[PATCH 2/2] arm64: Allow changing of attributes outside of modules

[Laura Abbott]

Currently, the set_memory_* functions that are implemented for arm64
are restricted to module addresses only. This was mostly done
because arm64 maps normal zone memory with larger page sizes to
improve TLB performance. This has the side effect though of making it
difficult to adjust attributes at the PAGE_SIZE granularity. There are
an increasing number of use cases related to security where it is
necessary to change the attributes of kernel memory. Add functionality
to the page attribute changing code under a Kconfig to let systems
designers decide if they want to make the trade off of security for TLB
pressure.

Signed-off-by: Laura Abbott <labbott at fedoraproject.org>
---
 arch/arm64/Kconfig.debug | 11 +++++++
 arch/arm64/mm/mm.h       |  3 ++
 arch/arm64/mm/mmu.c      |  2 +-
 arch/arm64/mm/pageattr.c | 74 ++++++++++++++++++++++++++++++++++++++++++++----
 4 files changed, 84 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index d6285ef..abc6922 100644
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -89,6 +89,17 @@ config DEBUG_ALIGN_RODATA
 
 	  If in doubt, say N
 
+config DEBUG_CHANGE_PAGEATTR
+	bool "Allow all kernel memory to have attributes changed"
+	help
+	  If this option is selected, APIs that change page attributes
+	  (RW <-> RO, X <-> NX) will be valid for all memory mapped in
+	  the kernel space. The trade off is that there may be increased
+	  TLB pressure from finer grained page mapping. Turn on this option
+	  if performance is more important than security
+
+	  If in doubt, say N
+
 source "drivers/hwtracing/coresight/Kconfig"
 
 endmenu
diff --git a/arch/arm64/mm/mm.h b/arch/arm64/mm/mm.h
index ef47d99..7b0dcc4 100644
--- a/arch/arm64/mm/mm.h
+++ b/arch/arm64/mm/mm.h
@@ -1,3 +1,6 @@
 extern void __init bootmem_init(void);
 
 void fixup_init(void);
+
+void split_pud(pud_t *old_pud, pmd_t *pmd);
+void split_pmd(pmd_t *pmd, pte_t *pte);
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index ff41efa..cefad2d 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -72,7 +72,7 @@ static void __init *early_alloc(unsigned long sz)
 /*
  * remap a PMD into pages
  */
-static void split_pmd(pmd_t *pmd, pte_t *pte)
+void split_pmd(pmd_t *pmd, pte_t *pte)
 {
 	unsigned long pfn = pmd_pfn(*pmd);
 	unsigned long addr = pfn << PAGE_SHIFT;
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index e47ed1c..48a4ce9 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -15,9 +15,12 @@
 #include <linux/module.h>
 #include <linux/sched.h>
 
+#include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
 
+#include "mm.h"
+
 struct page_change_data {
 	pgprot_t set_mask;
 	pgprot_t clear_mask;
@@ -36,6 +39,66 @@ static int change_page_range(pte_t *ptep, pgtable_t token, unsigned long addr,
 	return 0;
 }
 
+#ifdef CONFIG_DEBUG_CHANGE_PAGEATTR
+static int check_address(unsigned long addr)
+{
+	pgd_t *pgd = pgd_offset_k(addr);
+	pud_t *pud;
+	pmd_t *pmd;
+	pte_t *pte;
+	int ret = -EFAULT;
+
+	if (pgd_none(*pgd))
+		goto out;
+
+	pud = pud_offset(pgd, addr);
+	if (pud_none(*pud))
+		goto out;
+
+	if (pud_sect(*pud)) {
+		pmd = pmd_alloc_one(&init_mm, addr);
+		if (!pmd) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		split_pud(pud, pmd);
+		pud_populate(&init_mm, pud, pmd);
+	}
+
+	pmd = pmd_offset(pud, addr);
+	if (pmd_none(*pmd))
+		goto out;
+
+	if (pmd_sect(*pmd)) {
+		pte = pte_alloc_one_kernel(&init_mm, addr);
+		if (!pte) {
+			ret = -ENOMEM;
+			goto out;
+		}
+		split_pmd(pmd, pte);
+		__pmd_populate(pmd, __pa(pte), PMD_TYPE_TABLE);
+	}
+
+	pte = pte_offset_kernel(pmd, addr);
+	if (pte_none(*pte))
+		goto out;
+
+	flush_tlb_all();
+	ret = 0;
+
+out:
+	return ret;
+}
+#else
+static int check_address(unsigned long addr)
+{
+	if (addr < MODULES_VADDR || addr >= MODULES_END)
+		return -EINVAL;
+
+	return 0;
+}
+#endif
+
 static int change_memory_common(unsigned long addr, int numpages,
 				pgprot_t set_mask, pgprot_t clear_mask)
 {
@@ -45,17 +108,18 @@ static int change_memory_common(unsigned long addr, int numpages,
 	int ret;
 	struct page_change_data data;
 
+	if (addr < PAGE_OFFSET && !is_vmalloc_addr((void *)addr))
+		return -EINVAL;
+
 	if (!IS_ALIGNED(addr, PAGE_SIZE)) {
 		start &= PAGE_MASK;
 		end = start + size;
 		WARN_ON_ONCE(1);
 	}
 
-	if (start < MODULES_VADDR || start >= MODULES_END)
-		return -EINVAL;
-
-	if (end < MODULES_VADDR || end >= MODULES_END)
-		return -EINVAL;
+	ret = check_address(addr);
+	if (ret)
+		return ret;
 
 	data.set_mask = set_mask;
 	data.clear_mask = clear_mask;
-- 
2.4.3