[RFC PATCH 7/8] mm: secretmem: use AS_INACCESSIBLE to prohibit GUP

[Patrick Roy <roypat@amazon.co.uk>]

Inside of vma_is_secretmem and secretmem_mapping, instead of checking
whether a vm_area_struct/address_space has the secretmem ops structure
attached to it, check whether the address_space has the AS_INACCESSIBLE
bit set. Then set the AS_INACCESSIBLE flag for secretmem's
address_space.

This means that get_user_pages and friends are disables for all
adress_spaces that set AS_INACCESIBLE. The AS_INACCESSIBLE flag was
introduced in commit c72ceafbd12c ("mm: Introduce AS_INACCESSIBLE for
encrypted/confidential memory") specifically for guest_memfd to indicate
that no reads and writes should ever be done to guest_memfd
address_spaces. Disallowing gup seems like a reasonable semantic
extension, and means that potential future mmaps of guest_memfd cannot
be GUP'd.

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
---
 include/linux/secretmem.h | 13 +++++++++++--
 mm/secretmem.c            |  6 +-----
 2 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/include/linux/secretmem.h b/include/linux/secretmem.h
index e918f96881f5..886c8f7eb63e 100644
--- a/include/linux/secretmem.h
+++ b/include/linux/secretmem.h
@@ -8,10 +8,19 @@ extern const struct address_space_operations secretmem_aops;
 
 static inline bool secretmem_mapping(struct address_space *mapping)
 {
-	return mapping->a_ops == &secretmem_aops;
+	return mapping->flags & AS_INACCESSIBLE;
+}
+
+static inline bool vma_is_secretmem(struct vm_area_struct *vma)
+{
+	struct file *file = vma->vm_file;
+
+	if (!file)
+		return false;
+
+	return secretmem_mapping(file->f_inode->i_mapping);
 }
 
-bool vma_is_secretmem(struct vm_area_struct *vma);
 bool secretmem_active(void);
 
 #else
diff --git a/mm/secretmem.c b/mm/secretmem.c
index 3afb5ad701e1..fd03a84a1cb5 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -136,11 +136,6 @@ static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
 	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 = {
 	.release	= secretmem_release,
 	.mmap		= secretmem_mmap,
@@ -218,6 +213,7 @@ static struct file *secretmem_file_create(unsigned long flags)
 
 	inode->i_op = &secretmem_iops;
 	inode->i_mapping->a_ops = &secretmem_aops;
+	inode->i_mapping->flags |= AS_INACCESSIBLE;
 
 	/* pretend we are a normal file with zero size */
 	inode->i_mode |= S_IFREG;
-- 
2.45.2