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t=1783436753; bh=U6SbfFdui2/7cITPPGTWciEVmjsDv8a6ho9k9ea7zPU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=dGFTG7RwnKilQwvtW8NwFRMNYwqKv8j9M25Kg36Pzeda4MfK9gjY9uXjS4G7/mR1k Lym4airyudQ85+awNxabpr9cLx+tMmiyA/rdVi+x8tUwXzuhVWrm9ExQDK7Vzng3BW 6fpGlC5jpZvhWpYfQ1+TAjjHMbO/9rHLPmF0guNxcC4xpm/IiCbHAKjm7wAxJkG1gK 4HJYeEPv+frXnk1rLOu8KzqI66WYPWJDIV0x3fXkZic/HzW+OgRlaj6B8i5AZQr0V5 n+7dBCmnHHwbqlKvSBDLoG1DeN7V0kc9Fq6WXeiVKawIqkJjEj0Kax7OGqJzkZE5Ft RXtz78VY4vuQQ== Message-ID: <678ecae4-cda7-4683-9012-ed7c5c5b879f@kernel.org> Date: Tue, 7 Jul 2026 17:05:49 +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 , akpm@linux-foundation.org Cc: 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, kas@kernel.org References: <20260703161833.57416-1-kirill@shutemov.name> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 8594418000F X-Stat-Signature: 3gcc3m6tg8s3ja9x1cz573gtn9myqe1c X-HE-Tag: 1783436754-836398 X-HE-Meta: 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 BDeSGLFr oQSk5SQK4jNZ1uHD5EPDP2hE/ulYzTefZ4TesjCVqq+rrnVXINtQNOUcO1T1mD8Uv+xktn4qIoX8+1ZTAmDhZGbKYsJioOy/ztL4lwTLIJAaHgZb7riDWntiNqve+7pnR/IL6KAaLyhxaTq/J6xGPFwFgHqx9y3HS9eeIxrCXnw3O5kknmm4JBBmoeWCFzHy5BgndvdSNyxi4pK38Ys7/V0HbLygxBM5Zz+RDXXmLKW+B2T/w+cTSnYKlYQ0PCMPeafLNMEv6imUUZsDaJXjFWzcv/w== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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. > set_huge_pte_at(dst, addr, dst_pte, entry, sz); > } else if (unlikely(softleaf_is_migration(softleaf))) { > bool uffd_wp = pte_swp_uffd_wp(entry); > @@ -4936,7 +4936,7 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, > set_huge_pte_at(src, addr, src_pte, entry, sz); > } > if (!userfaultfd_wp(dst_vma)) > - entry = huge_pte_clear_uffd_wp(entry); > + entry = pte_swp_clear_uffd_wp(entry); That looks correct. -- Cheers, David