Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

From: Vishal Annapurve
Date: Thu Dec 01 2022 - 21:17:09 EST

On Tue, Oct 25, 2022 at 8:18 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote:
>
> From: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
>
> Introduce 'memfd_restricted' system call with the ability to create
> memory areas that are restricted from userspace access through ordinary
> MMU operations (e.g. read/write/mmap). The memory content is expected to
> be used through a new in-kernel interface by a third kernel module.
>
> memfd_restricted() is useful for scenarios where a file descriptor(fd)
> can be used as an interface into mm but want to restrict userspace's
> ability on the fd. Initially it is designed to provide protections for
> KVM encrypted guest memory.
>
> Normally KVM uses memfd memory via mmapping the memfd into KVM userspace
> (e.g. QEMU) and then using the mmaped virtual address to setup the
> mapping in the KVM secondary page table (e.g. EPT). With confidential
> computing technologies like Intel TDX, the memfd memory may be encrypted
> with special key for special software domain (e.g. KVM guest) and is not
> expected to be directly accessed by userspace. Precisely, userspace
> access to such encrypted memory may lead to host crash so should be
> prevented.
>
> memfd_restricted() provides semantics required for KVM guest encrypted
> memory support that a fd created with memfd_restricted() is going to be
> used as the source of guest memory in confidential computing environment
> and KVM can directly interact with core-mm without the need to expose
> the memoy content into KVM userspace.
>
> KVM userspace is still in charge of the lifecycle of the fd. It should
> pass the created fd to KVM. KVM uses the new restrictedmem_get_page() to
> obtain the physical memory page and then uses it to populate the KVM
> secondary page table entries.
>
> The userspace restricted memfd can be fallocate-ed or hole-punched
> from userspace. When these operations happen, KVM can get notified
> through restrictedmem_notifier, it then gets chance to remove any
> mapped entries of the range in the secondary page tables.
>
> memfd_restricted() itself is implemented as a shim layer on top of real
> memory file systems (currently tmpfs). Pages in restrictedmem are marked
> as unmovable and unevictable, this is required for current confidential
> usage. But in future this might be changed.
>
> By default memfd_restricted() prevents userspace read, write and mmap.
> By defining new bit in the 'flags', it can be extended to support other
> restricted semantics in the future.
>
> The system call is currently wired up for x86 arch.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
> arch/x86/entry/syscalls/syscall_32.tbl | 1 +
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> include/linux/restrictedmem.h | 62 ++++++
> include/linux/syscalls.h | 1 +
> include/uapi/asm-generic/unistd.h | 5 +-
> include/uapi/linux/magic.h | 1 +
> kernel/sys_ni.c | 3 +
> mm/Kconfig | 4 +
> mm/Makefile | 1 +
> mm/restrictedmem.c | 250 +++++++++++++++++++++++++
> 10 files changed, 328 insertions(+), 1 deletion(-)
> create mode 100644 include/linux/restrictedmem.h
> create mode 100644 mm/restrictedmem.c
>
> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> index 320480a8db4f..dc70ba90247e 100644
> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> @@ -455,3 +455,4 @@
> 448 i386 process_mrelease sys_process_mrelease
> 449 i386 futex_waitv sys_futex_waitv
> 450 i386 set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 i386 memfd_restricted sys_memfd_restricted
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index c84d12608cd2..06516abc8318 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -372,6 +372,7 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common memfd_restricted sys_memfd_restricted
>
> #
> # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/include/linux/restrictedmem.h b/include/linux/restrictedmem.h
