From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793024848; b=rKNNmv6/MOtaPpaQa2LOQ0WeLFev+S6dE0ly/kpIUU+B0cNyWQHKttMIeDtSn8hxXhSH xCqmdJsaaqzqmq40StyrhwKtiblC9SsHUjoKGjTXX66dFMEXV7CetR06mfLT6lx/Xy5iW L1gb9eodbQ3RA2C4xeK4xvpCOrjxqtkGM1QNXVqDxApeNtremadjb4SXoAujulrZLlNjW 6tfd+oziUnTwLg2VUe4iSsGhYE5xGS8S5I5vUMmwFHNE3w8HZxIRTQ9FVwc4oxNXOiyJX GnPOQ9NdRmeMJadJylhgab/PGdPaRffT3m7W62caKnZbY9OsKHCD7VL5rGX6GBHbHrA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793024848; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=6gmCDvdn2nsesgNhAh+epn0sztNU7HD2eJkAnfX6dkM=; b=sE8nHaHuViYVYe+hFVJg0INVk9dSqgyFt9KWfRry9PKqtnSljDH7gbFeZzUZpZ8AGDHF Co+kTt9pTjyCz1r5aO7yXarRbKmKmUGr9B90X23ED53vGQoqEHEySOSMOyT1P29oPyfdp 1Wq8Q3SJQImoJ87hs1PI1aq7aW9bLiy6Re0t9J/eJBV+q/YAbNgUvZLS//0JuxMC4bCWw g2NxenrhDtjKnu1UNSbKYQLwm6lnVRF4bqEPIek21P+w1C+lhUvIyXPilSxIolN1yYZ2D 4L0Xqeo633DnlWbFERXK/lcPz0SsDQGO9+GbAHbrZvoWIVu9YZ6VtUMaZUbkInu/5Xg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="IxP+N3i/"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F169B606A31A; Mon, 26 Oct 2026 14:27:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=IxP+N3i/ Received: by smtp.kernel.org (Postfix) with ESMTPSA id 15B491F01825; Mon, 26 Oct 2026 14:27:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793024847; bh=6gmCDvdn2nsesgNhAh+epn0sztNU7HD2eJkAnfX6dkM=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=IxP+N3i/n2LONGGX+afxhWlsWhOTpuuACZjEFunZuKXhHM/2qN4hqqU64faaUXtI2 b92CvQVadIqScamGEi3o9eCIrTfxGrFzZlCrrDKgiAtx3T5/tzM1rR5eMIeW2po5Pa urKkl72EYR0eQoR0RdsG8tBvJ4Ol1kKz+lj5Dtmb6mncXA0dTNhWibCCxMDWCYHhEm JjcPAXaw3eOLZGJ+tQaXLV+RTDRlHI3bdmuftngAJbZneOyCTJ7Qf7FQ5aTrFkvkd/ ctTKXd5FH7EQpXUMrcra1UQ+oWmBwBaVAaQqPbqfNg2IdzzJuTG8DG5krtiJ1R7e/r RmLfgQDjpJKJg== Date: Mon, 26 Oct 2026 14:27:16 +0000 From: "Lorenzo Stoakes (ARM)" To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, xu.xin@linux.dev, chengming.zhou@linux.dev, 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 Subject: Re: [PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge Message-ID: References: <20261026130054.91461-1-lance.yang@linux.dev> 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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261026130054.91461-1-lance.yang@linux.dev> On Mon, Oct 26, 2026 at 09:00:54PM +0800, Lance Yang wrote: > 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. I feel we probably shouldn't have allowed an extension to UFFD while it's still fundamentally a broken feature :)) Which is evidenced by the SLEW of AI bug reports, and human-reported bug reports, and endless history of bug reports and it infecting the rest of mm, etc. etc. /rant > > 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 Looks sensible to me so: Acked-by: Lorenzo Stoakes (ARM) > --- > 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); OK, so by definition, RWP pages are protnone, and this resets it correctly. > + } > > 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); > + } Could this be abstracted? You're doing the same pattern here, which also exists in copy_present_page() and presumably many other places. But maybe that's one for a follow up? > } 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 > -- Cheers, Lorenzo