[RFC PATCH 1/6] mm: Add F_SEAL_GUEST to shmem/memfd

From: Chao Peng
Date: Thu Nov 11 2021 - 09:15:12 EST

The new seal is only allowed if there's no pre-existing pages in the fd
and there's no existing mapping of the file. After the seal is set, no
read/write/mmap from userspace is allowed.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
---
include/linux/memfd.h | 22 +++++++
include/linux/shmem_fs.h | 9 +++
include/uapi/linux/fcntl.h | 1 +
mm/memfd.c | 34 +++++++++-
mm/shmem.c | 127 ++++++++++++++++++++++++++++++++++++-
5 files changed, 189 insertions(+), 4 deletions(-)

diff --git a/include/linux/memfd.h b/include/linux/memfd.h
index 4f1600413f91..ea213f5e3f95 100644
--- a/include/linux/memfd.h
+++ b/include/linux/memfd.h
@@ -4,13 +4,35 @@

#include <linux/file.h>

+struct guest_ops {
+ void (*invalidate_page_range)(struct inode *inode, void *owner,
+ pgoff_t start, pgoff_t end);
+};
+
+struct guest_mem_ops {
+ unsigned long (*get_lock_pfn)(struct inode *inode, pgoff_t offset,
+ int *page_level);
+ void (*put_unlock_pfn)(unsigned long pfn);
+
+};
+
#ifdef CONFIG_MEMFD_CREATE
extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg);
+
+extern inline int memfd_register_guest(struct inode *inode, void *owner,
+ const struct guest_ops *guest_ops,
+ const struct guest_mem_ops **guest_mem_ops);
#else
static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned long a)
{
return -EINVAL;
}
+static inline int memfd_register_guest(struct inode *inode, void *owner,
+ const struct guest_ops *guest_ops,
+ const struct guest_mem_ops **guest_mem_ops)
+{
+ return -EINVAL;
+}
#endif

#endif /* __LINUX_MEMFD_H */
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index d82b6f396588..1b4c032680d5 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -12,6 +12,9 @@

/* inode in-kernel data */

+struct guest_ops;
+struct guest_mem_ops;
+
struct shmem_inode_info {
spinlock_t lock;
unsigned int seals; /* shmem seals */
@@ -24,6 +27,8 @@ struct shmem_inode_info {
struct simple_xattrs xattrs; /* list of xattrs */
atomic_t stop_eviction; /* hold when working on inode */
struct inode vfs_inode;
+ void *guest_owner;
+ const struct guest_ops *guest_ops;
};

struct shmem_sb_info {
@@ -90,6 +95,10 @@ extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
pgoff_t start, pgoff_t end);

+extern int shmem_register_guest(struct inode *inode, void *owner,
+ const struct guest_ops *guest_ops,
+ const struct guest_mem_ops **guest_mem_ops);
+
/* Flag allocation requirements to shmem_getpage */
enum sgp_type {
SGP_READ, /* don't exceed i_size, don't allocate page */
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 2f86b2ad6d7e..c79bc8572721 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -43,6 +43,7 @@
#define F_SEAL_GROW 0x0004 /* prevent file from growing */
#define F_SEAL_WRITE 0x0008 /* prevent writes */
#define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */
+#define F_SEAL_GUEST 0x0020
/* (1U << 31) is reserved for signed error codes */

/*
diff --git a/mm/memfd.c b/mm/memfd.c
index 2647c898990c..5a34173f55f4 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -130,11 +130,26 @@ static unsigned int *memfd_file_seals_ptr(struct file *file)
return NULL;
}

+int memfd_register_guest(struct inode *inode, void *owner,
+ const struct guest_ops *guest_ops,
+ const struct guest_mem_ops **guest_mem_ops)
+{
+ if (shmem_mapping(inode->i_mapping)) {
+ return shmem_register_guest(inode, owner,
+ guest_ops, guest_mem_ops);
+ }
+
+ return -EINVAL;
+}
+
+EXPORT_SYMBOL_GPL(memfd_register_guest);
+
#define F_ALL_SEALS (F_SEAL_SEAL | \
F_SEAL_SHRINK | \
F_SEAL_GROW | \
F_SEAL_WRITE | \
- F_SEAL_FUTURE_WRITE)
+ F_SEAL_FUTURE_WRITE | \
+ F_SEAL_GUEST)

static int memfd_add_seals(struct file *file, unsigned int seals)
{
@@ -203,10 +218,27 @@ static int memfd_add_seals(struct file *file, unsigned int seals)
}
}

