linux-kernel - [PATCH 1/9] mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory

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Date: Tue,  7 May 2024 14:07:21 -0400
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: vbabka@...e.cz,
	isaku.yamahata@...el.com,
	xiaoyao.li@...el.com,
	binbin.wu@...ux.intel.com,
	seanjc@...gle.com,
	rick.p.edgecombe@...el.com,
	michael.roth@....com,
	yilun.xu@...el.com,
	Matthew Wilcox <willy@...radead.org>,
	linux-mm@...ck.org
Subject: [PATCH 1/9] mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory

From: Michael Roth <michael.roth@....com>

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 as
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.

This is admittedly a rather blunt solution, but it seems like
there are no other places that should take into account the
flag to keep its promise.

Link: https://lore.kernel.org/lkml/ZR9LYhpxTaTk6PJX@google.com/
Cc: Matthew Wilcox <willy@...radead.org>
Cc: linux-mm@...ck.org
Suggested-by: Sean Christopherson <seanjc@...gle.com>
Signed-off-by: Michael Roth <michael.roth@....com>
Message-ID: <20240329212444.395559-5-michael.roth@....com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.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 2df35e65557d..f879c1d54da7 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);
-- 
2.43.0