From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=91.218.175.232 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793019670; b=lpMyDYU2X6yGnwt9RSOjJR72hpWyMewpSzXve+jtts1lbCMMlgJ6uv0tl7DefBVqkRe+ 5RnJd7qB4PG3Skly0IDWH6D6w499+Y2uSnEsSWgUAONWgikRB5iibFXJ+Yfb8eARMFfLS dOMc+KDerABMCdRc60lKef0iAwZrMxmOsY4ZHsCTTZGAyOdJRypLWGYopeDPu+dgvMPIQ 2E0IAUAAVsqBcydBJUXKLZvseqyUjDzNQHQimoxapAGdgfxKxAaeHIV1rC+uBnvbbQbSN SM07eHw8sxkppxEnVppX9NkMo5tnfnLfRPVyryn+q9EJJbgpMz14Fiqp5msauGzf8sA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793019670; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:From:To:Cc:Subject:Date:Message-ID: X-Mailer:MIME-Version:Content-Transfer-Encoding; bh=mXgMFETLS+maZru+AaxXX7HtH++GszgJXY1ChWAfpak=; b=gee7TDceZGYpu2pRw3QjfdJWdIh/00rqTr4mxtIXG+PMiW9kRUiVexBor6GbdYXwfc3z T+l751YRlFQLcdcrhsCmkqqf30aoYMdwXstDdzfEdMV4Z+Cowh2XCbfZcQ9zIAw497TSp xFZwTOcVyhohvGtbkTwUvZN5mUEH7c3Tw2WM1XVd1gCB0YurZ5GYRFATcfroQsWh5NyD3 0NYwXXgix0ud93uWB27k0PJAcL9Aq/TOgAVZHDmEdQtYOa0/FIAJ1EaZuMFdOlM0QbEqc UiEIULOauN8ECWz5tBMQJRwgPBbCgaGQpenr9nezh690xBLMLhzdMiMjzC+pftC3DNw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=eOS7lClG; arc=none smtp.remote-ip=91.218.175.232 Received: from mta0.migadu.com (out-232.mta0.migadu.com [91.218.175.232]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 40051606A324 for ; Mon, 26 Oct 2026 13:01:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 40051606A324 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux.dev header.i=@linux.dev header.a=rsa-sha256 header.s=key1 header.b=eOS7lClG X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=B6SFPPfTE9xynzH7YlPHvBs9zmRqMXuyeuvm0EVIU4Y=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793019666; v=1; x=1793624466; b=eOS7lClGi/NgQp0DtV9ezX0Fjrcv+6UoB7uzRiTJeRH14SUuo4u4a9RC5pVgPSjcbTtW8SC6 yUNkIsOLnD7KBu6erCpsdvH9d1JLKpJTHFkt+SHnavlqYfecG2mlwCMZj659jXq4LPBi6bYD1UF b3aveIxdgeuLmwvWytaWVTW0= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id f415b154d1059a70; 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-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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