Re: [PATCH v4 02/12] mm/memfd: Introduce MFD_INACCESSIBLE flag

[Steven Price]

On 18/01/2022 13:21, Chao Peng wrote:
> Introduce a new memfd_create() flag indicating the content of the
> created memfd is inaccessible from userspace. It does this by force
> setting F_SEAL_INACCESSIBLE seal when the file is created. It also set
> F_SEAL_SEAL to prevent future sealing, which means, it can not coexist
> with MFD_ALLOW_SEALING.
> 
> The pages backed by such memfd will be used as guest private memory in
> confidential computing environments such as Intel TDX/AMD SEV. Since
> page migration/swapping is not yet supported for such usages so these
> pages are currently marked as UNMOVABLE and UNEVICTABLE which makes
> them behave like long-term pinned pages.
> 
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/uapi/linux/memfd.h |  1 +
>  mm/memfd.c                 | 20 +++++++++++++++++++-
>  2 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/memfd.h b/include/uapi/linux/memfd.h
> index 7a8a26751c23..48750474b904 100644
> --- a/include/uapi/linux/memfd.h
> +++ b/include/uapi/linux/memfd.h
> @@ -8,6 +8,7 @@
>  #define MFD_CLOEXEC          0x0001U
>  #define MFD_ALLOW_SEALING    0x0002U
>  #define MFD_HUGETLB          0x0004U
> +#define MFD_INACCESSIBLE     0x0008U
>  
>  /*
>   * Huge page size encoding when MFD_HUGETLB is specified, and a huge page
> diff --git a/mm/memfd.c b/mm/memfd.c
> index 9f80f162791a..26998d96dc11 100644
> --- a/mm/memfd.c
> +++ b/mm/memfd.c
> @@ -245,16 +245,19 @@ long memfd_fcntl(struct file *file, unsigned int cmd, 
> unsigned long arg)
>  #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
>  #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
>  
> -#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
> +#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | \
> +                    MFD_INACCESSIBLE)
>  
>  SYSCALL_DEFINE2(memfd_create,
>               const char __user *, uname,
>               unsigned int, flags)
>  {
> +     struct address_space *mapping;
>       unsigned int *file_seals;
>       struct file *file;
>       int fd, error;
>       char *name;
> +     gfp_t gfp;
>       long len;
>  
>       if (!(flags & MFD_HUGETLB)) {
> @@ -267,6 +270,10 @@ SYSCALL_DEFINE2(memfd_create,
>                       return -EINVAL;
>       }
>  
> +     /* Disallow sealing when MFD_INACCESSIBLE is set. */
> +     if (flags & MFD_INACCESSIBLE && flags & MFD_ALLOW_SEALING)
> +             return -EINVAL;
> +
>       /* length includes terminating zero */
>       len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1);
>       if (len <= 0)
> @@ -315,6 +322,17 @@ SYSCALL_DEFINE2(memfd_create,
>               *file_seals &= ~F_SEAL_SEAL;
>       }
>  
> +     if (flags & MFD_INACCESSIBLE) {
> +             mapping = file_inode(file)->i_mapping;
> +             gfp = mapping_gfp_mask(mapping);
> +             gfp &= ~__GFP_MOVABLE;
> +             mapping_set_gfp_mask(mapping, gfp);
> +             mapping_set_unevictable(mapping);
> +
> +             file_seals = memfd_file_seals_ptr(file);
> +             *file_seals &= F_SEAL_SEAL | F_SEAL_INACCESSIBLE;

This looks backwards - the flags should be set on *file_seals, but here
you are unsetting all other flags.

Steve

> +     }
> +
>       fd_install(fd, file);
>       kfree(name);
>       return fd;
>