Skip to content


mm: introduce memfd_secret system call to create "secret" memory areas
Browse files Browse the repository at this point in the history
Introduce "memfd_secret" system call with the ability to create memory
areas visible only in the context of the owning process and not mapped not
only to other processes but in the kernel page tables as well.

The secretmem feature is off by default and the user must explicitly
enable it at the boot time.

Once secretmem is enabled, the user will be able to create a file
descriptor using the memfd_secret() system call.  The memory areas created
by mmap() calls from this file descriptor will be unmapped from the kernel
direct map and they will be only mapped in the page table of the processes
that have access to the file descriptor.

Secretmem is designed to provide the following protections:

* Enhanced protection (in conjunction with all the other in-kernel
  attack prevention systems) against ROP attacks.  Seceretmem makes
  "simple" ROP insufficient to perform exfiltration, which increases the
  required complexity of the attack.  Along with other protections like
  the kernel stack size limit and address space layout randomization which
  make finding gadgets is really hard, absence of any in-kernel primitive
  for accessing secret memory means the one gadget ROP attack can't work.
  Since the only way to access secret memory is to reconstruct the missing
  mapping entry, the attacker has to recover the physical page and insert
  a PTE pointing to it in the kernel and then retrieve the contents.  That
  takes at least three gadgets which is a level of difficulty beyond most
  standard attacks.

* Prevent cross-process secret userspace memory exposures.  Once the
  secret memory is allocated, the user can't accidentally pass it into the
  kernel to be transmitted somewhere.  The secreremem pages cannot be
  accessed via the direct map and they are disallowed in GUP.

* Harden against exploited kernel flaws.  In order to access secretmem,
  a kernel-side attack would need to either walk the page tables and
  create new ones, or spawn a new privileged uiserspace process to perform
  secrets exfiltration using ptrace.

The file descriptor based memory has several advantages over the
"traditional" mm interfaces, such as mlock(), mprotect(), madvise().  File
descriptor approach allows explicit and controlled sharing of the memory
areas, it allows to seal the operations.  Besides, file descriptor based
memory paves the way for VMMs to remove the secret memory range from the
userspace hipervisor process, for instance QEMU.  Andy Lutomirski says:

  "Getting fd-backed memory into a guest will take some possibly major
  work in the kernel, but getting vma-backed memory into a guest without
  mapping it in the host user address space seems much, much worse."

memfd_secret() is made a dedicated system call rather than an extension to
memfd_create() because it's purpose is to allow the user to create more
secure memory mappings rather than to simply allow file based access to
the memory.  Nowadays a new system call cost is negligible while it is way
simpler for userspace to deal with a clear-cut system calls than with a
multiplexer or an overloaded syscall.  Moreover, the initial
implementation of memfd_secret() is completely distinct from
memfd_create() so there is no much sense in overloading memfd_create() to
begin with.  If there will be a need for code sharing between these
implementation it can be easily achieved without a need to adjust user
visible APIs.

The secret memory remains accessible in the process context using uaccess
primitives, but it is not exposed to the kernel otherwise; secret memory
areas are removed from the direct map and functions in the
follow_page()/get_user_page() family will refuse to return a page that
belongs to the secret memory area.

Once there will be a use case that will require exposing secretmem to the
kernel it will be an opt-in request in the system call flags so that user
would have to decide what data can be exposed to the kernel.

Removing of the pages from the direct map may cause its fragmentation on
architectures that use large pages to map the physical memory which
affects the system performance.  However, the original Kconfig text for
CONFIG_DIRECT_GBPAGES said that gigabyte pages in the direct map "...  can
improve the kernel's performance a tiny bit ..." (commit 00d1c5e
("x86: add gbpages switches")) and the recent report [1] showed that "...
although 1G mappings are a good default choice, there is no compelling
evidence that it must be the only choice".  Hence, it is sufficient to
have secretmem disabled by default with the ability of a system
administrator to enable it at boot time.

