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

[Chao Peng]

On Fri, Aug 26, 2022 at 04:19:32PM +0100, Fuad Tabba wrote:
> Hi Chao,
> 
> On Wed, Jul 6, 2022 at 9:25 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> 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 */
> 
> I think this should also be added to tools/include/uapi/linux/fcntl.h

Yes, thanks.

Chao
> 
> Cheers,
> /fuad
> 
> 
> >  /* (1U << 31) is reserved for signed error codes */
> >
> >  /*
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 08f5f8304746..2afd898798e4 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> > @@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
> >                      F_SEAL_SHRINK | \
> >                      F_SEAL_GROW | \
> >                      F_SEAL_WRITE | \
> > -                    F_SEAL_FUTURE_WRITE)
> > +                    F_SEAL_FUTURE_WRITE | \
> > +                    F_SEAL_AUTO_ALLOCATE)
> >
> >  static int memfd_add_seals(struct file *file, unsigned int seals)
> >  {
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index a6f565308133..6c8aef15a17d 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2051,6 +2051,8 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
> >         struct vm_area_struct *vma = vmf->vma;
> >         struct inode *inode = file_inode(vma->vm_file);
> >         gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
> > +       struct shmem_inode_info *info = SHMEM_I(inode);
> > +       enum sgp_type sgp;
> >         int err;
> >         vm_fault_t ret = VM_FAULT_LOCKED;
> >
> > @@ -2113,7 +2115,12 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
> >                 spin_unlock(&inode->i_lock);
> >         }
> >
> > -       err = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, SGP_CACHE,
> > +       if (unlikely(info->seals & F_SEAL_AUTO_ALLOCATE))
> > +               sgp = SGP_NOALLOC;
> > +       else
> > +               sgp = SGP_CACHE;
> > +
> > +       err = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, sgp,
> >                                   gfp, vma, vmf, &ret);
> >         if (err)
> >                 return vmf_error(err);
> > @@ -2459,6 +2466,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> >         struct inode *inode = mapping->host;
> >         struct shmem_inode_info *info = SHMEM_I(inode);
> >         pgoff_t index = pos >> PAGE_SHIFT;
> > +       enum sgp_type sgp;
> >         int ret = 0;
> >
> >         /* i_rwsem is held by caller */
> > @@ -2470,7 +2478,11 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> >                         return -EPERM;
> >         }
> >
> > -       ret = shmem_getpage(inode, index, pagep, SGP_WRITE);
> > +       if (unlikely(info->seals & F_SEAL_AUTO_ALLOCATE))
> > +               sgp = SGP_NOALLOC;
> > +       else
> > +               sgp = SGP_WRITE;
> > +       ret = shmem_getpage(inode, index, pagep, sgp);
> >
> >         if (ret)
> >                 return ret;
> > --
> > 2.25.1
> >