[PATCH] x86: ignore spurious faults

[Jeremy Fitzhardinge]

When changing a kernel page from RO->RW, it's OK to leave stale TLB
entries around, since doing a global flush is expensive and they pose
no security problem.  They can, however, generate a spurious fault,
which we should catch and simply return from (which will have the
side-effect of reloading the TLB to the current PTE).

This can occur when running under Xen, because it frequently changes
kernel pages from RW->RO->RW to implement Xen's pagetable semantics.
It could also occur when using CONFIG_DEBUG_PAGEALLOC, since it avoids
doing a global TLB flush after changing page permissions.

[ Changes to fault_32.c and fault_64.c are identical, and should be
 easy unify when the time comes. ]

Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
Cc: Harvey Harrison <harvey.harrison@xxxxxxxxx>
---
arch/x86/mm/fault_32.c |   52 ++++++++++++++++++++++++++++++++++++++++++++++++
arch/x86/mm/fault_64.c |   52 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 104 insertions(+)

===================================================================
--- a/arch/x86/mm/fault_32.c
+++ b/arch/x86/mm/fault_32.c
@@ -290,6 +290,53 @@ static int is_errata93(struct pt_regs *r


/*
+ * Handle a spurious fault caused by a stale TLB entry.  This allows
+ * us to lazily refresh the TLB when increasing the permissions of a
+ * kernel page (RO -> RW or NX -> X).  Doing it eagerly is very
+ * expensive since that implies doing a full cross-processor TLB
+ * flush, even if no stale TLB entries exist on other processors.
+ * There are no security implications to leaving a stale TLB when
+ * increasing the permissions on a page.
+ */
+static int spurious_fault(unsigned long address,
+			  unsigned long error_code)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+
+	/* Reserved-bit violation or user access to kernel space? */
+	if (error_code & (PF_USER | PF_RSVD))
+		return 0;
+
+	pgd = init_mm.pgd + pgd_index(address);
+	if (!pgd_present(*pgd))
+		return 0;
+
+	pud = pud_offset(pgd, address);
+	if (!pud_present(*pud))
+		return 0;
+
+	pmd = pmd_offset(pud, address);
+	if (!pmd_present(*pmd))
+		return 0;
+
+	pte = pte_offset_kernel(pmd, address);
+	if (!pte_present(*pte))
+		return 0;
+	if ((error_code & 0x02) && !pte_write(*pte))
+		return 0;
+
+#if _PAGE_NX
+	if ((error_code & PF_INSTR) && !pte_exec(*pte))
+		return 0;
+#endif
+
+	return 1;
+}
+
+/*
 * Handle a fault on the vmalloc or module mapping area
 *
 * This assumes no large pages in there.
@@ -412,6 +459,11 @@ void __kprobes do_page_fault(struct pt_r
		if (!(error_code & (PF_RSVD|PF_USER|PF_PROT)) &&
		    vmalloc_fault(address) >= 0)
			return;
+
+		/* Can handle a stale RO->RW TLB */
+		if (spurious_fault(address, error_code))
+			return;
+
		/*
		 * Don't take the mm semaphore here. If we fixup a prefetch
		 * fault we could otherwise deadlock.
===================================================================
--- a/arch/x86/mm/fault_64.c
+++ b/arch/x86/mm/fault_64.c
@@ -275,6 +275,53 @@ static noinline void pgtable_bad(unsigne
}

/*
+ * Handle a spurious fault caused by a stale TLB entry.  This allows
+ * us to lazily refresh the TLB when increasing the permissions of a
+ * kernel page (RO -> RW or NX -> X).  Doing it eagerly is very
+ * expensive since that implies doing a full cross-processor TLB
+ * flush, even if no stale TLB entries exist on other processors.
+ * There are no security implications to leaving a stale TLB when
+ * increasing the permissions on a page.
+ */
+static int spurious_fault(unsigned long address,
+			  unsigned long error_code)
+{
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+
+	/* Reserved-bit violation or user access to kernel space? */
+	if (error_code & (PF_USER | PF_RSVD))
+		return 0;
+
+	pgd = init_mm.pgd + pgd_index(address);
+	if (!pgd_present(*pgd))
+		return 0;
+
+	pud = pud_offset(pgd, address);
+	if (!pud_present(*pud))
+		return 0;
+
+	pmd = pmd_offset(pud, address);
+	if (!pmd_present(*pmd))
+		return 0;
+
+	pte = pte_offset_kernel(pmd, address);
+	if (!pte_present(*pte))
+		return 0;
+	if ((error_code & 0x02) && !pte_write(*pte))
+		return 0;
+
+#if _PAGE_NX
+	if ((error_code & PF_INSTR) && !pte_exec(*pte))
+		return 0;
+#endif
+
+	return 1;
+}
+
+/*
 * Handle a fault on the vmalloc area
 *
 * This assumes no large pages in there.
@@ -406,6 +453,11 @@ asmlinkage void __kprobes do_page_fault(
			if (vmalloc_fault(address) >= 0)
				return;
		}
+
+		/* Can handle a stale RO->RW TLB */
+		if (spurious_fault(address, error_code))
+			return;
+
		/*
		 * Don't take the mm semaphore here. If we fixup a prefetch
		 * fault we could otherwise deadlock.

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