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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b="HL/XciLR"; 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 61FE2606A31A; Mon, 26 Oct 2026 12:20:31 +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=HL/XciLR Received: by smtp.kernel.org (Postfix) with ESMTPSA id 39F081F01825; Mon, 26 Oct 2026 12:20:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793017231; bh=M2b8Cpwjp/vQVDnm2g9Ld/DjAGfbM4iZVTCxpoq7yDU=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=HL/XciLRn2tWpwwLLrUJl0MVaUmqH+DsJ3qccRi03nf4Pd24xS17o7ct81jnxMoRa CGLYLax5txB8YcN4JV7kFYIuxLu+sZIOExdpF2HwQVkVfFghNtlZ2O30DnSu4B6tzW Hm0zPBxxdYYFg6aBgZtmpv/x0l4tRlKMinyFhc09gK94gf7GUar32ygwA0iuMI9ZPE nQIrerybF1GR8oFAwx7ioyXS+cXXiSxmeDP1x+/fkDjhSdEdk5aZkNThsFao+EGdI8 yftOFqss3J7a7J/zoSbg88XCRxZVWr86wk1BqOwud6QzcMw+O3lvE6KPvugPeSNj0m P+1pwxzfL9vgw== Message-ID: <78802ed9-9cc6-41c4-b131-7c7d38f7ff39@kernel.org> Date: Mon, 26 Oct 2026 13:20:25 +0100 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] RDMA/uverbs: Don't return the shared zero page from rdma_umap_fault() To: Zi Yan , Jason Gunthorpe , Leon Romanovsky Cc: linux-mm@kvack.org, Lorenzo Stoakes , Lance Yang , Kiryl Shutsemau , linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261025-fix-rdma_umap_fault-zero-page-v1-1-186255624f1f@nvidia.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 10/26/26 03:04, Zi Yan wrote: > After disassociation, rdma_umap_fault() returns the shared zero page from > ->fault for VMAs without VM_WRITE and VM_MAYWRITE. Such VMAs come from > rdma_user_mmap_io() in drivers that clear VM_MAYWRITE and they are shared > VM_PFNMAP. A page returned from ->fault is mapped as a normal page and goes > through rmap code. Two issues: > > 1. shared zero page causes VM_WARN_ONs in rmap code, > > 2. normal pages in shared VM_PFNMAP VMAs cause accounting leak on arch > without pte_special(), namely the page refcount, mapcount, and vm stats > are never dropped. > > Map ufile->disassociate_page for VMAs without VM_WRITE and VM_MAYWRITE, > with vmf_insert_pfn(). The page lives until ib_uverbs_release_file(), which > cannot run while a VMA holds a reference to the file. > > Writable VMAs keep returning disassociate_page in vmf->page, thus its > accounting leak on arch without pte_special() needs a separate fix. > > Userspace can see two differences after the patch. A mapping without > VM_WRITE and VM_MAYWRITE now reads what was written through writable > mappings of the same uverbs file instead of zeroes; after disassociation > both are dummy data. The first fault now allocates the page and fails with > VM_FAULT_SIGBUS if the allocation fails, as a write fault already does. > > Fixes: 67f269b37f9b ("RDMA/ucontext: Fix regression with disassociate") > Signed-off-by: Zi Yan > --- > The issue was discovered during PG_folio development when making sure only > rmappable folios can enter rmap code path. > > Alternative: map the zero page with vmf_insert_pfn_prot() > === > > Read-only mappings could keep reading zeroes if rdma_umap_fault() > installed the zero page itself as a special PTE: > > return vmf_insert_pfn_prot(vmf->vma, vmf->address, zero_pfn(vmf->address)); > > It does not work today. Commit 7b466529307be ("mm: disallow raw PFN > mappings of huge/shared zeropage") makes vmf_insert_pfn_prot() return > VM_FAULT_SIGBUS for the zero pfn. vmf_insert_mixed() does not support > VM_PFNMAP, since vm_mixed_zeropage_allowed() warns on VM_PFNMAP. > > The