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

Patrick Roy
2024-07-09T13:21:25Z

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