arm64: Support execute-only permissions with Enhanced PAN

Enhanced Privileged Access Never (EPAN) allows Privileged Access Never
to be used with Execute-only mappings.

Absence of such support was a reason for 24cecc377463 ("arm64: Revert
support for execute-only user mappings"). Thus now it can be revisited
and re-enabled.

Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210312173811.58284-2-vladimir.murzin@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 5656e7a..c4d7baf 100644 (file)
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1058,6 +1058,9 @@ config SYS_SUPPORTS_HUGETLBFS
 config ARCH_HAS_CACHE_LINE_SIZE
        def_bool y
 
+config ARCH_HAS_FILTER_PGPROT
+       def_bool y
+
 config ARCH_ENABLE_SPLIT_PMD_PTLOCK
        def_bool y if PGTABLE_LEVELS > 2
 
@@ -1681,6 +1684,20 @@ config ARM64_MTE
 
 endmenu
 
+menu "ARMv8.7 architectural features"
+
+config ARM64_EPAN
+       bool "Enable support for Enhanced Privileged Access Never (EPAN)"
+       default y
+       depends on ARM64_PAN
+       help
+        Enhanced Privileged Access Never (EPAN) allows Privileged
+        Access Never to be used with Execute-only mappings.
+
+        The feature is detected at runtime, and will remain disabled
+        if the cpu does not implement the feature.
+endmenu
+
 config ARM64_SVE
        bool "ARM Scalable Vector Extension support"
        default y

diff --git a/arch/arm64/include/asm/cpucaps.h b/arch/arm64/include/asm/cpucaps.h
index b77d997..9e3ec4d 100644 (file)
--- a/arch/arm64/include/asm/cpucaps.h
+++ b/arch/arm64/include/asm/cpucaps.h
@@ -66,7 +66,8 @@
 #define ARM64_WORKAROUND_1508412               58
 #define ARM64_HAS_LDAPR                                59
 #define ARM64_KVM_PROTECTED_MODE               60
+#define ARM64_HAS_EPAN                         61
 
-#define ARM64_NCAPS                            61
+#define ARM64_NCAPS                            62
 
 #endif /* __ASM_CPUCAPS_H */

diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index 9a65fb5..fab2f57 100644 (file)
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -87,12 +87,13 @@ extern bool arm64_use_ng_mappings;
 #define PAGE_SHARED_EXEC       __pgprot(_PAGE_DEFAULT | PTE_USER | PTE_RDONLY | PTE_NG | PTE_PXN | PTE_WRITE)
 #define PAGE_READONLY          __pgprot(_PAGE_DEFAULT | PTE_USER | PTE_RDONLY | PTE_NG | PTE_PXN | PTE_UXN)
 #define PAGE_READONLY_EXEC     __pgprot(_PAGE_DEFAULT | PTE_USER | PTE_RDONLY | PTE_NG | PTE_PXN)
+#define PAGE_EXECONLY          __pgprot(_PAGE_DEFAULT | PTE_RDONLY | PTE_NG | PTE_PXN)
 
 #define __P000  PAGE_NONE
 #define __P001  PAGE_READONLY
 #define __P010  PAGE_READONLY
 #define __P011  PAGE_READONLY
-#define __P100  PAGE_READONLY_EXEC
+#define __P100  PAGE_EXECONLY
 #define __P101  PAGE_READONLY_EXEC
 #define __P110  PAGE_READONLY_EXEC
 #define __P111  PAGE_READONLY_EXEC
@@ -101,7 +102,7 @@ extern bool arm64_use_ng_mappings;
 #define __S001  PAGE_READONLY
 #define __S010  PAGE_SHARED
 #define __S011  PAGE_SHARED
-#define __S100  PAGE_READONLY_EXEC
+#define __S100  PAGE_EXECONLY
 #define __S101  PAGE_READONLY_EXEC
 #define __S110  PAGE_SHARED_EXEC
 #define __S111  PAGE_SHARED_EXEC

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 4702779..0b10204 100644 (file)
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -113,11 +113,12 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 #define pte_dirty(pte)         (pte_sw_dirty(pte) || pte_hw_dirty(pte))
 
 #define pte_valid(pte)         (!!(pte_val(pte) & PTE_VALID))
+/*
+ * Execute-only user mappings do not have the PTE_USER bit set. All valid
+ * kernel mappings have the PTE_UXN bit set.
+ */
 #define pte_valid_not_user(pte) \
-       ((pte_val(pte) & (PTE_VALID | PTE_USER)) == PTE_VALID)
-#define pte_valid_user(pte) \
-       ((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER))
-
+       ((pte_val(pte) & (PTE_VALID | PTE_USER | PTE_UXN)) == (PTE_VALID | PTE_UXN))
 /*
  * Could the pte be present in the TLB? We must check mm_tlb_flush_pending
  * so that we don't erroneously return false for pages that have been
@@ -130,12 +131,14 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
        (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
 
 /*
- * p??_access_permitted() is true for valid user mappings (subject to the
- * write permission check). PROT_NONE mappings do not have the PTE_VALID bit
- * set.
+ * p??_access_permitted() is true for valid user mappings (PTE_USER
+ * bit set, subject to the write permission check). For execute-only
+ * mappings, like PROT_EXEC with EPAN (both PTE_USER and PTE_UXN bits
+ * not set) must return false. PROT_NONE mappings do not have the
+ * PTE_VALID bit set.
  */
 #define pte_access_permitted(pte, write) \
-       (pte_valid_user(pte) && (!(write) || pte_write(pte)))
+       (((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER)) && (!(write) || pte_write(pte)))
 #define pmd_access_permitted(pmd, write) \
        (pte_access_permitted(pmd_pte(pmd), (write)))
 #define pud_access_permitted(pud, write) \
@@ -995,6 +998,18 @@ static inline bool arch_wants_old_prefaulted_pte(void)
 }
 #define arch_wants_old_prefaulted_pte  arch_wants_old_prefaulted_pte
 