+ if (seals & F_SEAL_GUEST) {
+ i_mmap_lock_read(inode->i_mapping);
+
+ if (!RB_EMPTY_ROOT(&inode->i_mapping->i_mmap.rb_root)) {
+ error = -EBUSY;
+ goto unlock;
+ }
+
+ if (i_size_read(inode)) {
+ error = -EBUSY;
+ goto unlock;
+ }
+ }
+
*file_seals |= seals;
error = 0;

unlock:
+ if (seals & F_SEAL_GUEST)
+ i_mmap_unlock_read(inode->i_mapping);
+
inode_unlock(inode);
return error;
}
diff --git a/mm/shmem.c b/mm/shmem.c
index b2db4ed0fbc7..978c841c42c4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -80,6 +80,7 @@ static struct vfsmount *shm_mnt;
#include <linux/userfaultfd_k.h>
#include <linux/rmap.h>
#include <linux/uuid.h>
+#include <linux/memfd.h>

#include <linux/uaccess.h>

@@ -883,6 +884,21 @@ static bool shmem_punch_compound(struct page *page, pgoff_t start, pgoff_t end)
return split_huge_page(page) >= 0;
}

+static void guest_invalidate_page(struct inode *inode,
+ struct page *page, pgoff_t start, pgoff_t end)
+{
+ struct shmem_inode_info *info = SHMEM_I(inode);
+
+ if (!info->guest_ops || !info->guest_ops->invalidate_page_range)
+ return;
+
+ start = max(start, page->index);
+ end = min(end, page->index + HPAGE_PMD_NR) - 1;
+
+ info->guest_ops->invalidate_page_range(inode, info->guest_owner,
+ start, end);
+}
+
/*
* Remove range of pages and swap entries from page cache, and free them.
* If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate.
@@ -923,6 +939,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
}
index += thp_nr_pages(page) - 1;

+ guest_invalidate_page(inode, page, start, end);
+
if (!unfalloc || !PageUptodate(page))
truncate_inode_page(mapping, page);
unlock_page(page);
@@ -999,6 +1017,9 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
index--;
break;
}
+
+ guest_invalidate_page(inode, page, start, end);
+
VM_BUG_ON_PAGE(PageWriteback(page), page);
if (shmem_punch_compound(page, start, end))
truncate_inode_page(mapping, page);
@@ -1074,6 +1095,9 @@ static int shmem_setattr(struct user_namespace *mnt_userns,
(newsize > oldsize && (info->seals & F_SEAL_GROW)))
return -EPERM;

+ if ((info->seals & F_SEAL_GUEST) && (newsize & ~PAGE_MASK))
+ return -EINVAL;
+
if (newsize != oldsize) {
error = shmem_reacct_size(SHMEM_I(inode)->flags,
oldsize, newsize);
@@ -1348,6 +1372,8 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
goto redirty;
if (!total_swap_pages)
goto redirty;
+ if (info->seals & F_SEAL_GUEST)
+ goto redirty;

/*
* Our capabilities prevent regular writeback or sync from ever calling
@@ -2278,6 +2304,9 @@ static int shmem_mmap(struct file *file, struct vm_area_struct *vma)
vma->vm_flags &= ~(VM_MAYWRITE);
}

+ if (info->seals & F_SEAL_GUEST)
+ return -EPERM;
+
/* arm64 - allow memory tagging on RAM-based files */
vma->vm_flags |= VM_MTE_ALLOWED;

@@ -2519,12 +2548,14 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
pgoff_t index = pos >> PAGE_SHIFT;

