Re: [RFC PATCH v2 3/5] mm: extend memfd with ability to create "secret" memory areas

[Mike Rapoport <rppt@kernel.org>]

On Mon, Jul 13, 2020 at 01:58:12PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jul 06, 2020 at 08:20:49PM +0300, Mike Rapoport wrote:
> > From: Mike Rapoport <rppt@linux.ibm.com>
> > 
> > Extend memfd_create() 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_create system call.
> > The user than has to use ioctl() to define the desired protection mode for
> > the memory associated with that file descriptor and only when the mode is
> > set it is possible to mmap() the memory. For instance, the following
> > exapmple will create an uncached mapping (error handling is omitted):
> > 
> >         fd = memfd_create("secret", MFD_SECRET);
> 
> I'm not convinced that it belong to memfd. You don't share anything with
> memfd, but the syscall.
 
I've chosen memfd because it implements file descriptor for memory
access. Indeed, there similarities end and this can be entirely new
system call.
And, TBH, I was too lazy to wire up a new syscall for RFC :)

> >         ioctl(fd, MFD_SECRET_UNCACHED);
> > 	ftruncate(fd. MAP_SIZE);
> 
> Mix of tabs and spaces?

Will fix.

> >         ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED,
> > 		   fd, 0);
> > 
> > Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> > ---
> >  include/linux/memfd.h      |   9 ++
> >  include/uapi/linux/magic.h |   1 +
> >  include/uapi/linux/memfd.h |   6 +
> >  mm/Kconfig                 |   3 +
> >  mm/Makefile                |   1 +
> >  mm/memfd.c                 |  10 +-
> >  mm/secretmem.c             | 247 +++++++++++++++++++++++++++++++++++++
> >  7 files changed, 275 insertions(+), 2 deletions(-)
> >  create mode 100644 mm/secretmem.c
> > 
> > diff --git a/include/linux/memfd.h b/include/linux/memfd.h
> > index 4f1600413f91..d3ca7285f51a 100644
> > --- a/include/linux/memfd.h
> > +++ b/include/linux/memfd.h
> > @@ -13,4 +13,13 @@ static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned long a)
> >  }
> >  #endif
> >  
> > +#ifdef CONFIG_MEMFD_SECRETMEM
> > +extern struct file *secretmem_file_create(const char *name, unsigned int flags);
> > +#else
> > +static inline struct file *secretmem_file_create(const char *name, unsigned int flags)
> > +{
> > +	return ERR_PTR(-EINVAL);
> > +}
> > +#endif
> > +
> >  #endif /* __LINUX_MEMFD_H */
> > 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/memfd.h b/include/uapi/linux/memfd.h
> > index 7a8a26751c23..3320a79b638d 100644
> > --- a/include/uapi/linux/memfd.h
> > +++ b/include/uapi/linux/memfd.h
> > @@ -8,6 +8,12 @@
> >  #define MFD_CLOEXEC		0x0001U
> >  #define MFD_ALLOW_SEALING	0x0002U
> >  #define MFD_HUGETLB		0x0004U
> > +#define MFD_SECRET		0x0008U
> > +
> > +/* ioctls for secret memory */
> > +#define MFD_SECRET_IOCTL '-'
> > +#define MFD_SECRET_EXCLUSIVE	_IOW(MFD_SECRET_IOCTL, 0x13, unsigned long)
> > +#define MFD_SECRET_UNCACHED	_IOW(MFD_SECRET_IOCTL, 0x14, unsigned long)
> >  
> >  /*
> >   * Huge page size encoding when MFD_HUGETLB is specified, and a huge page
> > diff --git a/mm/Kconfig b/mm/Kconfig
> > index f2104cc0d35c..20dfcc54cc7a 100644
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -872,4 +872,7 @@ config ARCH_HAS_HUGEPD
> >  config MAPPING_DIRTY_HELPERS
> >          bool
> >  
> > +config MEMFD_SECRETMEM
> > +        def_bool MEMFD_CREATE && ARCH_HAS_SET_DIRECT_MAP
> > +
> >  endmenu
> > diff --git a/mm/Makefile b/mm/Makefile
> > index 6e9d46b2efc9..a9459c8a655a 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_MEMFD_SECRETMEM) += secretmem.o
> > diff --git a/mm/memfd.c b/mm/memfd.c
> > index 2647c898990c..3e1cc37e0389 100644
> > --- a/mm/memfd.c
> > +++ b/mm/memfd.c
> > @@ -245,7 +245,8 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
> >  #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
> >  #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
> >  
> > -#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB)
> > +#define MFD_SECRET_MASK (MFD_CLOEXEC | MFD_SECRET)
> > +#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB | MFD_SECRET)
> >  
> >  SYSCALL_DEFINE2(memfd_create,
> >  		const char __user *, uname,
> > @@ -257,6 +258,9 @@ SYSCALL_DEFINE2(memfd_create,
> >  	char *name;
> >  	long len;
> >  
> > +	if (flags & ~(unsigned int)MFD_SECRET_MASK)
> > +		return -EINVAL;
> > +
> 
> Didn't you just broke MFD_ALLOW_SEALING and MFD_HUGETLB with this?
> I guess the check has to be under 'if (flags & MFD_SECRET) {' check, no?

