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    Subject[PATCH 05/10] x86/pks: Add PKS kernel API
    Date
    From: Fenghua Yu <fenghua.yu@intel.com>

    PKS allows kernel users to define domains of page mappings which have
    additional protections beyond the paging protections.

    Add an API to allocate, use, and free a protection key which identifies
    such a domain. Export 5 new symbols pks_key_alloc(), pks_mknoaccess(),
    pks_mkread(), pks_mkrdwr(), and pks_key_free(). Add 2 new macros;
    PAGE_KERNEL_PKEY(key) and _PAGE_PKEY(pkey).

    Update the protection key documentation to cover pkeys on supervisor
    pages.

    Co-developed-by: Ira Weiny <ira.weiny@intel.com>
    Signed-off-by: Ira Weiny <ira.weiny@intel.com>
    Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>

    ---
    Changes from RFC V3
    Per Dave Hansen
    Put WARN_ON_ONCE in pks_key_free()
    s/pks_mknoaccess/pks_mk_noaccess/
    s/pks_mkread/pks_mk_readonly/
    s/pks_mkrdwr/pks_mk_readwrite/
    Change return pks_key_alloc() to EOPNOTSUPP when not
    supported or configured
    Per Peter Zijlstra
    Remove unneeded preempt disable/enable
    ---
    Documentation/core-api/protection-keys.rst | 101 ++++++++++++++---
    arch/x86/include/asm/pgtable_types.h | 12 ++
    arch/x86/include/asm/pkeys.h | 11 ++
    arch/x86/include/asm/pkeys_common.h | 4 +
    arch/x86/mm/pkeys.c | 123 +++++++++++++++++++++
    include/linux/pgtable.h | 4 +
    include/linux/pkeys.h | 22 ++++
    7 files changed, 259 insertions(+), 18 deletions(-)

    diff --git a/Documentation/core-api/protection-keys.rst b/Documentation/core-api/protection-keys.rst
    index ec575e72d0b2..e6cb29dda5b8 100644
    --- a/Documentation/core-api/protection-keys.rst
    +++ b/Documentation/core-api/protection-keys.rst
    @@ -4,25 +4,33 @@
    Memory Protection Keys
    ======================

    -Memory Protection Keys for Userspace (PKU aka PKEYs) is a feature
    -which is found on Intel's Skylake (and later) "Scalable Processor"
    -Server CPUs. It will be available in future non-server Intel parts
    -and future AMD processors.
    -
    -For anyone wishing to test or use this feature, it is available in
    -Amazon's EC2 C5 instances and is known to work there using an Ubuntu
    -17.04 image.
    -
    Memory Protection Keys provides a mechanism for enforcing page-based
    protections, but without requiring modification of the page tables
    -when an application changes protection domains. It works by
    -dedicating 4 previously ignored bits in each page table entry to a
    -"protection key", giving 16 possible keys.
    +when an application changes protection domains.
    +
    +PKeys Userspace (PKU) is a feature which is found on Intel's Skylake "Scalable
    +Processor" Server CPUs and later. And It will be available in future
    +non-server Intel parts and future AMD processors.
    +
    +Future Intel processors will support Protection Keys for Supervisor pages
    +(PKS).
    +
    +For anyone wishing to test or use user space pkeys, it is available in Amazon's
    +EC2 C5 instances and is known to work there using an Ubuntu 17.04 image.
    +
    +pkeys work by dedicating 4 previously Reserved bits in each page table entry to
    +a "protection key", giving 16 possible keys. User and Supervisor pages are
    +treated separately.
    +
    +Protections for each page are controlled with per CPU registers for each type
    +of page User and Supervisor. Each of these 32 bit register stores two separate
    +bits (Access Disable and Write Disable) for each key.

    -There is also a new user-accessible register (PKRU) with two separate
    -bits (Access Disable and Write Disable) for each key. Being a CPU
    -register, PKRU is inherently thread-local, potentially giving each
    -thread a different set of protections from every other thread.
    +For Userspace the register is user-accessible (rdpkru/wrpkru). For
    +Supervisor, the register (MSR_IA32_PKRS) is accessible only to the kernel.
    +
    +Being a CPU register, pkeys are inherently thread-local, potentially giving
    +each thread an independent set of protections from every other thread.

    There are two new instructions (RDPKRU/WRPKRU) for reading and writing
    to the new register. The feature is only available in 64-bit mode,
    @@ -30,8 +38,11 @@ even though there is theoretically space in the PAE PTEs. These
    permissions are enforced on data access only and have no effect on
    instruction fetches.