Pages in the secretmem regions are unevictable and unmovable to avoid
accidental exposure of the sensitive data via swap or during page

Since the secretmem mappings are locked in memory they cannot exceed
RLIMIT_MEMLOCK.  Since these mappings are already locked independently
from mlock(), an attempt to mlock()/munlock() secretmem range would fail
and mlockall()/munlockall() will ignore secretmem mappings.

However, unlike mlock()ed memory, secretmem currently behaves more like
long-term GUP: secretmem mappings are unmovable mappings directly consumed
by user space.  With default limits, there is no excessive use of
secretmem and it poses no real problem in combination with
ZONE_MOVABLE/CMA, but in the future this should be addressed to allow
balanced use of large amounts of secretmem along with ZONE_MOVABLE/CMA.

A page that was a part of the secret memory area is cleared when it is
freed to ensure the data is not exposed to the next user of that page.

The following example demonstrates creation of a secret mapping (error
handling is omitted):

	fd = memfd_secret(0);
	ftruncate(fd, MAP_SIZE);
		   MAP_SHARED, fd, 0);


[ suppress Kconfig whine]

Signed-off-by: Mike Rapoport <>
Acked-by: Hagen Paul Pfeifer <>
Acked-by: James Bottomley <>
Cc: Alexander Viro <>
Cc: Andy Lutomirski <>
Cc: Arnd Bergmann <>
Cc: Borislav Petkov <>
Cc: Catalin Marinas <>
Cc: Christopher Lameter <>
Cc: Dan Williams <>
Cc: Dave Hansen <>
Cc: Elena Reshetova <>
Cc: "H. Peter Anvin" <>
Cc: Ingo Molnar <>
Cc: James Bottomley <>
Cc: "Kirill A. Shutemov" <>
Cc: Matthew Wilcox <>
Cc: Mark Rutland <>
Cc: Michael Kerrisk <>
Cc: Palmer Dabbelt <>
Cc: Palmer Dabbelt <>
Cc: Paul Walmsley <>
Cc: Peter Zijlstra <>
Cc: Rick Edgecombe <>
Cc: Roman Gushchin <>
Cc: Shakeel Butt <>
Cc: Shuah Khan <>
Cc: Thomas Gleixner <>
Cc: Tycho Andersen <>
Cc: Will Deacon <>
Cc: David Hildenbrand <>
Cc: kernel test robot <>
Signed-off-by: Andrew Morton <>
Signed-off-by: Linus Torvalds <>
  • Loading branch information
rppt authored and torvalds committed Jul 8, 2021
1 parent 6d47c23 commit 1507f51
Show file tree
Hide file tree
Showing 8 changed files with 309 additions and 1 deletion.
48 changes: 48 additions & 0 deletions include/linux/secretmem.h
Original file line number Diff line number Diff line change
@@ -0,0 +1,48 @@
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */


extern const struct address_space_operations secretmem_aops;

static inline bool page_is_secretmem(struct page *page)
struct address_space *mapping;

* Using page_mapping() is quite slow because of the actual call
* instruction and repeated compound_head(page) inside the
* page_mapping() function.
* We know that secretmem pages are not compound and LRU so we can
* save a couple of cycles here.
if (PageCompound(page) || !PageLRU(page))
return false;

mapping = (struct address_space *)
((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS);

if (mapping != page->mapping)
return false;

return mapping->a_ops == &secretmem_aops;

bool vma_is_secretmem(struct vm_area_struct *vma);


static inline bool vma_is_secretmem(struct vm_area_struct *vma)
return false;

static inline bool page_is_secretmem(struct page *page)
return false;


#endif /* _LINUX_SECRETMEM_H */
1 change: 1 addition & 0 deletions include/uapi/linux/magic.h
Original file line number Diff line number Diff line change
Expand Up @@ -97,5 +97,6 @@
#define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
#define Z3FOLD_MAGIC 0x33
#define PPC_CMM_MAGIC 0xc7571590
#define SECRETMEM_MAGIC 0x5345434d /* "SECM" */