Apparently I did. Thanks!

> And (unsigned int) case looks redundant to me.
> 
> >  	if (!(flags & MFD_HUGETLB)) {
> >  		if (flags & ~(unsigned int)MFD_ALL_FLAGS)
> >  			return -EINVAL;
> > @@ -296,7 +300,9 @@ SYSCALL_DEFINE2(memfd_create,
> >  		goto err_name;
> >  	}
> >  
> > -	if (flags & MFD_HUGETLB) {
> > +	if (flags & MFD_SECRET) {
> > +		file = secretmem_file_create(name, flags);
> > +	} else if (flags & MFD_HUGETLB) {
> >  		struct user_struct *user = NULL;
> >  
> >  		file = hugetlb_file_setup(name, 0, VM_NORESERVE, &user,
> > diff --git a/mm/secretmem.c b/mm/secretmem.c
> > new file mode 100644
> > index 000000000000..df8f8c958cc2
> > --- /dev/null
> > +++ b/mm/secretmem.c
> > @@ -0,0 +1,247 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +#include <linux/mm.h>
> > +#include <linux/fs.h>
> > +#include <linux/mount.h>
> > +#include <linux/memfd.h>
> > +#include <linux/printk.h>
> > +#include <linux/pagemap.h>
> > +#include <linux/pseudo_fs.h>
> > +#include <linux/set_memory.h>
> > +#include <linux/sched/signal.h>
> > +
> > +#include <uapi/linux/memfd.h>
> > +#include <uapi/linux/magic.h>
> > +
> > +#include <asm/tlbflush.h>
> > +
> > +#include "internal.h"
> > +
> > +#undef pr_fmt
> > +#define pr_fmt(fmt) "secretmem: " fmt
> > +
> > +#define SECRETMEM_EXCLUSIVE	0x1
> > +#define SECRETMEM_UNCACHED	0x2
> > +
> > +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_lru(page, mapping, offset, vmf->gfp_mask);
> > +		if (unlikely(ret))
> > +			goto err_put_page;
> 
> What the reason to add it to LRU? These pages never evictable. Do we have
> some PageLRU() check that needs to be satisfied or what?

Hmm, most probably not. I'll recheck and will replace with
add_to_page_cache().

> > +
> > +		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 mode = ctx->mode;
> > +	unsigned long len = vma->vm_end - vma->vm_start;
> > +
> > +	if (!mode)
> > +		return -EINVAL;
> > +
> > +	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
> > +		return -EAGAIN;
> > +
> > +	switch (mode) {
> > +	case SECRETMEM_UNCACHED:
> > +		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> > +		fallthrough;
> > +	case SECRETMEM_EXCLUSIVE:
> > +		vma->vm_ops = &secretmem_vm_ops;
> > +		break;
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +
> > +	vma->vm_flags |= VM_LOCKED;
> > +
> > +	return 0;
> > +}
> > +
> > +static long secretmem_ioctl(struct file *file, unsigned cmd, unsigned long arg)
> > +{
> > +	struct secretmem_ctx *ctx = file->private_data;
> > +	unsigned long mode = ctx->mode;
> > +
> > +	if (mode)
> > +		return -EINVAL;
> > +
> > +	switch (cmd) {
> > +	case MFD_SECRET_EXCLUSIVE:
> > +		mode = SECRETMEM_EXCLUSIVE;
> > +		break;
> > +	case MFD_SECRET_UNCACHED:
> > +		mode = SECRETMEM_UNCACHED;
> > +		break;
> > +	default:
> > +		return -EINVAL;
> > +	}
> > +
> > +	ctx->mode = mode;
> > +
> > +	return 0;
> > +}
> > +
> > +const struct file_operations secretmem_fops = {
> > +	.mmap		= secretmem_mmap,
> > +	.unlocked_ioctl = secretmem_ioctl,
> > +	.compat_ioctl	= secretmem_ioctl,
> > +};
> > +
> > +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;
> > +
> > +struct file *secretmem_file_create(const char *name, unsigned int flags)
> > +{
> > +	struct inode *inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
> > +	struct file *file = ERR_PTR(-ENOMEM);
> > +	struct secretmem_ctx *ctx;
> > +
> > +	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;
> > +
> > +	return file;
> > +
> > +err_free_ctx:
> > +	kfree(ctx);
> > +err_free_inode:
> > +	iput(inode);
> > +	return file;
> > +}
> > +
> > +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
> > 
> 
> -- 
>  Kirill A. Shutemov

-- 
Sincerely yours,
Mike.