linux-kernel - Re: [PATCH v15 06/11] mm: introduce memfd_secret system call to create "secret" memory areas

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Date:   Wed, 20 Jan 2021 20:35:04 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v15 06/11] mm: introduce memfd_secret system call to
 create "secret" memory areas

On Wed, Jan 20, 2021 at 08:06:07PM +0200, Mike Rapoport wrote:
> +static struct page *secretmem_alloc_page(gfp_t gfp)
> +{
> +	/*
> +	 * FIXME: use a cache of large pages to reduce the direct map
> +	 * fragmentation
> +	 */
> +	return alloc_page(gfp);
> +}
> +
> +static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> +{
> +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> +	struct inode *inode = file_inode(vmf->vma->vm_file);
> +	pgoff_t offset = vmf->pgoff;
> +	unsigned long addr;
> +	struct page *page;
> +	int err;
> +
> +	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
> +		return vmf_error(-EINVAL);
> +
> +retry:
> +	page = find_lock_page(mapping, offset);
> +	if (!page) {
> +		page = secretmem_alloc_page(vmf->gfp_mask);
> +		if (!page)
> +			return VM_FAULT_OOM;
> +
> +		err = set_direct_map_invalid_noflush(page, 1);
> +		if (err)
> +			return vmf_error(err);

Haven't we leaked the page at this point?

> +		__SetPageUptodate(page);
> +		err = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);

At this point, doesn't the page contain data from the last person to use
the page?  ie we've leaked data to this process?  I don't see anywhere
that we write data to the page.