> new file mode 100644
> index 000000000000..9c37c3ea3180
> --- /dev/null
> +++ b/include/linux/restrictedmem.h
> @@ -0,0 +1,62 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +#ifndef _LINUX_RESTRICTEDMEM_H
> +
> +#include <linux/file.h>
> +#include <linux/magic.h>
> +#include <linux/pfn_t.h>
> +
> +struct restrictedmem_notifier;
> +
> +struct restrictedmem_notifier_ops {
> + void (*invalidate_start)(struct restrictedmem_notifier *notifier,
> + pgoff_t start, pgoff_t end);
> + void (*invalidate_end)(struct restrictedmem_notifier *notifier,
> + pgoff_t start, pgoff_t end);
> +};
> +
> +struct restrictedmem_notifier {
> + struct list_head list;
> + const struct restrictedmem_notifier_ops *ops;
> +};
> +
> +#ifdef CONFIG_RESTRICTEDMEM
> +
> +void restrictedmem_register_notifier(struct file *file,
> + struct restrictedmem_notifier *notifier);
> +void restrictedmem_unregister_notifier(struct file *file,
> + struct restrictedmem_notifier *notifier);
> +
> +int restrictedmem_get_page(struct file *file, pgoff_t offset,
> + struct page **pagep, int *order);
> +
> +static inline bool file_is_restrictedmem(struct file *file)
> +{
> + return file->f_inode->i_sb->s_magic == RESTRICTEDMEM_MAGIC;
> +}
> +
> +#else
> +
> +static inline void restrictedmem_register_notifier(struct file *file,
> + struct restrictedmem_notifier *notifier)
> +{
> +}
> +
> +static inline void restrictedmem_unregister_notifier(struct file *file,
> + struct restrictedmem_notifier *notifier)
> +{
> +}
> +
> +static inline int restrictedmem_get_page(struct file *file, pgoff_t offset,
> + struct page **pagep, int *order)
> +{
> + return -1;
> +}
> +
> +static inline bool file_is_restrictedmem(struct file *file)
> +{
> + return false;
> +}
> +
> +#endif /* CONFIG_RESTRICTEDMEM */
> +
> +#endif /* _LINUX_RESTRICTEDMEM_H */
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index a34b0f9a9972..f9e9e0c820c5 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1056,6 +1056,7 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> unsigned long home_node,
> unsigned long flags);
> +asmlinkage long sys_memfd_restricted(unsigned int flags);
>
> /*
> * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 45fa180cc56a..e93cd35e46d0 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
> #define __NR_set_mempolicy_home_node 450
> __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
>
> +#define __NR_memfd_restricted 451
> +__SYSCALL(__NR_memfd_restricted, sys_memfd_restricted)
> +
> #undef __NR_syscalls
> -#define __NR_syscalls 451
> +#define __NR_syscalls 452
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
> index 6325d1d0e90f..8aa38324b90a 100644
> --- a/include/uapi/linux/magic.h
> +++ b/include/uapi/linux/magic.h
> @@ -101,5 +101,6 @@
> #define DMA_BUF_MAGIC 0x444d4142 /* "DMAB" */
> #define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
> #define SECRETMEM_MAGIC 0x5345434d /* "SECM" */
> +#define RESTRICTEDMEM_MAGIC 0x5245534d /* "RESM" */
>
> #endif /* __LINUX_MAGIC_H__ */
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 860b2dcf3ac4..7c4a32cbd2e7 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -360,6 +360,9 @@ COND_SYSCALL(pkey_free);
> /* memfd_secret */
> COND_SYSCALL(memfd_secret);
>
> +/* memfd_restricted */
> +COND_SYSCALL(memfd_restricted);
> +
> /*
> * Architecture specific weak syscall entries.
> */
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 0331f1461f81..0177d53676c7 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -1076,6 +1076,10 @@ config IO_MAPPING
> config SECRETMEM
> def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
>
> +config RESTRICTEDMEM
> + bool
> + depends on TMPFS
> +
> config ANON_VMA_NAME
> bool "Anonymous VMA name support"
> depends on PROC_FS && ADVISE_SYSCALLS && MMU
> diff --git a/mm/Makefile b/mm/Makefile
> index 9a564f836403..6cb6403ffd40 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -117,6 +117,7 @@ obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