/* i_mutex is held by caller */
- if (unlikely(info->seals & (F_SEAL_GROW |
- F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {
+ if (unlikely(info->seals & (F_SEAL_GROW | F_SEAL_WRITE |
+ F_SEAL_FUTURE_WRITE | F_SEAL_GUEST))) {
if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))
return -EPERM;
if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
return -EPERM;
+ if (info->seals & F_SEAL_GUEST)
+ return -EPERM;
}

return shmem_getpage(inode, index, pagep, SGP_WRITE);
@@ -2598,6 +2629,20 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
end_index = i_size >> PAGE_SHIFT;
if (index > end_index)
break;
+
+ /*
+ * inode_lock protects setting up seals as well as write to
+ * i_size. Setting F_SEAL_GUEST only allowed with i_size == 0.
+ *
+ * Check F_SEAL_GUEST after i_size. It effectively serialize
+ * read vs. setting F_SEAL_GUEST without taking inode_lock in
+ * read path.
+ */
+ if (SHMEM_I(inode)->seals & F_SEAL_GUEST) {
+ error = -EPERM;
+ break;
+ }
+
if (index == end_index) {
nr = i_size & ~PAGE_MASK;
if (nr <= offset)
@@ -2723,6 +2768,12 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
goto out;
}

+ if ((info->seals & F_SEAL_GUEST) &&
+ (offset & ~PAGE_MASK || len & ~PAGE_MASK)) {
+ error = -EINVAL;
+ goto out;
+ }
+
shmem_falloc.waitq = &shmem_falloc_waitq;
shmem_falloc.start = (u64)unmap_start >> PAGE_SHIFT;
shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
@@ -3806,6 +3857,20 @@ static void shmem_destroy_inodecache(void)
kmem_cache_destroy(shmem_inode_cachep);
}

+#ifdef CONFIG_MIGRATION
+int shmem_migrate_page(struct address_space *mapping,
+ struct page *newpage, struct page *page,
+ enum migrate_mode mode)
+{
+ struct inode *inode = mapping->host;
+ struct shmem_inode_info *info = SHMEM_I(inode);
+
+ if (info->seals & F_SEAL_GUEST)
+ return -ENOTSUPP;
+ return migrate_page(mapping, newpage, page, mode);
+}
+#endif
+
const struct address_space_operations shmem_aops = {
.writepage = shmem_writepage,
.set_page_dirty = __set_page_dirty_no_writeback,
@@ -3814,12 +3879,68 @@ const struct address_space_operations shmem_aops = {
.write_end = shmem_write_end,
#endif
#ifdef CONFIG_MIGRATION
- .migratepage = migrate_page,
+ .migratepage = shmem_migrate_page,
#endif
.error_remove_page = generic_error_remove_page,
};
EXPORT_SYMBOL(shmem_aops);

+static unsigned long shmem_get_lock_pfn(struct inode *inode, pgoff_t offset,
+ int *page_level)
+{
+ struct page *page;
+ int ret;
+
+ ret = shmem_getpage(inode, offset, &page, SGP_WRITE);
+ if (ret)
+ return ret;
+
+ if (is_transparent_hugepage(page))
+ *page_level = PG_LEVEL_2M;
+ else
+ *page_level = PG_LEVEL_4K;
+
+ return page_to_pfn(page);
+}
+
+static void shmem_put_unlock_pfn(unsigned long pfn)
+{
+ struct page *page = pfn_to_page(pfn);
+
+ VM_BUG_ON_PAGE(!PageLocked(page), page);
+
+ set_page_dirty(page);
+ unlock_page(page);
+ put_page(page);
+}
+
+static const struct guest_mem_ops shmem_guest_ops = {
+ .get_lock_pfn = shmem_get_lock_pfn,
+ .put_unlock_pfn = shmem_put_unlock_pfn,
+};
+
+int shmem_register_guest(struct inode *inode, void *owner,
+ const struct guest_ops *guest_ops,
+ const struct guest_mem_ops **guest_mem_ops)
+{
+ struct shmem_inode_info *info = SHMEM_I(inode);
+
+ if (!owner)
+ return -EINVAL;
+
+ if (info->guest_owner) {
+ if (info->guest_owner == owner)
+ return 0;
+ else
+ return -EPERM;
+ }
+
+ info->guest_owner = owner;
+ info->guest_ops = guest_ops;
+ *guest_mem_ops = &shmem_guest_ops;
+ return 0;
+}
+
static const struct file_operations shmem_file_operations = {
.mmap = shmem_mmap,
.get_unmapped_area = shmem_get_unmapped_area,
--
2.17.1