Re: [PATCH v4 17/29] arm64: implement PKEYS support

[Amit Daniel Kachhap <amitdaniel.kachhap@arm.com>]

On 5/3/24 18:31, Joey Gouly wrote:
> 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>
> ---
>   arch/arm64/include/asm/mmu.h         |   1 +
>   arch/arm64/include/asm/mmu_context.h |  51 ++++++++++++-
>   arch/arm64/include/asm/pgtable.h     |  22 +++++-
>   arch/arm64/include/asm/pkeys.h       | 110 +++++++++++++++++++++++++++
>   arch/arm64/include/asm/por.h         |  33 ++++++++
>   arch/arm64/mm/mmu.c                  |  40 ++++++++++
>   6 files changed, 255 insertions(+), 2 deletions(-)
>   create mode 100644 arch/arm64/include/asm/pkeys.h
>   create mode 100644 arch/arm64/include/asm/por.h
> 
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 65977c7783c5..983afeb4eba5 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/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;
>   
>   /*
> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index c768d16b81a4..cb499db7a97b 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/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 = 0x1;
> +
> +	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 +294,28 @@ static inline unsigned long mm_untag_mask(struct mm_struct *mm)
>   	return -1UL >> 8;
>   }
>   
> +/*
> + * We only want to enforce protection keys on the current process
> + * because we effectively have no access to POR_EL0 for other
> + * processes or any way to tell *which * POR_EL0 in a threaded
> + * process we could use.
> + *
> + * So do not enforce things if the VMA is not from the current
> + * mm, or if we are in a kernel thread.
> + */
> +static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
> +		bool write, bool execute, bool foreign)
> +{
> +	if (!arch_pkeys_enabled())
> +		return true;

The above check can be dropped as the caller of this function
fault_from_pkey() does the same check.

Thanks,
Amit

> +
> +	/* 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__ */
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 2449e4e27ea6..8ee68ff03016 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/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>
> @@ -153,6 +154,24 @@ 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
> @@ -163,7 +182,8 @@ 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) \
> diff --git a/arch/arm64/include/asm/pkeys.h b/arch/arm64/include/asm/pkeys.h
> new file mode 100644
> index 000000000000..a284508a4d02
> --- /dev/null
> +++ b/arch/arm64/include/asm/pkeys.h
> @@ -0,0 +1,110 @@
> +/* 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() 7
> +
> +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)
> +		return false;
> +	if (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 = ((1U << arch_max_pkey()) - 1);
> +	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 */
> diff --git a/arch/arm64/include/asm/por.h b/arch/arm64/include/asm/por.h
> new file mode 100644
> index 000000000000..d6604e0c5c54
> --- /dev/null
> +++ b/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 */
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 495b732d5af3..e50ccc86d150 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/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>
> @@ -1535,3 +1536,42 @@ 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 (!arch_pkeys_enabled())
> +		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:  */
> +	if (init_val & PKEY_DISABLE_ACCESS)
> +		new_por = POE_X;
> +	else if (init_val & PKEY_DISABLE_WRITE)
> +		new_por = POE_RX;
> +
> +	/* 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