> obj-$(CONFIG_PAGE_TABLE_CHECK) += page_table_check.o
> obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
> obj-$(CONFIG_SECRETMEM) += secretmem.o
> +obj-$(CONFIG_RESTRICTEDMEM) += restrictedmem.o
> obj-$(CONFIG_CMA_SYSFS) += cma_sysfs.o
> obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
> obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
> diff --git a/mm/restrictedmem.c b/mm/restrictedmem.c
> new file mode 100644
> index 000000000000..e5bf8907e0f8
> --- /dev/null
> +++ b/mm/restrictedmem.c
> @@ -0,0 +1,250 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include "linux/sbitmap.h"
> +#include <linux/pagemap.h>
> +#include <linux/pseudo_fs.h>
> +#include <linux/shmem_fs.h>
> +#include <linux/syscalls.h>
> +#include <uapi/linux/falloc.h>
> +#include <uapi/linux/magic.h>
> +#include <linux/restrictedmem.h>
> +
> +struct restrictedmem_data {
> + struct mutex lock;
> + struct file *memfd;
> + struct list_head notifiers;
> +};
> +
> +static void restrictedmem_notifier_invalidate(struct restrictedmem_data *data,
> + pgoff_t start, pgoff_t end, bool notify_start)
> +{
> + struct restrictedmem_notifier *notifier;
> +
> + mutex_lock(&data->lock);
> + list_for_each_entry(notifier, &data->notifiers, list) {
> + if (notify_start)
> + notifier->ops->invalidate_start(notifier, start, end);
> + else
> + notifier->ops->invalidate_end(notifier, start, end);
> + }
> + mutex_unlock(&data->lock);
> +}
> +
> +static int restrictedmem_release(struct inode *inode, struct file *file)
> +{
> + struct restrictedmem_data *data = inode->i_mapping->private_data;
> +
> + fput(data->memfd);
> + kfree(data);
> + return 0;
> +}
> +
> +static long restrictedmem_fallocate(struct file *file, int mode,
> + loff_t offset, loff_t len)
> +{
> + struct restrictedmem_data *data = file->f_mapping->private_data;
> + struct file *memfd = data->memfd;
> + int ret;
> +
> + if (mode & FALLOC_FL_PUNCH_HOLE) {
> + if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> + return -EINVAL;
> + }
> +
> + restrictedmem_notifier_invalidate(data, offset, offset + len, true);
> + ret = memfd->f_op->fallocate(memfd, mode, offset, len);
> + restrictedmem_notifier_invalidate(data, offset, offset + len, false);
> + return ret;
> +}
> +
> +static const struct file_operations restrictedmem_fops = {
> + .release = restrictedmem_release,
> + .fallocate = restrictedmem_fallocate,
> +};
> +
> +static int restrictedmem_getattr(struct user_namespace *mnt_userns,
> + const struct path *path, struct kstat *stat,
> + u32 request_mask, unsigned int query_flags)
> +{
> + struct inode *inode = d_inode(path->dentry);
> + struct restrictedmem_data *data = inode->i_mapping->private_data;
> + struct file *memfd = data->memfd;
> +
> + return memfd->f_inode->i_op->getattr(mnt_userns, path, stat,
> + request_mask, query_flags);
> +}
> +
> +static int restrictedmem_setattr(struct user_namespace *mnt_userns,
> + struct dentry *dentry, struct iattr *attr)
> +{
> + struct inode *inode = d_inode(dentry);
> + struct restrictedmem_data *data = inode->i_mapping->private_data;
> + struct file *memfd = data->memfd;
> + int ret;
> +
> + if (attr->ia_valid & ATTR_SIZE) {
> + if (memfd->f_inode->i_size)
> + return -EPERM;
> +
> + if (!PAGE_ALIGNED(attr->ia_size))
> + return -EINVAL;
> + }
> +
> + ret = memfd->f_inode->i_op->setattr(mnt_userns,
> + file_dentry(memfd), attr);
> + return ret;
> +}
> +
> +static const struct inode_operations restrictedmem_iops = {
> + .getattr = restrictedmem_getattr,
> + .setattr = restrictedmem_setattr,
> +};
> +
> +static int restrictedmem_init_fs_context(struct fs_context *fc)
> +{
> + if (!init_pseudo(fc, RESTRICTEDMEM_MAGIC))
> + return -ENOMEM;
> +
> + fc->s_iflags |= SB_I_NOEXEC;
> + return 0;
> +}
> +
> +static struct file_system_type restrictedmem_fs = {
> + .owner = THIS_MODULE,
> + .name = "memfd:restrictedmem",
> + .init_fs_context = restrictedmem_init_fs_context,
> + .kill_sb = kill_anon_super,
> +};
> +
> +static struct vfsmount *restrictedmem_mnt;
