arm64: implement PKEYS support

Implement the PKEYS interface, using the Permission Overlay Extension.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20240822151113.1479789-19-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/mmu.h

@@ -25,6 +25,7 @@ typedef struct {
 	refcount_t	pinned;
 	void		*vdso;
 	unsigned long	flags;
+	u8		pkey_allocation_map;
 } mm_context_t;
 
 /*

arch/arm64/include/asm/mmu_context.h

@@ -15,12 +15,12 @@
 #include <linux/sched/hotplug.h>
 #include <linux/mm_types.h>
 #include <linux/pgtable.h>
+#include <linux/pkeys.h>
 
 #include <asm/cacheflush.h>
 #include <asm/cpufeature.h>
 #include <asm/daifflags.h>
 #include <asm/proc-fns.h>
-#include <asm-generic/mm_hooks.h>
 #include <asm/cputype.h>
 #include <asm/sysreg.h>
 #include <asm/tlbflush.h>
@@ -175,9 +175,36 @@ init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 {
 	atomic64_set(&mm->context.id, 0);
 	refcount_set(&mm->context.pinned, 0);
+
+	/* pkey 0 is the default, so always reserve it. */
+	mm->context.pkey_allocation_map = BIT(0);
+
 	return 0;
 }
 
+static inline void arch_dup_pkeys(struct mm_struct *oldmm,
+				  struct mm_struct *mm)
+{
+	/* Duplicate the oldmm pkey state in mm: */
+	mm->context.pkey_allocation_map = oldmm->context.pkey_allocation_map;
+}
+
+static inline int arch_dup_mmap(struct mm_struct *oldmm, struct mm_struct *mm)
+{
+	arch_dup_pkeys(oldmm, mm);
+
+	return 0;
+}
+
+static inline void arch_exit_mmap(struct mm_struct *mm)
+{
+}
+
+static inline void arch_unmap(struct mm_struct *mm,
+			unsigned long start, unsigned long end)
+{
+}
+
 #ifdef CONFIG_ARM64_SW_TTBR0_PAN
 static inline void update_saved_ttbr0(struct task_struct *tsk,
 				      struct mm_struct *mm)
@@ -267,6 +294,23 @@ static inline unsigned long mm_untag_mask(struct mm_struct *mm)
 	return -1UL >> 8;
 }
 
+/*
+ * Only enforce protection keys on the current process, because there is no
+ * user context to access POR_EL0 for another address space.
+ */
+static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
+		bool write, bool execute, bool foreign)
+{
+	if (!system_supports_poe())
+		return true;
+
+	/* allow access if the VMA is not one from this process */
+	if (foreign || vma_is_foreign(vma))
+		return true;
+
+	return por_el0_allows_pkey(vma_pkey(vma), write, execute);
+}
+
 #include <asm-generic/mmu_context.h>
 
 #endif /* !__ASSEMBLY__ */

arch/arm64/include/asm/pgtable.h

@@ -34,6 +34,7 @@
 
 #include <asm/cmpxchg.h>
 #include <asm/fixmap.h>
+#include <asm/por.h>
 #include <linux/mmdebug.h>
 #include <linux/mm_types.h>
 #include <linux/sched.h>
@@ -149,6 +150,24 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 #define pte_accessible(mm, pte)	\
 	(mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
 
+static inline bool por_el0_allows_pkey(u8 pkey, bool write, bool execute)
+{
+	u64 por;
+
+	if (!system_supports_poe())
+		return true;
+
+	por = read_sysreg_s(SYS_POR_EL0);
+
+	if (write)
+		return por_elx_allows_write(por, pkey);
+
+	if (execute)
+		return por_elx_allows_exec(por, pkey);
+
+	return por_elx_allows_read(por, pkey);
+}
+
 /*
  * p??_access_permitted() is true for valid user mappings (PTE_USER
  * bit set, subject to the write permission check). For execute-only
@@ -159,7 +178,8 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 #define pte_access_permitted_no_overlay(pte, write) \
 	(((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER)) && (!(write) || pte_write(pte)))
 #define pte_access_permitted(pte, write) \
-	pte_access_permitted_no_overlay(pte, write)
+	(pte_access_permitted_no_overlay(pte, write) && \
+	por_el0_allows_pkey(FIELD_GET(PTE_PO_IDX_MASK, pte_val(pte)), write, false))
 #define pmd_access_permitted(pmd, write) \
 	(pte_access_permitted(pmd_pte(pmd), (write)))
 #define pud_access_permitted(pud, write) \

