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b=Pnl3raGOoJkCwbnzFm64lOMWPNyEnR+6u9+7fd989jtUVH/maOI9HvwzXCprAFwRd yj2xtf+KcrkVmrtUnSRrNgOANy/ipsDJGogspMH2MhUh12ZlWB4lWdCo9pX+nweAdq zs2xJSy2+eaiwb8qC8GMpJLuQIHKpAE4o4Q6Rqabc30+8O5qL6of8fUaQX6bH6XVbZ JY5W5kY9yUH9upf1RwbM2/HTnHc1lq7RTSqNmRCTAELq7TVY3KYYDn+3R50TZGi5YA SRrw+Bg7LkezQyWX75bJqhWDEw0EABo6/wu3+et1PoS/R3DZvJCuGArG/lCUN2pKk/ EwWpwozAAcx1w== Message-ID: Date: Tue, 7 Jul 2026 21:25:20 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/hugetlb: fix swap entry corruption when clearing uffd-wp at fork() To: Kiryl Shutsemau Cc: akpm@linux-foundation.org, muchun.song@linux.dev, osalvador@suse.de, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, kernel-team@meta.com References: <20260703161833.57416-1-kirill@shutemov.name> <678ecae4-cda7-4683-9012-ed7c5c5b879f@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 42F4F18000D X-Stat-Signature: 5ntkpxayhsozfxz9jbf5ysnah6jc6wx9 X-HE-Tag: 1783452325-68840 X-HE-Meta: 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 mFHVavUf zMe7tg8PBH7SaA6R2so4z53j4TshkzzYK+XHcfHc4J45MRYVvWDZBtib5KYAsyGqCd8aha6htPlh3NO4lxtpTWkUGHMisitCYTb/bGPhdwRgtnehK4oDDZzsM7Kt1V949ayMu8JXWnnz7saLQEz5IryQuZNU4VlzGPQLlQlJL6UPtKJ9gVy8ATjPOSohv0XoaScCaHoTIrtAPg+/Y6CEuEPhqqraECEonmWgW9FknkHgghT2yP6hjAx+Se59RBfrj20bPWkK6XSw5/0GgishcTgJUVA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/7/26 19:05, Kiryl Shutsemau wrote: > On Tue, Jul 07, 2026 at 05:05:49PM +0200, David Hildenbrand (Arm) wrote: >> On 7/3/26 18:18, Kiryl Shutsemau wrote: >>> From: "Kiryl Shutsemau (Meta)" >>> >>> copy_hugetlb_page_range() clears the uffd-wp bit of hwpoison and >>> migration entries with huge_pte_clear_uffd_wp(), which operates on the >>> present-PTE bit position. Swap entries keep the uffd-wp state elsewhere >>> -- the same branches read and set it with pte_swp_uffd_wp() and >>> pte_swp_mkuffd_wp() -- and the present-PTE position falls into the swap >>> payload. On x86-64 it lands in the inverted swap offset, where a >>> naturally-aligned hugetlb PFN always has the affected bit set, so the >>> clear advances the encoded PFN by two pages. >>> >>> No userfaultfd needs to be involved: the clear is guarded only by the >>> child VMA not being uffd-wp registered, so a plain fork() with an >>> in-flight hugetlb migration entry (or a poisoned hugetlb page) corrupts >>> the entry copied into the child. Instrumenting the hwpoison branch and >>> forking after MADV_HWPOISON on a 2MB anon hugetlb page shows: >>> >>> offset before=120e00 >>> offset after =120e02 >>> >>> The fallout is mostly latent: rmap walks match migration entries by >>> folio range and remove_migration_pte() rebuilds the PTE from the folio, >>> so a within-folio PFN skew heals once migration completes. But any path >>> that re-encodes the corrupted offset -- e.g. hugetlb_change_protection() >>> rewriting a writable migration entry via >>> make_readable_migration_entry(swp_offset(entry)) -- propagates it, and >>> an hwpoison entry misidentifies which page is poisoned. >>> >>> Use pte_swp_clear_uffd_wp(), matching copy_nonpresent_pte() and >>> move_huge_pte(). >>> >>> Reported-by: Sashiko AI review >>> Closes: https://lore.kernel.org/all/20260703140011.99E601F000E9@smtp.kernel.org/ >>> Fixes: bc70fbf269fd ("mm/hugetlb: handle uffd-wp during fork()") >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Kiryl Shutsemau >>> Assisted-by: Claude:claude-fable-5 >>> --- >>> mm/hugetlb.c | 4 ++-- >>> 1 file changed, 2 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c >>> index 571212b80835..a4e6dd3a82f4 100644 >>> --- a/mm/hugetlb.c >>> +++ b/mm/hugetlb.c >>> @@ -4918,7 +4918,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, >>> softleaf = softleaf_from_pte(entry); >>> if (unlikely(softleaf_is_hwpoison(softleaf))) { >>> if (!userfaultfd_wp(dst_vma)) >>> - entry = huge_pte_clear_uffd_wp(entry); >>> + entry = pte_swp_clear_uffd_wp(entry); >> >> I think installing a hwpoison pte will actually drop the uffd marker. >> >> hugetlb_change_protection() does nothing on hwpoison entrues. >> >> So how could be possibly get a hwpoison entry with an uffd-wp bit set here? >> >> If we indeed can't, Id assume there is nothing to clear here at all. > > You are right. > > I am inclined to remove it in a separate cleanup patch. Any objections? I'd just do it in this patch -- removing unnecessary code instead of fixing it. However, if there are strong opinions, I don't care that much. -- Cheers, David