#endif /* __LINUX_MAGIC_H__ */
2 changes: 2 additions & 0 deletions kernel/sys_ni.c
Original file line number Diff line number Diff line change
Expand Up @@ -358,6 +358,8 @@ COND_SYSCALL(pkey_mprotect);

/* memfd_secret */

* Architecture specific weak syscall entries.
Expand Down
4 changes: 4 additions & 0 deletions mm/Kconfig
Original file line number Diff line number Diff line change
Expand Up @@ -885,4 +885,8 @@ config KMAP_LOCAL
# struct io_mapping based helper. Selected by drivers that need them


1 change: 1 addition & 0 deletions mm/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -113,6 +113,7 @@ obj-$(CONFIG_CMA) += cma.o
obj-$(CONFIG_MEMORY_BALLOON) += balloon_compaction.o
obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
obj-$(CONFIG_SECRETMEM) += secretmem.o
obj-$(CONFIG_CMA_SYSFS) += cma_sysfs.o
obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
Expand Down
12 changes: 12 additions & 0 deletions mm/gup.c
Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@
#include <linux/rmap.h>
#include <linux/swap.h>
#include <linux/swapops.h>
#include <linux/secretmem.h>

#include <linux/sched/signal.h>
#include <linux/rwsem.h>
Expand Down Expand Up @@ -855,6 +856,9 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
struct follow_page_context ctx = { NULL };
struct page *page;

if (vma_is_secretmem(vma))
return NULL;

page = follow_page_mask(vma, address, foll_flags, &ctx);
if (ctx.pgmap)
Expand Down Expand Up @@ -988,6 +992,9 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))

if (vma_is_secretmem(vma))
return -EFAULT;

if (write) {
if (!(vm_flags & VM_WRITE)) {
if (!(gup_flags & FOLL_FORCE))
Expand Down Expand Up @@ -2170,6 +2177,11 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
if (!head)
goto pte_unmap;

if (unlikely(page_is_secretmem(page))) {
put_compound_head(head, 1, flags);
goto pte_unmap;

if (unlikely(pte_val(pte) != pte_val(*ptep))) {
put_compound_head(head, 1, flags);
goto pte_unmap;
Expand Down
3 changes: 2 additions & 1 deletion mm/mlock.c
Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,7 @@
#include <linux/hugetlb.h>
#include <linux/memcontrol.h>
#include <linux/mm_inline.h>
#include <linux/secretmem.h>

#include "internal.h"

Expand Down Expand Up @@ -503,7 +504,7 @@ static int mlock_fixup(struct vm_area_struct *vma, struct vm_area_struct **prev,

if (newflags == vma->vm_flags || (vma->vm_flags & VM_SPECIAL) ||
is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
vma_is_dax(vma) || vma_is_secretmem(vma))
/* don't set VM_LOCKED or VM_LOCKONFAULT and don't count */
goto out;

Expand Down
239 changes: 239 additions & 0 deletions mm/secretmem.c
Original file line number Diff line number Diff line change
@@ -0,0 +1,239 @@
// SPDX-License-Identifier: GPL-2.0
* Copyright IBM Corporation, 2021
* Author: Mike Rapoport <>

#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/swap.h>
#include <linux/mount.h>
#include <linux/memfd.h>
#include <linux/bitops.h>
#include <linux/printk.h>
#include <linux/pagemap.h>
#include <linux/syscalls.h>
#include <linux/pseudo_fs.h>
#include <linux/secretmem.h>
#include <linux/set_memory.h>
#include <linux/sched/signal.h>

#include <uapi/linux/magic.h>

#include <asm/tlbflush.h>

#include "internal.h"

#undef pr_fmt
#define pr_fmt(fmt) "secretmem: " fmt

* Define mode and flag masks to allow validation of the system call
* parameters.

