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

[Fuad Tabba]

Hi Chao,

On Wed, Jul 6, 2022 at 9:25 AM Chao Peng <chao.p.peng@linux.intel.com> 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 */

I think this should also be added to tools/include/uapi/linux/fcntl.h

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
>