linux-kernel - Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO

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Message-ID: <YtlrJR3uP6940tjd@google.com>
Date:   Thu, 21 Jul 2022 15:05:09 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        linux-kselftest@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, aarcange@...hat.com, ddutile@...hat.com,
        dhildenb@...hat.com, Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

On Thu, Jul 21, 2022, David Hildenbrand wrote:
> On 21.07.22 11:44, David Hildenbrand wrote:
> > On 06.07.22 10:20, Chao Peng wrote:
> >> Normally, a write to unallocated space of a file or the hole of a sparse
> >> file automatically causes space allocation, for memfd, this equals to
> >> memory allocation. This new seal prevents such automatically allocating,
> >> either this is from a direct write() or a write on the previously
> >> mmap-ed area. The seal does not prevent fallocate() so an explicit
> >> fallocate() can still cause allocating and can be used to reserve
> >> memory.
> >>
> >> This is used to prevent unintentional allocation from userspace on a
> >> stray or careless write and any intentional allocation should use an
> >> explicit fallocate(). One of the main usecases is to avoid memory double
> >> allocation for confidential computing usage where we use two memfds to
> >> back guest memory and at a single point only one memfd is alive and we
> >> want to prevent memory allocation for the other memfd which may have
> >> been mmap-ed previously. More discussion can be found at:
> >>
> >>   https://lkml.org/lkml/2022/6/14/1255
> >>
> >> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> >> Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> >> ---
> >>  include/uapi/linux/fcntl.h |  1 +
> >>  mm/memfd.c                 |  3 ++-
> >>  mm/shmem.c                 | 16 ++++++++++++++--
> >>  3 files changed, 17 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> >> index 2f86b2ad6d7e..98bdabc8e309 100644
> >> --- a/include/uapi/linux/fcntl.h
> >> +++ b/include/uapi/linux/fcntl.h
> >> @@ -43,6 +43,7 @@
> >>  #define F_SEAL_GROW	0x0004	/* prevent file from growing */
> >>  #define F_SEAL_WRITE	0x0008	/* prevent writes */
> >>  #define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */
> >> +#define F_SEAL_AUTO_ALLOCATE	0x0020  /* prevent allocation for writes */
> > 
> > Why only "on writes" and not "on reads". IIRC, shmem doesn't support the
> > shared zeropage, so you'll simply allocate a new page via read() or on
> > read faults.
> 
> Correction: on read() we don't allocate a fresh page. But on read faults
> we would. So this comment here needs clarification.

Not just the comment, the code too.  The intent of F_SEAL_AUTO_ALLOCATE is very
much to block _all_ implicit allocations (or maybe just fault-based allocations
if "implicit" is too broad of a description).