On 1/18/22 14:21, 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.
Shouldn't the amount of such memory allocations be restricted? E.g. similar
to secretmem_mmap() doing mlock_future_check().
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
> 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;
> + }
> +
> fd_install(fd, file);
> kfree(name);
> return fd;
