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

[Michael Roth]

On Tue, Oct 25, 2022 at 11:13:37PM +0800, Chao Peng 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);

The KVM restrictedmem ops seem to expect pgoff_t, but here we pass
loff_t. For SNP we've made this strange as part of the following patch
and it seems to produce the expected behavior:

  https://github.com/mdroth/linux/commit/d669c7d3003ff7a7a47e73e8c3b4eeadbd2c4eb6

> +	ret = memfd->f_op->fallocate(memfd, mode, offset, len);
> +	restrictedmem_notifier_invalidate(data, offset, offset + len, false);
> +	return ret;
> +}
> +

<snip>

> +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);

This will result in KVM allocating pages that userspace hasn't necessary
fallocate()'d. In the case of SNP we need to get the PFN so we can clean
up the RMP entries when restrictedmem invalidations are issued for a GFN
range.

If the guest supports lazy-acceptance however, these pages may not have
been faulted in yet, and if the VMM defers actually fallocate()'ing space
until the guest actually tries to issue a shared->private for that GFN
(to support lazy-pinning), then there may never be a need to allocate
pages for these backends.

However, the restrictedmem invalidations are for GFN ranges so there's
no way to know inadvance whether it's been allocated yet or not. The
xarray is one option but currently it defaults to 'private' so that
doesn't help us here. It might if we introduced a 'uninitialized' state
or something along that line instead of just the binary
'shared'/'private' though...

But for now we added a restrictedmem_get_page_noalloc() that uses
SGP_NONE instead of SGP_WRITE to avoid accidentally allocating a bunch
of memory as part of guest shutdown, and a
kvm_restrictedmem_get_pfn_noalloc() variant to go along with that. But
maybe a boolean param is better? Or maybe SGP_NOALLOC is the better
default, and we just propagate an error to userspace if they didn't
fallocate() in advance?

-Mike

> +	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
>