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arm: hwcap: add new hwcap for identifying PXN bits

PATCH RESEND New Jungseung Lee 2 participants 3 messages Oct 24, 2014 Arm (32-bit)

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Jungseung Lee

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#0 lore raw

PATCH RESEND 1/2 arm: hwcap: add new hwcap for identifying PXN bits

Modern ARMv7-A/R cores can optionally implement below new hardware feature:

- PXN:
Privileged execute-never(PXN) is a security feature. PXN bit determines whether the processor can execute software from the region. This is effective solution against ret2usr attack. On an implementation that does not include the LPAE, PXN is optionally supported.

This patch adds new HWCAP defines to describe new feature.

In ARM world, elf_hwcap is used both internally in the kernel and for user space to keep track of optional features. HWCAP_PXN is exceptionally not exposed to user space since the user doesn't need to know.

Signed-off-by Jungseung Lee <[email protected]>
---
 arch/arm/include/uapi/asm/hwcap.h | 1 +
 arch/arm/kernel/setup.c           | 5 +++++
 2 files changed, 6 insertions(+)
arch/arm/include/uapi/asm/hwcap.h +1 -0
index 20d12f2..7075207 100644
@@ -27,6 +27,7 @@
 #define HWCAP_IDIV	(HWCAP_IDIVA | HWCAP_IDIVT)
 #define HWCAP_LPAE	(1 << 20)
 #define HWCAP_EVTSTRM	(1 << 21)
+#define HWCAP_PXN   (1 << 22)       /* set if support the PXN attribute */
 
 /*
  * HWCAP2 flags - for elf_hwcap2 (in kernel) and AT_HWCAP2
arch/arm/kernel/setup.c +5 -0
index c031063..22cd19a 100644
@@ -390,6 +390,10 @@ static void __init cpuid_init_hwcaps(void)
 
 	/* LPAE implies atomic ldrd/strd instructions */
 	vmsa = (read_cpuid_ext(CPUID_EXT_MMFR0) & 0xf) >> 0;
+
+	if (vmsa >= 4)
+		elf_hwcap |= HWCAP_PXN;
+
 	if (vmsa >= 5)
 		elf_hwcap |= HWCAP_LPAE;
 }

@@ -1015,6 +1019,7 @@ static const char *hwcap_str[] = {
 	"vfpd32",
 	"lpae",
 	"evtstrm",
+	"",	/* HWCAP_PXN : not expose to user */
 	NULL
 };
Signature · 1 line
1.9.1
author
#1 lore raw

PATCH RESEND 2/2 arm: Support for the PXN CPU feature on ARMv7.

This patch set PXN bit on user page table for preventing user code execution with privilege mode. This is effective solution against ret2usr attack.

Signed-off-by Jungseung Lee <[email protected]>
---
 arch/arm/include/asm/pgalloc.h              | 8 +++++++-
 arch/arm/include/asm/pgtable-2level-hwdef.h | 2 ++
 2 files changed, 9 insertions(+), 1 deletion(-)
arch/arm/include/asm/pgalloc.h +7 -1
index 78a7793..f4f4fd3 100644
@@ -157,7 +157,13 @@ pmd_populate_kernel(struct mm_struct *mm, pmd_t *pmdp, pte_t *ptep)
 static inline void
 pmd_populate(struct mm_struct *mm, pmd_t *pmdp, pgtable_t ptep)
 {
-	__pmd_populate(pmdp, page_to_phys(ptep), _PAGE_USER_TABLE);
+#ifdef CONFIG_CPU_V7
+	if (elf_hwcap & HWCAP_PXN)
+		__pmd_populate(pmdp, page_to_phys(ptep),
+				_PAGE_USER_TABLE | PMD_PXNTABLE);
+	else
+#endif
+		__pmd_populate(pmdp, page_to_phys(ptep), _PAGE_USER_TABLE);
 }
 #define pmd_pgtable(pmd) pmd_page(pmd)
arch/arm/include/asm/pgtable-2level-hwdef.h +2 -0
index 5cfba15..5e68278 100644
@@ -20,12 +20,14 @@
 #define PMD_TYPE_FAULT		(_AT(pmdval_t, 0) << 0)
 #define PMD_TYPE_TABLE		(_AT(pmdval_t, 1) << 0)
 #define PMD_TYPE_SECT		(_AT(pmdval_t, 2) << 0)
+#define PMD_PXNTABLE		(_AT(pmdval_t, 1) << 2)     /* v7 */
 #define PMD_BIT4		(_AT(pmdval_t, 1) << 4)
 #define PMD_DOMAIN(x)		(_AT(pmdval_t, (x)) << 5)
 #define PMD_PROTECTION		(_AT(pmdval_t, 1) << 9)		/* v5 */
 /*
  *   - section
  */
+#define PMD_SECT_PXN    (_AT(pmdval_t, 1) << 0)     /* v7 */
 #define PMD_SECT_BUFFERABLE	(_AT(pmdval_t, 1) << 2)
 #define PMD_SECT_CACHEABLE	(_AT(pmdval_t, 1) << 3)
 #define PMD_SECT_XN		(_AT(pmdval_t, 1) << 4)		/* v6 */
Signature · 1 line
1.9.1
#2 lore raw

PATCH RESEND 1/2 arm: hwcap: add new hwcap for identifying PXN bits

In reply to Jungseung Lee
Quoted 10 lines · Jungseung Lee

Modern ARMv7-A/R cores can optionally implement below new hardware feature:

- PXN:
Privileged execute-never(PXN) is a security feature. PXN bit determines whether the processor can execute software from the region. This is effective solution against ret2usr attack. On an implementation that does not include the LPAE, PXN is optionally supported.

Quoted 6 lines

This patch adds new HWCAP defines to describe new feature.

In ARM world, elf_hwcap is used both internally in the kernel and for user space to keep track of optional features. HWCAP_PXN is exceptionally not exposed to user space since the user doesn't need to know.

Still no good. Adding it into elf_hwcap always exposes it to the user. Plus, these are a limited commodity, so we don't want to waste them.

Signature · 2 lines
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