Date:   Mon, 13 Jul 2020 13:58:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Alan Cox <alan@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christopher Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Idan Yaniv <idan.yaniv@....com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [RFC PATCH v2 3/5] mm: extend memfd with ability to create
 "secret" memory areas

On Mon, Jul 06, 2020 at 08:20:49PM +0300, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.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.

>         ioctl(fd, MFD_SECRET_UNCACHED);
> 	ftruncate(fd. MAP_SIZE);

Mix of tabs and spaces?

>         ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED,
> 		   fd, 0);
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.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?

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?

> +
> +		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

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