+static inline pgprot_t arch_filter_pgprot(pgprot_t prot)
+{
+       if (cpus_have_const_cap(ARM64_HAS_EPAN))
+               return prot;
+
+       if (pgprot_val(prot) != pgprot_val(PAGE_EXECONLY))
+               return prot;
+
+       return PAGE_READONLY_EXEC;
+}
+
+
 #endif /* !__ASSEMBLY__ */
 
 #endif /* __ASM_PGTABLE_H */

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index d4a5fca..94291c7 100644 (file)
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -597,6 +597,7 @@
        (SCTLR_EL2_RES1 | ENDIAN_SET_EL2)
 
 /* SCTLR_EL1 specific flags. */
+#define SCTLR_EL1_EPAN         (BIT(57))
 #define SCTLR_EL1_ATA0         (BIT(42))
 
 #define SCTLR_EL1_TCF0_SHIFT   38
@@ -637,7 +638,7 @@
         SCTLR_EL1_SED  | SCTLR_ELx_I    | SCTLR_EL1_DZE  | SCTLR_EL1_UCT   | \
         SCTLR_EL1_NTWE | SCTLR_ELx_IESB | SCTLR_EL1_SPAN | SCTLR_ELx_ITFSB | \
         SCTLR_ELx_ATA  | SCTLR_EL1_ATA0 | ENDIAN_SET_EL1 | SCTLR_EL1_UCI   | \
-        SCTLR_EL1_RES1)
+        SCTLR_EL1_EPAN | SCTLR_EL1_RES1)
 
 /* MAIR_ELx memory attributes (used by Linux) */
 #define MAIR_ATTR_DEVICE_nGnRnE                UL(0x00)

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 0660307..2ab0496 100644 (file)
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -1821,6 +1821,18 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
                .cpu_enable = cpu_enable_pan,
        },
 #endif /* CONFIG_ARM64_PAN */
+#ifdef CONFIG_ARM64_EPAN
+       {
+               .desc = "Enhanced Privileged Access Never",
+               .capability = ARM64_HAS_EPAN,
+               .type = ARM64_CPUCAP_SYSTEM_FEATURE,
+               .matches = has_cpuid_feature,
+               .sys_reg = SYS_ID_AA64MMFR1_EL1,
+               .field_pos = ID_AA64MMFR1_PAN_SHIFT,
+               .sign = FTR_UNSIGNED,
+               .min_field_value = 3,
+       },
+#endif /* CONFIG_ARM64_EPAN */
 #ifdef CONFIG_ARM64_LSE_ATOMICS
        {
                .desc = "LSE atomic instructions",

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index f37d4e3..871c82a 100644 (file)
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -527,7 +527,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
        const struct fault_info *inf;
        struct mm_struct *mm = current->mm;
        vm_fault_t fault;
-       unsigned long vm_flags = VM_ACCESS_FLAGS;
+       unsigned long vm_flags;
        unsigned int mm_flags = FAULT_FLAG_DEFAULT;
        unsigned long addr = untagged_addr(far);
 
@@ -544,12 +544,28 @@ static int __kprobes do_page_fault(unsigned long far, unsigned int esr,
        if (user_mode(regs))
                mm_flags |= FAULT_FLAG_USER;
 
+       /*
+        * vm_flags tells us what bits we must have in vma->vm_flags
+        * for the fault to be benign, __do_page_fault() would check
+        * vma->vm_flags & vm_flags and returns an error if the
+        * intersection is empty
+        */
        if (is_el0_instruction_abort(esr)) {
+               /* It was exec fault */
                vm_flags = VM_EXEC;
                mm_flags |= FAULT_FLAG_INSTRUCTION;
        } else if (is_write_abort(esr)) {
+               /* It was write fault */
                vm_flags = VM_WRITE;
                mm_flags |= FAULT_FLAG_WRITE;
+       } else {
+               /* It was read fault */
+               vm_flags = VM_READ;
+               /* Write implies read */
+               vm_flags |= VM_WRITE;
+               /* If EPAN is absent then exec implies read */
+               if (!cpus_have_const_cap(ARM64_HAS_EPAN))
+                       vm_flags |= VM_EXEC;
        }
 
        if (is_ttbr0_addr(addr) && is_el1_permission_fault(addr, esr, regs)) {

diff --git a/mm/mmap.c b/mm/mmap.c
index 3f28759..1d96a21 100644 (file)
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -93,6 +93,12 @@ static void unmap_region(struct mm_struct *mm,
  * MAP_PRIVATE r: (no) no      r: (yes) yes    r: (no) yes     r: (no) yes
  *             w: (no) no      w: (no) no      w: (copy) copy  w: (no) no
  *             x: (no) no      x: (no) yes     x: (no) yes     x: (yes) yes
+ *
+ * On arm64, PROT_EXEC has the following behaviour for both MAP_SHARED and
+ * MAP_PRIVATE (with Enhanced PAN supported):
+ *                                                             r: (no) no
+ *                                                             w: (no) no
+ *                                                             x: (yes) yes
  */
 pgprot_t protection_map[16] __ro_after_init = {
        __P000, __P001, __P010, __P011, __P100, __P101, __P110, __P111,