Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create "secret" memory areas

From: Catalin Marinas <catalin.marinas@arm•com>
To: Mark Rutland <mark.rutland@arm•com>
Cc: Peter Zijlstra <peterz@infradead•org>,
	Dave Hansen <dave.hansen@linux•intel.com>,
	linux-mm@kvack•org, "H. Peter Anvin" <hpa@zytor•com>,
	Christopher Lameter <cl@linux•com>,
	Idan Yaniv <idan.yaniv@ibm•com>,
	Thomas Gleixner <tglx@linutronix•de>,
	Elena Reshetova <elena.reshetova@intel•com>,
	linux-arch@vger•kernel.org, Tycho Andersen <tycho@tycho•ws>,
	linux-nvdimm@lists•01.org, Will Deacon <will@kernel•org>,
	x86@kernel•org, Matthew Wilcox <willy@infradead•org>,
	Mike Rapoport <rppt@linux•ibm.com>,
	Ingo Molnar <mingo@redhat•com>,
	Michael Kerrisk <mtk.manpages@gmail•com>,
	Arnd Bergmann <arnd@arndb•de>,
	James Bottomley <jejb@linux•ibm.com>,
	Borislav Petkov <bp@alien8•de>,
	Alexander Viro <viro@zeniv•linux.org.uk>,
	Andy Lutomirski <luto@kernel•org>,
	Paul Walmsley <paul.walmsley@sifive•com>,
	"Kirill A. Shutemov" <kirill@shutemov•name>,
	Dan Williams <dan.j.williams@intel•com>,
	linux-arm-kernel@lists•infradead.org, linux-api@vger•kernel.org,
	linux-kernel@vger•kernel.org, linux-riscv@lists•infradead.org,
	Palmer Dabbelt <palmer@dabbelt•com>,
	linux-fsdevel@vger•kernel.org,
	Andrew Morton <akpm@linux-foundation•org>,
	Mike Rapoport <rppt@kernel•org>
Subject: Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Fri, 31 Jul 2020 17:22:34 +0100	[thread overview]
Message-ID: <20200731162234.GF29569@gaia> (raw)
In-Reply-To: <20200731142905.GA67415@C02TD0UTHF1T.local>

On Fri, Jul 31, 2020 at 03:29:05PM +0100, Mark Rutland wrote:
> On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote:
> > On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:
> > > +static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
> > > +{
> > > +	struct secretmem_ctx *ctx = file->private_data;
> > > +	unsigned long mode = ctx->mode;
> > > +	unsigned long len = vma->vm_end - vma->vm_start;
> > > +
> > > +	if (!mode)
> > > +		return -EINVAL;
> > > +
> > > +	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
> > > +		return -EINVAL;
> > > +
> > > +	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
> > > +		return -EAGAIN;
> > > +
> > > +	switch (mode) {
> > > +	case SECRETMEM_UNCACHED:
> > > +		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> > > +		fallthrough;
> > > +	case SECRETMEM_EXCLUSIVE:
> > > +		vma->vm_ops = &secretmem_vm_ops;
> > > +		break;
> > > +	default:
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	vma->vm_flags |= VM_LOCKED;
> > > +
> > > +	return 0;
> > > +}
> > 
> > I think the uncached mapping is not the right thing for arm/arm64. First
> > of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
> > semantics together with not allowing unaligned accesses. I suspect the
> > semantics are different on x86.
> 
> > The second, more serious problem, is that I can't find any place where
> > the caches are flushed for the page mapped on fault. When a page is
> > allocated, assuming GFP_ZERO, only the caches are guaranteed to be
> > zeroed. Exposing this subsequently to user space as uncached would allow
> > the user to read stale data prior to zeroing. The arm64
> > set_direct_map_default_noflush() doesn't do any cache maintenance.
> 
> It's also worth noting that in a virtual machine this is liable to be
> either broken (with a potential loss of coherency if the host has a
> cacheable alias as existing KVM hosts have), or pointless (if the host
> uses S2FWB to upgrade Stage-1 attribues to cacheable as existing KVM
> hosts also have).
> 
> I think that trying to avoid the data caches creates many more problems
> than it solves, and I don't think there's a strong justification for
> trying to support that on arm64 to begin with, so I'd rather entirely
> opt-out on supporting SECRETMEM_UNCACHED.

Good point, I forgot the virtualisation aspect. So unless there is a
hypervisor API to unmap it from the host memory, the uncached option
isn't of much use on arm64.

-- 
Catalin

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