first version of that patch [1] did not change vmf_insert_pfn_prot(). > Due to no in-kernel user at that time and complication it can cause, > vmf_insert_pfn_prot() rejects zero pages now. > > Fix to writable VMA path > === > A new vmf_insert_pfn_mkwrite() is needed, since vmf_insert_pfn() has a > narrow failing scenario. When a process mapping the disassociated page has > soft-dirty tracking enabled, vm_page_prot becomes read-only; > vmf_insert_pfn() then uses a read-only PTE instead of a writable one; > fixup_user_fault() is used to write to the page (e.g., KVM with the uverbs > mapping as guest memory) and fails, because it does not take the second > fault to upgrade the read-only PTE to a writable one. > > LLM was used for investigation and code development. > > [1] https://lore.kernel.org/all/20260917121010.60966-1-lance.yang@linux.dev/ > --- > drivers/infiniband/core/uverbs_main.c | 44 ++++++++++++++++++++++------------- > 1 file changed, 28 insertions(+), 16 deletions(-) > > diff --git a/drivers/infiniband/core/uverbs_main.c b/drivers/infiniband/core/uverbs_main.c > index 2a046c888dfab..c3638bf031b15 100644 > --- a/drivers/infiniband/core/uverbs_main.c > +++ b/drivers/infiniband/core/uverbs_main.c > @@ -725,39 +725,51 @@ static void rdma_umap_close(struct vm_area_struct *vma) > > /* > * Once the zap_special_vma_range has been called touches to the VMA will come here and > - * we return a dummy writable zero page for all the pfns. > + * we map a dummy page for all the pfns. > */ > static vm_fault_t rdma_umap_fault(struct vm_fault *vmf) > { > struct ib_uverbs_file *ufile = vmf->vma->vm_file->private_data; > struct rdma_umap_priv *priv = vmf->vma->vm_private_data; > vm_fault_t ret = 0; > + struct page *page; > > if (!priv) > return VM_FAULT_SIGBUS; > > - /* Read only pages can just use the system zero page. */ > - if (!(vmf->vma->vm_flags & (VM_WRITE | VM_MAYWRITE))) { > - vmf->page = ZERO_PAGE(vmf->address); > - get_page(vmf->page); > - return 0; > - } > - > mutex_lock(&ufile->umap_lock); > if (!ufile->disassociate_page) > ufile->disassociate_page = > alloc_pages(vmf->gfp_mask | __GFP_ZERO, 0); > > - if (ufile->disassociate_page) { > - /* > - * This VMA is forced to always be shared so this doesn't have > - * to worry about COW. > - */ > - vmf->page = ufile->disassociate_page; > - get_page(vmf->page); > - } else { > + page = ufile->disassociate_page; > + if (!page) { > ret = VM_FAULT_SIGBUS; > + goto out; > + } > + > + /* Make sure the page is usable, like what __do_fault() does */ > + if (PageHWPoison(page)) { > + ret = VM_FAULT_HWPOISON; > + goto out; > } Is that really required? memory_failure() complains pretty hard on pages it doesn't know how to handle (random kernel memory). I'd assume that this here is not an ordinary pagecache page that mamoery_failure would know how to handle? > + > + /* > + * VMAs that cannot become writable map the page as a raw pfn, like > + * the device memory it replaces. The page lives as long as the file. > + */ > + if (!(vmf->vma->vm_flags & (VM_WRITE | VM_MAYWRITE))) { > + ret = vmf_insert_pfn(vmf->vma, vmf->address, page_to_pfn(page)); > + goto out; > + } > + > + /* > + * This VMA is forced to always be shared so this doesn't have > + * to worry about COW. > + */ > + vmf->page = page; > + get_page(page); This split handling is pretty horrible TBH. In once case we take a reference in the other not? Why not perform a PFN mapping in any case? (Does GUP have to work on these things?) -- Cheers, David