On Mon, Nov 28, 2022 at 4:37 PM Michael Roth <michael.roth@xxxxxxx> wrote: > > 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? > One caveat with SGP_NOALLOC being default: For performance reasons (to avoid frequent userspace exits), VMM will have to always preallocate all the guest restricted memory. In general this will prevent VMM from overcommitting. > -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 > >