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Subject[PATCH 1/3] x86/mm: factor out conditional pageattr PTE bit setting code
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From: Dave Hansen <dave.hansen@linux.intel.com>

The pageattr code has a pattern repeated where it sets a PTE bit
for present PTEs but clears it for non-present PTEs. This helps
to keep pte_none() from getting messed up. _PAGE_GLOBAL is the
most frequent target of this pattern.

This pattern also includes a nice, copy-and-pasted comment.

I want to do some special stuff with _PAGE_GLOBAL in a moment,
so refactor this a _bit_ to centralize the comment and the
bit operations.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.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
---

b/arch/x86/mm/pageattr.c | 65 ++++++++++++++---------------------------------
1 file changed, 20 insertions(+), 45 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 2018-02-13 15:17:55.602210062 -0800
+++ b/arch/x86/mm/pageattr.c 2018-02-13 15:17:55.606210062 -0800
@@ -512,6 +512,22 @@ static void __set_pmd_pte(pte_t *kpte, u
#endif
}

+static pgprot_t pgprot_set_on_present(pgprot_t prot, pteval_t flags)
+{
+ /*
+ * Set 'flags' only if PRESENT. Ensures that we do not
+ * set flags in an otherwise empty PTE breaking pte_none().
+ * A later function (such as canon_pgprot()) must clear
+ * possibly unsupported flags (like _PAGE_GLOBAL).
+ */
+ if (pgprot_val(prot) & _PAGE_PRESENT)
+ pgprot_val(prot) |= flags;
+ else
+ pgprot_val(prot) &= ~flags;
+
+ return prot;
+}
+
static int
try_preserve_large_page(pte_t *kpte, unsigned long address,
struct cpa_data *cpa)
@@ -577,18 +593,7 @@ 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;
- else
- pgprot_val(req_prot) &= ~(_PAGE_PSE | _PAGE_GLOBAL);
-
+ req_prot = pgprot_set_on_present(req_prot, _PAGE_GLOBAL | _PAGE_PSE);
req_prot = canon_pgprot(req_prot);

/*
@@ -698,16 +703,7 @@ __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_set_on_present(ref_prot, _PAGE_GLOBAL);

/*
* Get the target pfn from the original entry:
@@ -930,18 +926,7 @@ 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_set_on_present(pgprot, _PAGE_GLOBAL);
pgprot = canon_pgprot(pgprot);

while (num_pages-- && start < end) {
@@ -1234,17 +1219,7 @@ 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_set_on_present(new_prot, _PAGE_GLOBAL);

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