static bool secretmem_enable __ro_after_init;
module_param_named(enable, secretmem_enable, bool, 0400);
"Enable secretmem and memfd_secret(2) system call");

static vm_fault_t secretmem_fault(struct vm_fault *vmf)
struct address_space *mapping = vmf->vma->vm_file->f_mapping;
struct inode *inode = file_inode(vmf->vma->vm_file);
pgoff_t offset = vmf->pgoff;
gfp_t gfp = vmf->gfp_mask;
unsigned long addr;
struct page *page;
int err;

if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
return vmf_error(-EINVAL);

page = find_lock_page(mapping, offset);
if (!page) {
page = alloc_page(gfp | __GFP_ZERO);
if (!page)
return VM_FAULT_OOM;

err = set_direct_map_invalid_noflush(page);
if (err) {
return vmf_error(err);

err = add_to_page_cache_lru(page, mapping, offset, gfp);
if (unlikely(err)) {
* If a split of large page was required, it
* already happened when we marked the page invalid
* which guarantees that this call won't fail
if (err == -EEXIST)
goto retry;

return vmf_error(err);

addr = (unsigned long)page_address(page);
flush_tlb_kernel_range(addr, addr + PAGE_SIZE);

vmf->page = page;

static const struct vm_operations_struct secretmem_vm_ops = {
.fault = secretmem_fault,

static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
unsigned long len = vma->vm_end - vma->vm_start;

if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
return -EINVAL;

if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
return -EAGAIN;

vma->vm_flags |= VM_LOCKED | VM_DONTDUMP;
vma->vm_ops = &secretmem_vm_ops;

return 0;

bool vma_is_secretmem(struct vm_area_struct *vma)
return vma->vm_ops == &secretmem_vm_ops;

static const struct file_operations secretmem_fops = {
.mmap = secretmem_mmap,

static bool secretmem_isolate_page(struct page *page, isolate_mode_t mode)
return false;

static int secretmem_migratepage(struct address_space *mapping,
struct page *newpage, struct page *page,
enum migrate_mode mode)
return -EBUSY;

static void secretmem_freepage(struct page *page)

const struct address_space_operations secretmem_aops = {
.freepage = secretmem_freepage,
.migratepage = secretmem_migratepage,
.isolate_page = secretmem_isolate_page,

static struct vfsmount *secretmem_mnt;

static struct file *secretmem_file_create(unsigned long flags)
struct file *file = ERR_PTR(-ENOMEM);
struct inode *inode;

inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
if (IS_ERR(inode))
return ERR_CAST(inode);

file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
O_RDWR, &secretmem_fops);
if (IS_ERR(file))
goto err_free_inode;

mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);

inode->i_mapping->a_ops = &secretmem_aops;

/* pretend we are a normal file with zero size */
inode->i_mode |= S_IFREG;
inode->i_size = 0;

return file;

return file;

SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
struct file *file;
int fd, err;

/* make sure local flags do not confict with global fcntl.h */

if (!secretmem_enable)
return -ENOSYS;

return -EINVAL;

fd = get_unused_fd_flags(flags & O_CLOEXEC);
if (fd < 0)
return fd;

file = secretmem_file_create(flags);
if (IS_ERR(file)) {
err = PTR_ERR(file);
goto err_put_fd;

file->f_flags |= O_LARGEFILE;

fd_install(fd, file);
return fd;

return err;

static int secretmem_init_fs_context(struct fs_context *fc)
return init_pseudo(fc, SECRETMEM_MAGIC) ? 0 : -ENOMEM;

static struct file_system_type secretmem_fs = {
.name = "secretmem",
.init_fs_context = secretmem_init_fs_context,
.kill_sb = kill_anon_super,

static int secretmem_init(void)
int ret = 0;

if (!secretmem_enable)
return ret;

secretmem_mnt = kern_mount(&secretmem_fs);
if (IS_ERR(secretmem_mnt))
ret = PTR_ERR(secretmem_mnt);

/* prevent secretmem mappings from ever getting PROT_EXEC */
secretmem_mnt->mnt_flags |= MNT_NOEXEC;

return ret;

0 comments on commit 1507f51

Please sign in to comment.