[RFC][PATCH 01/10] x86/mm: factor out pageattr _PAGE_GLOBAL setting

Dave Hansen
2018-02-22T20:37:04Z

From: Dave Hansen <dave.hansen@linux.intel.com>

The pageattr code has a pattern repeated where it sets
_PAGE_GLOBAL for present PTEs but clears it for non-present PTEs.
The intention is to keep _PAGE_GLOBAL from getting confused
with _PAGE_PROTNONE since _PAGE_GLOBAL is for present PTEs and
_PAGE_PROTNONE is for non-present

But, this pattern makes no sense. Effectively, it says, if
you use the pageattr code, always set _PAGE_GLOBAL when
_PAGE_PRESENT. canon_pgprot() will clear it if unsupported,
but we *always* set it.

This gets confusing when we have PTI and non-PTI and we want
some areas to have _PAGE_GLOBAL and some not.

This updated version of the code says:
1. Clear _PAGE_GLOBAL when !_PAGE_PRESENT
2. Never set _PAGE_GLOBAL implicitly
3. Allow _PAGE_GLOBAL to be in cpa.set_mask
4. Allow _PAGE_GLOBAL to be inherited from previous PTE

Aside: _PAGE_GLOBAL is ignored when CR4.PGE=1, so why do we
even go to the trouble of filtering it anywhere?

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <keescook@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: x86@kernel.org
Cc: Nadav Amit <namit@vmware.com>
---

b/arch/x86/mm/pageattr.c | 68 ++++++++++++++++-------------------------------
1 file changed, 24 insertions(+), 44 deletions(-)

diff -puN arch/x86/mm/pageattr.c~kpti-centralize-global-setting arch/x86/mm/pageattr.c
--- a/arch/x86/mm/pageattr.c~kpti-centralize-global-setting 2018-02-22 12:36:17.531036556 -0800
+++ b/arch/x86/mm/pageattr.c 2018-02-22 12:36:17.535036556 -0800
@@ -512,6 +512,23 @@ static void __set_pmd_pte(pte_t *kpte, u
#endif
}

+static pgprot_t pgprot_clear_protnone_bits(pgprot_t prot)
+{
+ /*
+ * _PAGE_GLOBAL means "global page" for present PTEs.
+ * But, it is also used to indicate _PAGE_PROTNONE
+ * for non-present PTEs.
+ *
+ * This ensures that a _PAGE_GLOBAL PTE going from
+ * present to non-present is not confused as
+ * _PAGE_PROTNONE.
+ */
+ if (!(pgprot_val(prot) & _PAGE_PRESENT))
+ pgprot_val(prot) &= ~_PAGE_GLOBAL;
+
+ return prot;
+}
+
static int
try_preserve_large_page(pte_t *kpte, unsigned long address,
struct cpa_data *cpa)
@@ -577,18 +594,11 @@ try_preserve_large_page(pte_t *kpte, uns
* different bit positions in the two formats.
*/
req_prot = pgprot_4k_2_large(req_prot);
-
- /*
- * Set the PSE and GLOBAL flags only if the PRESENT flag is
- * set otherwise pmd_present/pmd_huge will return true even on
- * a non present pmd. The canon_pgprot will clear _PAGE_GLOBAL
- * for the ancient hardware that doesn't support it.
- */
- if (pgprot_val(req_prot) & _PAGE_PRESENT)
- pgprot_val(req_prot) |= _PAGE_PSE | _PAGE_GLOBAL;
+ req_prot = pgprot_clear_protnone_bits(req_prot);
+ if (pgprot_val(req_prot) & _PAGE_PRESENT)
+ pgprot_val(req_prot) |= _PAGE_PSE;
else
- pgprot_val(req_prot) &= ~(_PAGE_PSE | _PAGE_GLOBAL);
-
+ pgprot_val(req_prot) &= ~_PAGE_PSE;
req_prot = canon_pgprot(req_prot);

/*
@@ -698,16 +708,7 @@ __split_large_page(struct cpa_data *cpa,
return 1;
}

- /*
- * Set the GLOBAL flags only if the PRESENT flag is set
- * otherwise pmd/pte_present will return true even on a non
- * present pmd/pte. The canon_pgprot will clear _PAGE_GLOBAL
- * for the ancient hardware that doesn't support it.
- */
- if (pgprot_val(ref_prot) & _PAGE_PRESENT)
- pgprot_val(ref_prot) |= _PAGE_GLOBAL;
- else
- pgprot_val(ref_prot) &= ~_PAGE_GLOBAL;
+ ref_prot = pgprot_clear_protnone_bits(ref_prot);

/*
* Get the target pfn from the original entry:
@@ -930,18 +931,7 @@ static void populate_pte(struct cpa_data

pte = pte_offset_kernel(pmd, start);

- /*
- * Set the GLOBAL flags only if the PRESENT flag is
- * set otherwise pte_present will return true even on
- * a non present pte. The canon_pgprot will clear
- * _PAGE_GLOBAL for the ancient hardware that doesn't
- * support it.
- */
- if (pgprot_val(pgprot) & _PAGE_PRESENT)
- pgprot_val(pgprot) |= _PAGE_GLOBAL;
- else
- pgprot_val(pgprot) &= ~_PAGE_GLOBAL;
-
+ pgprot = pgprot_clear_protnone_bits(pgprot);
pgprot = canon_pgprot(pgprot);

while (num_pages-- && start < end) {
@@ -1234,17 +1224,7 @@ repeat:

new_prot = static_protections(new_prot, address, pfn);

- /*
- * Set the GLOBAL flags only if the PRESENT flag is
- * set otherwise pte_present will return true even on
- * a non present pte. The canon_pgprot will clear
- * _PAGE_GLOBAL for the ancient hardware that doesn't
- * support it.
- */
- if (pgprot_val(new_prot) & _PAGE_PRESENT)
- pgprot_val(new_prot) |= _PAGE_GLOBAL;
- else
- pgprot_val(new_prot) &= ~_PAGE_GLOBAL;
+ new_prot = pgprot_clear_protnone_bits(new_prot);

/*
* We need to keep the pfn from the existing PTE,
_