Re: [PATCH v11 4/9] mm: introduce memfd_secret system call to create "secret" memory areas

From: Mike Rapoport <rppt@kernel.org>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	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>,
	David Hildenbrand <david@redhat.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	James Bottomley <jejb@linux.ibm.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Wilcox <willy@infradead.org>,
	Mark Rutland <mark.rutland@arm.com>,
	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>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Roman Gushchin <guro@fb.com>, Shuah Khan <shuah@kernel.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-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
	x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>
Subject: Re: [PATCH v11 4/9] mm: introduce memfd_secret system call to create "secret" memory areas
Date: Tue, 24 Nov 2020 18:49:30 +0200	[thread overview]
Message-ID: <20201124164930.GK8537@kernel.org> (raw)
In-Reply-To: <20201124105947.GA5527@gaia>

On Tue, Nov 24, 2020 at 10:59:48AM +0000, Catalin Marinas wrote:
> Hi Mike,
> 
> On Tue, Nov 24, 2020 at 11:25:51AM +0200, Mike Rapoport wrote:
> > +static vm_fault_t secretmem_fault(struct vm_fault *vmf)
> > +{
> > +	struct address_space *mapping = vmf->vma->vm_file->f_mapping;

...

> > +
> > +		err = set_direct_map_invalid_noflush(page, 1);
> > +		if (err)
> > +			goto err_del_page_cache;
> 
> On arm64, set_direct_map_default_noflush() returns 0 if !rodata_full but
> no pgtable changes happen since the linear map can be a mix of small and
> huge pages. The arm64 implementation doesn't break large mappings. I
> presume we don't want to tell the user that the designated memory is
> "secret" but the kernel silently ignored it.
> 
> We could change the arm64 set_direct_map* to return an error, however, I
> think it would be pretty unexpected for the user to get a fault when
> trying to access it. It may be better to return a -ENOSYS or something
> on the actual syscall if the fault-in wouldn't be allowed later.
> 
> Alternatively, we could make the linear map always use pages on arm64,
> irrespective of other config or cmdline options (maybe not justified
> unless we have clear memsecret users). Yet another idea is to get
> set_direct_map* to break pmd/pud mappings into pte but that's not always
> possible without a stop_machine() and potentially disabling the MMU.

My preference would be to check at secretmem initialization if
set_direct_map_*() actually do anything and then return an error from
the syscall if they are essentially nop.

I'll update the patches with something like this in v12.

> -- 
> Catalin

-- 
Sincerely yours,
Mike.