On 05.08.24 20:34, Elliot Berman wrote:
This patch was reworked from Patrick's patch:
https://lore.kernel.org/all/20240709132041.3625501-6-roypat@xxxxxxxxxxxx/
While guest_memfd is not available to be mapped by userspace, it is
still accessible through the kernel's direct map. This means that in
scenarios where guest-private memory is not hardware protected, it can
be speculatively read and its contents potentially leaked through
hardware side-channels. Removing guest-private memory from the direct
map, thus mitigates a large class of speculative execution issues
[1, Table 1].
I think you have to point out here that the speculative execution issues
are primarily only an issue when guest_memfd private memory is used
without TDX and friends where the memory would be encrypted either way.
Or am I wrong?
Direct map removal do not reuse the `.prepare` machinery, since
`prepare` can be called multiple time, and it is the responsibility of
the preparation routine to not "prepare" the same folio twice [2]. Thus,
instead explicitly check if `filemap_grab_folio` allocated a new folio,
and remove the returned folio from the direct map only if this was the
case.
The patch uses release_folio instead of free_folio to reinsert pages
back into the direct map as by the time free_folio is called,
folio->mapping can already be NULL. This means that a call to
folio_inode inside free_folio might deference a NULL pointer, leaving no
way to access the inode which stores the flags that allow determining
whether the page was removed from the direct map in the first place.
[1]: https://download.vusec.net/papers/quarantine_raid23.pdf
Cc: Patrick Roy <roypat@xxxxxxxxxxxx>
Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
---
include/linux/guest_memfd.h | 8 ++++++
mm/guest_memfd.c | 65 ++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 72 insertions(+), 1 deletion(-)
diff --git a/include/linux/guest_memfd.h b/include/linux/guest_memfd.h
index be56d9d53067..f9e4a27aed67 100644
--- a/include/linux/guest_memfd.h
+++ b/include/linux/guest_memfd.h
@@ -25,6 +25,14 @@ struct guest_memfd_operations {
int (*release)(struct inode *inode);
};
+/**
+ * @GUEST_MEMFD_FLAG_NO_DIRECT_MAP: When making folios inaccessible by host, also
+ * remove them from the kernel's direct map.
+ */
Should we start introducing the concept of private and shared first,
such that we can then say that this only applies to private memory?
+enum {
+ GUEST_MEMFD_FLAG_NO_DIRECT_MAP = BIT(0),
+};
+
/**
* @GUEST_MEMFD_GRAB_UPTODATE: Ensure pages are zeroed/up to date.
* If trusted hyp will do it, can ommit this flag
diff --git a/mm/guest_memfd.c b/mm/guest_memfd.c
index 580138b0f9d4..e9d8cab72b28 100644
--- a/mm/guest_memfd.c
+++ b/mm/guest_memfd.c
@@ -7,9 +7,55 @@
#include <linux/falloc.h>
#include <linux/guest_memfd.h>
#include <linux/pagemap.h>
+#include <linux/set_memory.h>
+
+static inline int guest_memfd_folio_private(struct folio *folio)
+{
+ unsigned long nr_pages = folio_nr_pages(folio);
guest_memfd only supports small folios, this can be simplified.
+ unsigned long i;
+ int r;
+
+ for (i = 0; i < nr_pages; i++) {
+ struct page *page = folio_page(folio, i);
+
+ r = set_direct_map_invalid_noflush(page);
+ if (r < 0)
+ goto out_remap;
+ }
+
+ folio_set_private(folio);
+ return 0;
+out_remap:
+ for (; i > 0; i--) {
+ struct page *page = folio_page(folio, i - 1);
+
+ BUG_ON(set_direct_map_default_noflush(page));
+ }
+ return r;
+}
+
+static inline void guest_memfd_folio_clear_private(struct folio *folio)
Set set/clear private semantics in this context are a bit confusing. I
assume you mean "make inaccessible" "make accessible" and using the
PG_private flag is just an implementation detail.
+{
+ unsigned long start = (unsigned long)folio_address(folio);
+ unsigned long nr = folio_nr_pages(folio);
+ unsigned long i;
+
+ if (!folio_test_private(folio))
+ return;
+
+ for (i = 0; i < nr; i++) {
+ struct page *page = folio_page(folio, i);
+
+ BUG_ON(set_direct_map_default_noflush(page));
+ }
+ flush_tlb_kernel_range(start, start + folio_size(folio));
+
+ folio_clear_private(folio);
+}
struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags)
{
+ unsigned long gmem_flags = (unsigned long)file->private_data;
struct inode *inode = file_inode(file);
struct guest_memfd_operations *ops = inode->i_private;
struct folio *folio;
@@ -43,6 +89,12 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
goto out_err;
}
+ if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
+ r = guest_memfd_folio_private(folio);
+ if (r)
+ goto out_err;
+ }
+
/*
* Ignore accessed, referenced, and dirty flags. The memory is
* unevictable and there is no storage to write back to.
@@ -213,14 +265,25 @@ static bool gmem_release_folio(struct folio *folio, gfp_t gfp)
if (ops->invalidate_end)
ops->invalidate_end(inode, offset, nr);
+ guest_memfd_folio_clear_private(folio);
+
return true;
}
+static void gmem_invalidate_folio(struct folio *folio, size_t offset, size_t len)
+{
+ /* not yet supported */
+ BUG_ON(offset || len != folio_size(folio));
+
+ BUG_ON(!gmem_release_folio(folio, 0));
In general, no BUG_ON please. WARN_ON_ONCE() is sufficient.
+}
+
static const struct address_space_operations gmem_aops = {
.dirty_folio = noop_dirty_folio,
.migrate_folio = gmem_migrate_folio,
.error_remove_folio = gmem_error_folio,
.release_folio = gmem_release_folio,
+ .invalidate_folio = gmem_invalidate_folio,
};
static inline bool guest_memfd_check_ops(const struct guest_memfd_operations *ops)
@@ -241,7 +304,7 @@ struct file *guest_memfd_alloc(const char *name,
if (!guest_memfd_check_ops(ops))
return ERR_PTR(-EINVAL);
- if (flags)
+ if (flags & ~GUEST_MEMFD_FLAG_NO_DIRECT_MAP)
return ERR_PTR(-EINVAL);
/*
--
Cheers,
David / dhildenb
