[PATCH v11 02/35] mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory

Michael Roth
2023-12-30T17:32:44Z

filemap users like guest_memfd may use page cache pages to
allocate/manage memory that is only intended to be accessed by guests
via hardware protections like encryption. Writes to memory of this sort
in common paths like truncation may cause unexpected behavior such
writing garbage instead of zeros when attempting to zero pages, or
worse, triggering hardware protections that are considered fatal as far
as the kernel is concerned.

Introduce a new address_space flag, AS_INACCESSIBLE, and use this
initially to prevent zero'ing of pages during truncation, with the
understanding that it is up to the owner of the mapping to handle this
specially if needed.

Link: https://lore.kernel.org/lkml/ZR9LYhpxTaTk6PJX@google.com/
Cc: Matthew Wilcox <willy@infradead.org>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
---
include/linux/pagemap.h | 1 +
mm/truncate.c | 3 ++-
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index c2d90588c0bf..b56081a3512e 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -207,6 +207,7 @@ enum mapping_flags {
AS_STABLE_WRITES, /* must wait for writeback before modifying
folio contents */
AS_UNMOVABLE, /* The mapping cannot be moved, ever */
+ AS_INACCESSIBLE, /* Do not attempt direct R/W access to the mapping */
};

/**
diff --git a/mm/truncate.c b/mm/truncate.c
index 725b150e47ac..c501338c7ebd 100644
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -233,7 +233,8 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end)
* doing a complex calculation here, and then doing the zeroing
* anyway if the page split fails.
*/
- folio_zero_range(folio, offset, length);
+ if (!(folio->mapping->flags & AS_INACCESSIBLE))
+ folio_zero_range(folio, offset, length);

if (folio_has_private(folio))
folio_invalidate(folio, offset, length);
--
2.25.1