[PATCH v4 3/6] mm: introduce memfd_secret system call to create "secret" memory areas

Mike Rapoport
2020-08-18T14:16:50Z

From: Mike Rapoport <rppt@linux.ibm.com>

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 user will create a file descriptor using the memfd_secret() system call
where flags supplied as a parameter to this system call will define the
desired protection mode for the memory associated with that file
descriptor.

Currently there are two protection modes:

* exclusive - the memory area is unmapped from the kernel direct map and it
is present only in the page tables of the owning mm.
* uncached - the memory area is present only in the page tables of the
owning mm and it is mapped there as uncached.

The "exclusive" mode is enabled implicitly and it is the default mode for
memfd_secret().

The "uncached" mode requires architecture support and an architecture
should opt-in for this mode using HAVE_SECRETMEM_UNCACHED configuration
option.

For instance, the following example will create an uncached mapping (error
handling is omitted):

fd = memfd_secret(SECRETMEM_UNCACHED);
ftruncate(fd, MAP_SIZE);
ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
arch/Kconfig | 7 +
arch/x86/Kconfig | 1 +
include/uapi/linux/magic.h | 1 +
include/uapi/linux/secretmem.h | 8 +
kernel/sys_ni.c | 2 +
mm/Kconfig | 4 +
mm/Makefile | 1 +
mm/secretmem.c | 264 +++++++++++++++++++++++++++++++++
8 files changed, 288 insertions(+)
create mode 100644 include/uapi/linux/secretmem.h
create mode 100644 mm/secretmem.c

diff --git a/arch/Kconfig b/arch/Kconfig
index af14a567b493..8d161bd4142d 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -975,6 +975,13 @@ config HAVE_SPARSE_SYSCALL_NR
config ARCH_HAS_VDSO_DATA
bool

+config HAVE_SECRETMEM_UNCACHED
+ bool
+ help
+ An architecture can select this if its semantics of non-cached
+ mappings can be used to prevent speculative loads and it is
+ useful for secret protection.
+
source "kernel/gcov/Kconfig"

source "scripts/gcc-plugins/Kconfig"
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 7101ac64bb20..38ead8bd9909 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -220,6 +220,7 @@ config X86
select HAVE_UNSTABLE_SCHED_CLOCK
select HAVE_USER_RETURN_NOTIFIER
select HAVE_GENERIC_VDSO
+ select HAVE_SECRETMEM_UNCACHED
select HOTPLUG_SMT if SMP
select IRQ_FORCED_THREADING
select NEED_SG_DMA_LENGTH
diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index f3956fc11de6..35687dcb1a42 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h
@@ -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__ */
diff --git a/include/uapi/linux/secretmem.h b/include/uapi/linux/secretmem.h
new file mode 100644
index 000000000000..2b9675f5dea9
--- /dev/null
+++ b/include/uapi/linux/secretmem.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_SECRERTMEM_H
+#define _UAPI_LINUX_SECRERTMEM_H
+
+/* secretmem operation modes */
+#define SECRETMEM_UNCACHED 0x1
+
+#endif /* _UAPI_LINUX_SECRERTMEM_H */
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 4d59775ea79c..8ae8d0c2d381 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -349,6 +349,8 @@ COND_SYSCALL(pkey_mprotect);
COND_SYSCALL(pkey_alloc);
COND_SYSCALL(pkey_free);

+/* memfd_secret */
+COND_SYSCALL(memfd_secret);

/*
* Architecture specific weak syscall entries.
diff --git a/mm/Kconfig b/mm/Kconfig
index 6c974888f86f..70cfc20d7caa 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -868,4 +868,8 @@ config ARCH_HAS_HUGEPD
config MAPPING_DIRTY_HELPERS
bool

