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mm: Introduce AS_INACCESSIBLE for encrypted/confidential memory

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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@infradead.org>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240329212444.395559-5-michael.roth@amd.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Michael Roth 2024-03-29 16:24:42 -05:00 committed by Paolo Bonzini
parent 2b1f435505
commit c72ceafbd1
2 changed files with 3 additions and 1 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

1
include/linux/pagemap.h
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@ -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 */
};
/**

3
mm/truncate.c
View file

@ -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);