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

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Message-ID: <20201124164930.GK8537@kernel.org>
Date:   Tue, 24 Nov 2020 18:49:30 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
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>,
        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>,
        Matthew Wilcox <willy@...radead.org>,
        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>, 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>
Subject: Re: [PATCH v11 4/9] mm: introduce memfd_secret system call to create
 "secret" memory areas

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.