+config SECRETMEM
+ def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
+ select GENERIC_ALLOCATOR
+
endmenu
diff --git a/mm/Makefile b/mm/Makefile
index d5649f1c12c0..cae063dc8298 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -121,3 +121,4 @@ obj-$(CONFIG_MEMFD_CREATE) += memfd.o
obj-$(CONFIG_MAPPING_DIRTY_HELPERS) += mapping_dirty_helpers.o
obj-$(CONFIG_PTDUMP_CORE) += ptdump.o
obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
+obj-$(CONFIG_SECRETMEM) += secretmem.o
diff --git a/mm/secretmem.c b/mm/secretmem.c
new file mode 100644
index 000000000000..3293f761076e
--- /dev/null
+++ b/mm/secretmem.c
@@ -0,0 +1,264 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright IBM Corporation, 2020
+ *
+ * Author: Mike Rapoport <rppt@linux.ibm.com>
+ */
+
+#include <linux/mm.h>
+#include <linux/fs.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/set_memory.h>
+#include <linux/sched/signal.h>
+
+#include <uapi/linux/secretmem.h>
+#include <uapi/linux/magic.h>
+
+#include <asm/tlbflush.h>
+
+#include "internal.h"
+
+#undef pr_fmt
+#define pr_fmt(fmt) "secretmem: " fmt
+
+/*
+ * Secret memory areas are always exclusive to owning mm and they are
+ * removed from the direct map.
+ */
+#ifdef CONFIG_HAVE_SECRETMEM_UNCACHED
+#define SECRETMEM_MODE_MASK (SECRETMEM_UNCACHED)
+#else
+#define SECRETMEM_MODE_MASK (0x0)
+#endif
+
+#define SECRETMEM_FLAGS_MASK SECRETMEM_MODE_MASK
+
+struct secretmem_ctx {
+ unsigned int mode;
+};
+
+static struct page *secretmem_alloc_page(gfp_t gfp)
+{
+ /*
+ * FIXME: use a cache of large pages to reduce the direct map
+ * fragmentation
+ */
+ return alloc_page(gfp);
+}
+
+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;
+ unsigned long addr;
+ struct page *page;
+ int ret = 0;
+
+ if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+ return vmf_error(-EINVAL);
+
+ page = find_get_entry(mapping, offset);
+ if (!page) {
+ page = secretmem_alloc_page(vmf->gfp_mask);
+ if (!page)
+ return vmf_error(-ENOMEM);
+
+ ret = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
+ if (unlikely(ret))
+ goto err_put_page;
+
+ ret = set_direct_map_invalid_noflush(page);
+ if (ret)
+ goto err_del_page_cache;
+
+ addr = (unsigned long)page_address(page);
+ flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+ __SetPageUptodate(page);
+
+ ret = VM_FAULT_LOCKED;
+ }
+
+ vmf->page = page;
+ return ret;
+
+err_del_page_cache:
+ delete_from_page_cache(page);
+err_put_page:
+ put_page(page);
+ return vmf_error(ret);
+}
+
+static const struct vm_operations_struct secretmem_vm_ops = {
+ .fault = secretmem_fault,
+};
+
+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ struct secretmem_ctx *ctx = file->private_data;
+ 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;
+
+ if (ctx->mode & SECRETMEM_UNCACHED)
+ vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+
+ vma->vm_ops = &secretmem_vm_ops;
+ vma->vm_flags |= VM_LOCKED;
+
+ return 0;
+}
+
+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)
+{
+ set_direct_map_default_noflush(page);
+}
+
+static 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 secretmem_ctx *ctx;
+ struct inode *inode;
+
+ inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
+ if (IS_ERR(inode))
+ return ERR_CAST(inode);
+
+ ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+ if (!ctx)
+ goto err_free_inode;
+
+ file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
+ O_RDWR, &secretmem_fops);
+ if (IS_ERR(file))
+ goto err_free_ctx;
+
+ mapping_set_unevictable(inode->i_mapping);
+
+ inode->i_mapping->private_data = ctx;
+ 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;
+
+ file->private_data = ctx;
+
+ ctx->mode = flags & SECRETMEM_MODE_MASK;
+
+ return file;
+
+err_free_ctx:
+ kfree(ctx);
+err_free_inode:
+ iput(inode);
+ return file;
+}
+
+SYSCALL_DEFINE1(memfd_secret, unsigned long, flags)
+{
+ struct file *file;
+ int fd, err;
+
+ /* make sure local flags do not confict with global fcntl.h */
+ BUILD_BUG_ON(SECRETMEM_FLAGS_MASK & O_CLOEXEC);
+
+ if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
+ 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;
+
+err_put_fd:
+ put_unused_fd(fd);
+ return err;
+}
+
+static void secretmem_evict_inode(struct inode *inode)
+{
+ struct secretmem_ctx *ctx = inode->i_private;
+
+ truncate_inode_pages_final(&inode->i_data);
+ clear_inode(inode);
+ kfree(ctx);
+}
+
+static const struct super_operations secretmem_super_ops = {
+ .evict_inode = secretmem_evict_inode,
+};
+
+static int secretmem_init_fs_context(struct fs_context *fc)
+{
+ struct pseudo_fs_context *ctx = init_pseudo(fc, SECRETMEM_MAGIC);
+
+ if (!ctx)
+ return -ENOMEM;
+ ctx->ops = &secretmem_super_ops;
+
+ return 0;
+}
+
+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;
+
+ secretmem_mnt = kern_mount(&secretmem_fs);
+ if (IS_ERR(secretmem_mnt))
+ ret = PTR_ERR(secretmem_mnt);
+
+ return ret;
+}
+fs_initcall(secretmem_init);
--
2.26.2