    -Syscalls
    -========
    +For kernel space rdmsr/wrmsr are used to access the kernel MSRs.
    +
    +
    +Syscalls for user space keys
    +============================

    There are 3 system calls which directly interact with pkeys::

    @@ -98,3 +109,57 @@ with a read()::
    The kernel will send a SIGSEGV in both cases, but si_code will be set
    to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
    the plain mprotect() permissions are violated.
    +
    +
    +Kernel API for PKS support
    +==========================
    +
    +The following interface is used to allocate, use, and free a pkey which defines
    +a 'protection domain' within the kernel. Setting a pkey value in a supervisor
    +mapping adds that mapping to the protection domain.
    +
    + int pks_key_alloc(const char * const pkey_user);
    + #define PAGE_KERNEL_PKEY(pkey)
    + #define _PAGE_KEY(pkey)
    + void pks_mk_noaccess(int pkey);
    + void pks_mk_readonly(int pkey);
    + void pks_mk_readwrite(int pkey);
    + void pks_key_free(int pkey);
    +
    +pks_key_alloc() allocates keys dynamically to allow better use of the limited
    +key space.
    +
    +Callers of pks_key_alloc() _must_ be prepared for it to fail and take
    +appropriate action. This is due mainly to the fact that PKS may not be
    +available on all arch's. Failure to check the return of pks_key_alloc() and
    +using any of the rest of the API is undefined.
    +
    +Kernel users must set the PTE permissions in the page table entries for the
    +mappings they want to protect. This can be done with PAGE_KERNEL_PKEY() or
    +_PAGE_KEY().
    +
    +The pks_mk*() family of calls allows kernel users the ability to change the
    +protections for the domain identified by the pkey specified. 3 states are
    +available pks_mk_noaccess(), pks_mk_readonly(), and pks_mk_readwrite() which
    +set the access to none, read, and read/write respectively.
    +
    +Finally, pks_key_free() allows a user to return the key to the allocator for
    +use by others.
    +
    +The interface maintains pks_mk_noaccess() (Access Disabled (AD=1)) for all keys
    +not currently allocated. Therefore, the user can depend on access being
    +disabled when pks_key_alloc() returns a key and the user should remove mappings
    +from the domain (remove the pkey from the PTE) prior to calling pks_key_free().
    +
    +It should be noted that the underlying WRMSR(MSR_IA32_PKRS) is not serializing
    +but still maintains ordering properties similar to WRPKRU. Thus it is safe to
    +immediately use a mapping when the pks_mk*() functions returns.
    +
    +The current SDM section on PKRS needs updating but should be the same as that
    +of WRPKRU. So to quote from the WRPKRU text:
    +
    + WRPKRU will never execute transiently. Memory accesses
    + affected by PKRU register will not execute (even transiently)
    + until all prior executions of WRPKRU have completed execution
    + and updated the PKRU register.
    +
    diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
    index 816b31c68550..c9fdfbdcbbfb 100644
    --- a/arch/x86/include/asm/pgtable_types.h
    +++ b/arch/x86/include/asm/pgtable_types.h
    @@ -73,6 +73,12 @@
    _PAGE_PKEY_BIT2 | \
    _PAGE_PKEY_BIT3)

    +#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
    +#define _PAGE_PKEY(pkey) (_AT(pteval_t, pkey) << _PAGE_BIT_PKEY_BIT0)
    +#else
    +#define _PAGE_PKEY(pkey) (_AT(pteval_t, 0))
    +#endif
    +
    #if defined(CONFIG_X86_64) || defined(CONFIG_X86_PAE)
    #define _PAGE_KNL_ERRATUM_MASK (_PAGE_DIRTY | _PAGE_ACCESSED)
    #else
    @@ -229,6 +235,12 @@ enum page_cache_mode {
    #define PAGE_KERNEL_IO __pgprot_mask(__PAGE_KERNEL_IO)
    #define PAGE_KERNEL_IO_NOCACHE __pgprot_mask(__PAGE_KERNEL_IO_NOCACHE)

    +#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
    +#define PAGE_KERNEL_PKEY(pkey) __pgprot_mask(__PAGE_KERNEL | _PAGE_PKEY(pkey))
    +#else
    +#define PAGE_KERNEL_PKEY(pkey) PAGE_KERNEL
    +#endif
    +
    #endif /* __ASSEMBLY__ */

    /* xwr */
    diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
    index 4526245b03e5..92684092ee31 100644
    --- a/arch/x86/include/asm/pkeys.h
    +++ b/arch/x86/include/asm/pkeys.h
    @@ -3,6 +3,7 @@
    #define _ASM_X86_PKEYS_H

    #include <asm/pkeys_common.h>
    +#include <asm-generic/mman-common.h>

    #define ARCH_DEFAULT_PKEY 0

    @@ -138,4 +139,14 @@ static inline int vma_pkey(struct vm_area_struct *vma)

    u32 update_pkey_val(u32 pk_reg, int pkey, unsigned int flags);

    +#ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
    +int pks_key_alloc(const char *const pkey_user);
    +void pks_key_free(int pkey);
    +
    +void pks_mk_noaccess(int pkey);
    +void pks_mk_readonly(int pkey);
    +void pks_mk_readwrite(int pkey);
    +
    +#endif /* CONFIG_ARCH_HAS_SUPERVISOR_PKEYS */
    +
    #endif /*_ASM_X86_PKEYS_H */
    diff --git a/arch/x86/include/asm/pkeys_common.h b/arch/x86/include/asm/pkeys_common.h
    index 801a75615209..cd492c23b28c 100644
    --- a/arch/x86/include/asm/pkeys_common.h
    +++ b/arch/x86/include/asm/pkeys_common.h
    @@ -26,6 +26,10 @@
    PKR_AD_KEY(10) | PKR_AD_KEY(11) | PKR_AD_KEY(12) | \
    PKR_AD_KEY(13) | PKR_AD_KEY(14) | PKR_AD_KEY(15))