arch/arm64/include/asm/pkeys.h

@@ -0,0 +1,108 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2023 Arm Ltd.
+ *
+ * Based on arch/x86/include/asm/pkeys.h
+ */
+
+#ifndef _ASM_ARM64_PKEYS_H
+#define _ASM_ARM64_PKEYS_H
+
+#define ARCH_VM_PKEY_FLAGS (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2)
+
+#define arch_max_pkey() 8
+
+int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
+		unsigned long init_val);
+
+static inline bool arch_pkeys_enabled(void)
+{
+	return false;
+}
+
+static inline int vma_pkey(struct vm_area_struct *vma)
+{
+	return (vma->vm_flags & ARCH_VM_PKEY_FLAGS) >> VM_PKEY_SHIFT;
+}
+
+static inline int arch_override_mprotect_pkey(struct vm_area_struct *vma,
+		int prot, int pkey)
+{
+	if (pkey != -1)
+		return pkey;
+
+	return vma_pkey(vma);
+}
+
+static inline int execute_only_pkey(struct mm_struct *mm)
+{
+	// Execute-only mappings are handled by EPAN/FEAT_PAN3.
+	WARN_ON_ONCE(!cpus_have_final_cap(ARM64_HAS_EPAN));
+
+	return -1;
+}
+
+#define mm_pkey_allocation_map(mm)	(mm)->context.pkey_allocation_map
+#define mm_set_pkey_allocated(mm, pkey) do {		\
+	mm_pkey_allocation_map(mm) |= (1U << pkey);	\
+} while (0)
+#define mm_set_pkey_free(mm, pkey) do {			\
+	mm_pkey_allocation_map(mm) &= ~(1U << pkey);	\
+} while (0)
+
+static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
+{
+	/*
+	 * "Allocated" pkeys are those that have been returned
+	 * from pkey_alloc() or pkey 0 which is allocated
+	 * implicitly when the mm is created.
+	 */
+	if (pkey < 0 || pkey >= arch_max_pkey())
+		return false;
+
+	return mm_pkey_allocation_map(mm) & (1U << pkey);
+}
+
+/*
+ * Returns a positive, 3-bit key on success, or -1 on failure.
+ */
+static inline int mm_pkey_alloc(struct mm_struct *mm)
+{
+	/*
+	 * Note: this is the one and only place we make sure
+	 * that the pkey is valid as far as the hardware is
+	 * concerned.  The rest of the kernel trusts that
+	 * only good, valid pkeys come out of here.
+	 */
+	u8 all_pkeys_mask = GENMASK(arch_max_pkey() - 1, 0);
+	int ret;
+
+	if (!arch_pkeys_enabled())
+		return -1;
+
+	/*
+	 * Are we out of pkeys?  We must handle this specially
+	 * because ffz() behavior is undefined if there are no
+	 * zeros.
+	 */
+	if (mm_pkey_allocation_map(mm) == all_pkeys_mask)
+		return -1;
+
+	ret = ffz(mm_pkey_allocation_map(mm));
+
+	mm_set_pkey_allocated(mm, ret);
+
+	return ret;
+}
+
+static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
+{
+	if (!mm_pkey_is_allocated(mm, pkey))
+		return -EINVAL;
+
+	mm_set_pkey_free(mm, pkey);
+
+	return 0;
+}
+
+#endif /* _ASM_ARM64_PKEYS_H */

