From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Rapoport <rppt@kernel.org>,
linux-kernel@vger.kernel.org,
Alexander Viro <viro@zeniv.linux.org.uk>,
Andrew Morton <akpm@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
Borislav Petkov <bp@alien8.de>,
Christopher Lameter <cl@linux.com>,
Dan Williams <dan.j.williams@intel.com>,
Dave Hansen <dave.hansen@linux.intel.com>,
Elena Reshetova <elena.reshetova@intel.com>,
"H. Peter Anvin" <hpa@zytor.com>, Idan Yaniv <idan.yaniv@ibm.com>,
Ingo Molnar <mingo@redhat.com>,
James Bottomley <jejb@linux.ibm.com>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Matthew Wilcox <willy@infradead.org>,
Mike Rapoport <rppt@linux.ibm.com>,
Michael Kerrisk <mtk.manpages@gmail.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Paul Walmsley <paul.walmsley@sifive.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
x86@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
next prev parent reply other threads:[~2020-07-31 16:22 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-27 16:29 [PATCH v2 0/7] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2020-07-27 16:29 ` [PATCH v2 1/7] mm: add definition of PMD_PAGE_ORDER Mike Rapoport
2020-07-27 16:29 ` [PATCH v2 2/7] mmap: make mlock_future_check() global Mike Rapoport
2020-07-27 16:29 ` [PATCH v2 3/7] mm: introduce memfd_secret system call to create "secret" memory areas Mike Rapoport
2020-07-30 16:22 ` Catalin Marinas
2020-07-30 20:44 ` Mike Rapoport
2020-07-31 14:10 ` Catalin Marinas
2020-07-31 14:29 ` Mark Rutland
2020-07-31 16:22 ` Catalin Marinas [this message]
2020-07-27 16:29 ` [PATCH v2 4/7] arch, mm: wire up memfd_secret system call were relevant Mike Rapoport
2020-07-27 17:25 ` Arnd Bergmann
2020-07-27 16:29 ` [PATCH v2 5/7] mm: secretmem: use PMD-size pages to amortize direct map fragmentation Mike Rapoport
2020-07-27 16:29 ` [PATCH v2 6/7] mm: secretmem: add ability to reserve memory at boot Mike Rapoport
2020-07-27 16:29 ` [PATCH v2 7/7] " Mike Rapoport
2020-07-27 17:11 ` Mike Rapoport
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