> +
> +static __init int restrictedmem_init(void)
> +{
> + restrictedmem_mnt = kern_mount(&restrictedmem_fs);
> + if (IS_ERR(restrictedmem_mnt))
> + return PTR_ERR(restrictedmem_mnt);
> + return 0;
> +}
> +fs_initcall(restrictedmem_init);
> +
> +static struct file *restrictedmem_file_create(struct file *memfd)
> +{
> + struct restrictedmem_data *data;
> + struct address_space *mapping;
> + struct inode *inode;
> + struct file *file;
> +
> + data = kzalloc(sizeof(*data), GFP_KERNEL);
> + if (!data)
> + return ERR_PTR(-ENOMEM);
> +
> + data->memfd = memfd;
> + mutex_init(&data->lock);
> + INIT_LIST_HEAD(&data->notifiers);
> +
> + inode = alloc_anon_inode(restrictedmem_mnt->mnt_sb);
> + if (IS_ERR(inode)) {
> + kfree(data);
> + return ERR_CAST(inode);
> + }
> +
> + inode->i_mode |= S_IFREG;
> + inode->i_op = &restrictedmem_iops;
> + inode->i_mapping->private_data = data;
> +
> + file = alloc_file_pseudo(inode, restrictedmem_mnt,
> + "restrictedmem", O_RDWR,
> + &restrictedmem_fops);
> + if (IS_ERR(file)) {
> + iput(inode);
> + kfree(data);
> + return ERR_CAST(file);
> + }
> +
> + file->f_flags |= O_LARGEFILE;
> +
> + mapping = memfd->f_mapping;
> + mapping_set_unevictable(mapping);
> + mapping_set_gfp_mask(mapping,
> + mapping_gfp_mask(mapping) & ~__GFP_MOVABLE);
> +
> + return file;
> +}
> +
> +SYSCALL_DEFINE1(memfd_restricted, unsigned int, flags)
> +{

Looking at the underlying shmem implementation, there seems to be no
way to enable transparent huge pages specifically for restricted memfd
files.

Michael discussed earlier about tweaking
/sys/kernel/mm/transparent_hugepage/shmem_enabled setting to allow
hugepages to be used while backing restricted memfd. Such a change
will affect the rest of the shmem usecases as well. Even setting the
shmem_enabled policy to "advise" wouldn't help unless file based
advise for hugepage allocation is implemented.

Does it make sense to provide a flag here to allow creating restricted
memfds backed possibly by huge pages to give a more granular control?

> + struct file *file, *restricted_file;
> + int fd, err;
> +
> + if (flags)
> + return -EINVAL;
> +
> + fd = get_unused_fd_flags(0);
> + if (fd < 0)
> + return fd;
> +
> + file = shmem_file_setup("memfd:restrictedmem", 0, VM_NORESERVE);
> + if (IS_ERR(file)) {
> + err = PTR_ERR(file);
> + goto err_fd;
> + }
> + file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE;
> + file->f_flags |= O_LARGEFILE;
> +
> + restricted_file = restrictedmem_file_create(file);
> + if (IS_ERR(restricted_file)) {
> + err = PTR_ERR(restricted_file);
> + fput(file);
> + goto err_fd;
> + }
> +
> + fd_install(fd, restricted_file);
> + return fd;
> +err_fd:
> + put_unused_fd(fd);
> + return err;
> +}
> +
> +void restrictedmem_register_notifier(struct file *file,
> + struct restrictedmem_notifier *notifier)
> +{
> + struct restrictedmem_data *data = file->f_mapping->private_data;
> +
> + mutex_lock(&data->lock);
> + list_add(&notifier->list, &data->notifiers);
> + mutex_unlock(&data->lock);
> +}
> +EXPORT_SYMBOL_GPL(restrictedmem_register_notifier);
> +
> +void restrictedmem_unregister_notifier(struct file *file,
> + struct restrictedmem_notifier *notifier)
> +{
> + struct restrictedmem_data *data = file->f_mapping->private_data;
> +
> + mutex_lock(&data->lock);
> + list_del(&notifier->list);
> + mutex_unlock(&data->lock);
> +}
> +EXPORT_SYMBOL_GPL(restrictedmem_unregister_notifier);
> +
> +int restrictedmem_get_page(struct file *file, pgoff_t offset,
> + struct page **pagep, int *order)
> +{
> + struct restrictedmem_data *data = file->f_mapping->private_data;
> + struct file *memfd = data->memfd;
> + struct page *page;
> + int ret;
> +
> + ret = shmem_getpage(file_inode(memfd), offset, &page, SGP_WRITE);
> + if (ret)
> + return ret;
> +
> + *pagep = page;
> + if (order)
> + *order = thp_order(compound_head(page));
> +
> + SetPageUptodate(page);
> + unlock_page(page);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(restrictedmem_get_page);
> --
> 2.25.1
>