arch/arm64/include/asm/por.h

@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2023 Arm Ltd.
+ */
+
+#ifndef _ASM_ARM64_POR_H
+#define _ASM_ARM64_POR_H
+
+#define POR_BITS_PER_PKEY		4
+#define POR_ELx_IDX(por_elx, idx)	(((por_elx) >> ((idx) * POR_BITS_PER_PKEY)) & 0xf)
+
+static inline bool por_elx_allows_read(u64 por, u8 pkey)
+{
+	u8 perm = POR_ELx_IDX(por, pkey);
+
+	return perm & POE_R;
+}
+
+static inline bool por_elx_allows_write(u64 por, u8 pkey)
+{
+	u8 perm = POR_ELx_IDX(por, pkey);
+
+	return perm & POE_W;
+}
+
+static inline bool por_elx_allows_exec(u64 por, u8 pkey)
+{
+	u8 perm = POR_ELx_IDX(por, pkey);
+
+	return perm & POE_X;
+}
+
+#endif /* _ASM_ARM64_POR_H */

arch/arm64/include/uapi/asm/mman.h

@@ -7,4 +7,13 @@
 #define PROT_BTI	0x10		/* BTI guarded page */
 #define PROT_MTE	0x20		/* Normal Tagged mapping */
 
+/* Override any generic PKEY permission defines */
+#define PKEY_DISABLE_EXECUTE	0x4
+#define PKEY_DISABLE_READ	0x8
+#undef PKEY_ACCESS_MASK
+#define PKEY_ACCESS_MASK       (PKEY_DISABLE_ACCESS |\
+				PKEY_DISABLE_WRITE  |\
+				PKEY_DISABLE_READ   |\
+				PKEY_DISABLE_EXECUTE)
+
 #endif /* ! _UAPI__ASM_MMAN_H */

arch/arm64/mm/mmu.c

@@ -25,6 +25,7 @@
 #include <linux/vmalloc.h>
 #include <linux/set_memory.h>
 #include <linux/kfence.h>
+#include <linux/pkeys.h>
 
 #include <asm/barrier.h>
 #include <asm/cputype.h>
@@ -1549,3 +1550,47 @@ void __cpu_replace_ttbr1(pgd_t *pgdp, bool cnp)
 
 	cpu_uninstall_idmap();
 }
+
+#ifdef CONFIG_ARCH_HAS_PKEYS
+int arch_set_user_pkey_access(struct task_struct *tsk, int pkey, unsigned long init_val)
+{
+	u64 new_por = POE_RXW;
+	u64 old_por;
+	u64 pkey_shift;
+
+	if (!system_supports_poe())
+		return -ENOSPC;
+
+	/*
+	 * This code should only be called with valid 'pkey'
+	 * values originating from in-kernel users.  Complain
+	 * if a bad value is observed.
+	 */
+	if (WARN_ON_ONCE(pkey >= arch_max_pkey()))
+		return -EINVAL;
+
+	/* Set the bits we need in POR:  */
+	new_por = POE_RXW;
+	if (init_val & PKEY_DISABLE_WRITE)
+		new_por &= ~POE_W;
+	if (init_val & PKEY_DISABLE_ACCESS)
+		new_por &= ~POE_RW;
+	if (init_val & PKEY_DISABLE_READ)
+		new_por &= ~POE_R;
+	if (init_val & PKEY_DISABLE_EXECUTE)
+		new_por &= ~POE_X;
+
+	/* Shift the bits in to the correct place in POR for pkey: */
+	pkey_shift = pkey * POR_BITS_PER_PKEY;
+	new_por <<= pkey_shift;
+
+	/* Get old POR and mask off any old bits in place: */
+	old_por = read_sysreg_s(SYS_POR_EL0);
+	old_por &= ~(POE_MASK << pkey_shift);
+
+	/* Write old part along with new part: */
+	write_sysreg_s(old_por | new_por, SYS_POR_EL0);
+
+	return 0;
+}
+#endif