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Date: Fri, 31 Jul 2020 17:22:34 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create
 "secret" memory areas
Message-ID: <20200731162234.GF29569@gaia>
References: <20200727162935.31714-1-rppt@kernel.org>
 <20200727162935.31714-4-rppt@kernel.org>
 <20200730162209.GB3128@gaia>
 <20200731142905.GA67415@C02TD0UTHF1T.local>
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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
_______________________________________________
Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org
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From mboxrd@z Thu Jan  1 00:00:00 1970
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Date:   Fri, 31 Jul 2020 17:22:34 +0100
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
Message-ID: <20200731162234.GF29569@gaia>
References: <20200727162935.31714-1-rppt@kernel.org>
 <20200727162935.31714-4-rppt@kernel.org>
 <20200730162209.GB3128@gaia>
 <20200731142905.GA67415@C02TD0UTHF1T.local>
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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

From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas-5wv7dgnIgG8@public.gmane.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
Message-ID: <20200731162234.GF29569@gaia>
References: <20200727162935.31714-1-rppt@kernel.org>
 <20200727162935.31714-4-rppt@kernel.org>
 <20200730162209.GB3128@gaia>
 <20200731142905.GA67415@C02TD0UTHF1T.local>
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To: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>
Cc: Mike Rapoport <rppt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Andy Lutomirski <luto-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>, Borislav Petkov <bp-Gina5bIWoIWzQB+pC5nmwQ@public.gmane.org>, Christopher Lameter <cl-vYTEC60ixJUAvxtiuMwx3w@public.gmane.org>, Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, Dave Hansen <dave.hansen-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>, Elena Reshetova <elena.reshetova-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>, "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>, Idan Yaniv <idan.yaniv-qyM6JfAXOaQ@public.gmane.org>, Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, James Bottomley <jejb-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>, "Kirill A. Shutemov" <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>, Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>, Mike Rapoport <rppt-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>, Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Palmer Dabbelt <palmer-96lFi9zoCfxBDgjK7y7TUQ@public.gmane.org>, Paul
List-Id: linux-arch.vger.kernel.org

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

From mboxrd@z Thu Jan  1 00:00:00 1970
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Date: Fri, 31 Jul 2020 17:22:34 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create
 "secret" memory areas
Message-ID: <20200731162234.GF29569@gaia>
References: <20200727162935.31714-1-rppt@kernel.org>
 <20200727162935.31714-4-rppt@kernel.org>
 <20200730162209.GB3128@gaia>
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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>,
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 linux-riscv@lists.infradead.org, Palmer Dabbelt <palmer@dabbelt.com>,
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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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Date: Fri, 31 Jul 2020 17:22:34 +0100
From: Catalin Marinas <catalin.marinas@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH v2 3/7] mm: introduce memfd_secret system call to create
 "secret" memory areas
Message-ID: <20200731162234.GF29569@gaia>
References: <20200727162935.31714-1-rppt@kernel.org>
 <20200727162935.31714-4-rppt@kernel.org>
 <20200730162209.GB3128@gaia>
 <20200731142905.GA67415@C02TD0UTHF1T.local>
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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>,
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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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