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

[Sean Christopherson]

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@google.com>
> >> Signed-off-by: Chao Peng <chao.p.peng@linux.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).