 -0800 (PST) Date: Wed, 4 Dec 2024 11:13:06 -0500 From: Peter Xu To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Muchun Song , Andrew Morton , Guillaume Morin Subject: Re: [PATCH v1] mm/hugetlb: don't map folios writable without VM_WRITE when copying during fork() Message-ID: References: <20241204153100.1967364-1-david@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20241204153100.1967364-1-david@redhat.com> On Wed, Dec 04, 2024 at 04:31:00PM +0100, David Hildenbrand wrote: > If we have to trigger a hugetlb folio copy during fork() because the anon > folio might be pinned, we currently unconditionally create a writable > PTE. > > However, the VMA might not have write permissions (VM_WRITE) at that > point. > > Fix it by checking the VMA for VM_WRITE. Make the code less error prone > by moving checking for VM_WRITE into make_huge_pte(), and letting > callers only specify whether we should try making it writable. > > A simple reproducer that longter-pins the folios using liburing to then > mprotect(PROT_READ) the folios befor fork() [1] results in: > > Before: > [FAIL] access should not have worked > > After: > [PASS] access did not work as expected > > [1] https://gitlab.com/davidhildenbrand/scratchspace/-/raw/main/reproducers/hugetlb-mkwrite-fork.c > > This is rather a corner case, so stable might not be warranted. > > Fixes: 4eae4efa2c29 ("hugetlb: do early cow when page pinned on src mm") > Cc: Muchun Song > Cc: Andrew Morton > Cc: Peter Xu > Cc: Guillaume Morin > Signed-off-by: David Hildenbrand Acked-by: Peter Xu Thanks, -- Peter Xu