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Mon, 26 Oct 2026 13:01:06 +0000 X-Mizu-Trace-ID: f415b154d1059a70 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: akpm@linux-foundation.org Cc: david@kernel.org, xu.xin@linux.dev, chengming.zhou@linux.dev, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jglisse@redhat.com, aarcange@redhat.com, peterx@redhat.com, kas@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, bobbypowers@gmail.com, bgeffon@google.com, dplotnikov@virtuozzo.com, dgilbert@redhat.com, hughd@google.com, hannes@cmpxchg.org, kirill@shutemov.name, cracauer@cons.org, mcfadden8@llnl.gov, gokhale2@llnl.gov, mgorman@suse.de, mike.kravetz@oracle.com, rppt@linux.vnet.ibm.com, xemul@openvz.org, riel@redhat.com, shli@fb.com, jthoughton@google.com, corbet@lwn.net, pbonzini@redhat.com, seanjc@google.com, sj@kernel.org, ziy@nvidia.com, stable@vger.kernel.org, samarth.colleges@gmail.com, med08elkadiri@gmail.com, Lance Yang Subject: [PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge Date: Mon, 26 Oct 2026 21:00:54 +0800 Message-ID: <20261026130054.91461-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 X-Archived-By: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 964DD1A0011 X-Rspam-User: X-Stat-Signature: hjzfwx11cnesuctkyu7phg56qbwho67p X-Spam-Status: No, score=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam03 X-HE-Tag: 1793019667-38278 X-HE-Meta: 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 raSMkdqU 6/NWqFtsmS7SqMH91Rn9McuBVJ73eqlthQUGTKj2x0yPtaxMtm5RAf4XdN5yZsc3kSuyBKrEr0cW1UUpKLILoJVfXKt13vAIKOjKQk5MxyAstcPGPN/nzDGCi+Ex8sNSK6ZFD9PJ9fGXlClhqzYEqbIxAGW0nV6j9T9v72mGiSzipEjFshwkLGfOrtAh96JDnOlEe4EYe9K4MRpeMSo9j/WheAME1acoRUimdgf+iLf9e3Q43KpBizim4a8C224IlPG9R0jn7dgsRsoTvRUVceluOGlBsMPpOmv+5Gouo9b10OmHrUZYG9Nj4flqZCl4mwflvSvovK06AjD6fEwoY5J9Hfl2U0rEsMm9U Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 KSM loses userfaultfd protection when replace_page() builds a new PTE from vma->vm_page_prot. Dropping its uffd bit lets a subsequent write go through ordinary COW instead of userfaultfd write protection handling. For RWP, a merged page also becomes readable without faulting. Keeping the uffd bit and restoring PAGE_NONE fixes merging, but we also have to handle unmerging. An existing KSM page can be protected with RWP before MADV_UNMERGEABLE, so unmerging already needs fixing independently of preserving RWP during merging. Unsharing replaces a shared page with a private one and should leave userfaultfd protection in place, as do_wp_page() already does for write protection. Currently, an unshare fault reaches do_uffd_rwp() before it can proceed to do_wp_page(). - With synchronous RWP, handle_userfault() returns VM_FAULT_SIGBUS. break_ksm() then reports success with a KSM page still merged. - With asynchronous RWP, protection is cleared before unsharing. Preserve the uffd bit when merging, and restore PAGE_NONE for RWP. Let FAULT_FLAG_UNSHARE skip RWP fault handling and keep PAGE_NONE on each private copy as well. Unsharing can then complete with RWP still armed for a subsequent access. Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl") Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP") Link: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/ Link: https://lore.kernel.org/all/20261020055518.36087-1-lance.yang@linux.dev/ Cc: stable@vger.kernel.org Co-developed-by: Samarth Narang Signed-off-by: Samarth Narang Signed-off-by: Lance Yang --- mm/ksm.c | 8 +++++++- mm/memory.c | 14 ++++++++++---- 2 files changed, 17 insertions(+), 5 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index 910a1307ac9f..d6f736a1aa2f 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -34,6 +34,7 @@ #include #include #include +#include #include #include #include @@ -1463,9 +1464,14 @@ static int replace_page(struct vm_area_struct *vma, struct page *page, dec_mm_counter(mm, MM_ANONPAGES); } - /* Preserve soft-dirty when replacing the page. */ + /* Preserve soft-dirty and uffd protection when replacing the page. */ if (pte_soft_dirty(orig_pte)) newpte = pte_mksoft_dirty(newpte); + if (pte_uffd(orig_pte)) { + newpte = pte_mkuffd(newpte); + if (userfaultfd_rwp(vma)) + newpte = pte_modify(newpte, PAGE_NONE); + } flush_cache_page(vma, addr, pte_pfn(ptep_get(ptep))); /* diff --git a/mm/memory.c b/mm/memory.c index 6a93b7fa48b2..99439a7afa63 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4122,8 +4122,11 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) if (unlikely(unshare)) { if (pte_soft_dirty(vmf->orig_pte)) entry = pte_mksoft_dirty(entry); - if (pte_uffd(vmf->orig_pte)) + if (pte_uffd(vmf->orig_pte)) { entry = pte_mkuffd(entry); + if (userfaultfd_rwp(vma)) + entry = pte_modify(entry, PAGE_NONE); + } } else { entry = maybe_mkwrite(pte_mkdirty(entry), vma); } @@ -6659,9 +6662,12 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf) * VM_UFFD_RWP VMA, a protnone PTE without the uffd bit is * NUMA hinting and must still fall through to do_numa_page(). */ - if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte)) - return do_uffd_rwp(vmf); - return do_numa_page(vmf); + if (userfaultfd_pte_rwp(vmf->vma, vmf->orig_pte)) { + if (!(vmf->flags & FAULT_FLAG_UNSHARE)) + return do_uffd_rwp(vmf); + } else { + return do_numa_page(vmf); + } } spin_lock(vmf->ptl); -- 2.49.0