Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

[Chao Peng]

On Thu, Jul 21, 2022 at 03:05:09PM +0000, Sean Christopherson wrote:
> 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@xxxxxxxxxx>
> > >> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > >> ---
> > >>  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).

So maybe still your initial suggestion F_SEAL_FAULT_ALLOCATIONS? One
reason I don't like it is the write() ioctl also cause allocation and we
want to prevent it.

Chao