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

From: David Hildenbrand
Date: Tue Jul 09 2024 - 17:09:52 EST


On 09.07.24 15:20, Patrick Roy wrote:
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@xxxxxxxxxxxx>
---
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);
}

That sounds wrong. You should leave *secretmem alone and instead have something like inaccessible_mapping that is used where appropriate.

vma_is_secretmem() should not suddenly succeed on something that is not mm/secretmem.c

--
Cheers,

David / dhildenb