    +/* PKS supports 16 keys. Key 0 is reserved for the kernel. */
    +#define PKS_KERN_DEFAULT_KEY 0
    +#define PKS_NUM_KEYS 16
    +
    #ifdef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
    void write_pkrs(u32 new_pkrs);
    #else
    diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
    index 76a62419c446..fd5c4d34c3a5 100644
    --- a/arch/x86/mm/pkeys.c
    +++ b/arch/x86/mm/pkeys.c
    @@ -3,6 +3,9 @@
    * Intel Memory Protection Keys management
    * Copyright (c) 2015, Intel Corporation.
    */
    +#undef pr_fmt
    +#define pr_fmt(fmt) "x86/pkeys: " fmt
    +
    #include <linux/debugfs.h> /* debugfs_create_u32() */
    #include <linux/mm_types.h> /* mm_struct, vma, etc... */
    #include <linux/pkeys.h> /* PKEY_* */
    @@ -231,6 +234,7 @@ u32 update_pkey_val(u32 pk_reg, int pkey, unsigned int flags)

    return pk_reg;
    }
    +EXPORT_SYMBOL_GPL(update_pkey_val);

    DEFINE_PER_CPU(u32, pkrs_cache);

    @@ -262,3 +266,122 @@ void write_pkrs(u32 new_pkrs)
    }
    put_cpu_ptr(pkrs);
    }
    +EXPORT_SYMBOL_GPL(write_pkrs);
    +
    +/**
    + * Do not call this directly, see pks_mk*() below.
    + *
    + * @pkey: Key for the domain to change
    + * @protection: protection bits to be used
    + *
    + * Protection utilizes the same protection bits specified for User pkeys
    + * PKEY_DISABLE_ACCESS
    + * PKEY_DISABLE_WRITE
    + *
    + */
    +static inline void pks_update_protection(int pkey, unsigned long protection)
    +{
    + current->thread.saved_pkrs = update_pkey_val(current->thread.saved_pkrs,
    + pkey, protection);
    + write_pkrs(current->thread.saved_pkrs);
    +}
    +
    +/**
    + * PKS access control functions
    + *
    + * Change the access of the domain specified by the pkey. These are global
    + * updates. They only affects the current running thread. It is undefined and
    + * a bug for users to call this without having allocated a pkey and using it as
    + * pkey here.
    + *
    + * pks_mk_noaccess()
    + * Disable all access to the domain
    + * pks_mk_readonly()
    + * Make the domain Read only
    + * pks_mk_readwrite()
    + * Make the domain Read/Write
    + *
    + * @pkey the pkey for which the access should change.
    + *
    + */
    +void pks_mk_noaccess(int pkey)
    +{
    + pks_update_protection(pkey, PKEY_DISABLE_ACCESS);
    +}
    +EXPORT_SYMBOL_GPL(pks_mk_noaccess);
    +
    +void pks_mk_readonly(int pkey)
    +{
    + pks_update_protection(pkey, PKEY_DISABLE_WRITE);
    +}
    +EXPORT_SYMBOL_GPL(pks_mk_readonly);
    +
    +void pks_mk_readwrite(int pkey)
    +{
    + pks_update_protection(pkey, 0);
    +}
    +EXPORT_SYMBOL_GPL(pks_mk_readwrite);
    +
    +static const char pks_key_user0[] = "kernel";
    +
    +/* Store names of allocated keys for debug. Key 0 is reserved for the kernel. */
    +static const char *pks_key_users[PKS_NUM_KEYS] = {
    + pks_key_user0
    +};
    +
    +/*
    + * Each key is represented by a bit. Bit 0 is set for key 0 and reserved for
    + * its use. We use ulong for the bit operations but only 16 bits are used.
    + */
    +static unsigned long pks_key_allocation_map = 1 << PKS_KERN_DEFAULT_KEY;
    +
    +/*
    + * pks_key_alloc - Allocate a PKS key
    + *
    + * @pkey_user: String stored for debugging of key exhaustion. The caller is
    + * responsible to maintain this memory until pks_key_free().
    + */
    +int pks_key_alloc(const char * const pkey_user)
    +{
    + int nr;
    +
    + if (!cpu_feature_enabled(X86_FEATURE_PKS))
    + return -EOPNOTSUPP;
    +
    + while (1) {
    + nr = find_first_zero_bit(&pks_key_allocation_map, PKS_NUM_KEYS);
    + if (nr >= PKS_NUM_KEYS) {
    + pr_info("Cannot allocate supervisor key for %s.\n",
    + pkey_user);
    + return -ENOSPC;
    + }
    + if (!test_and_set_bit_lock(nr, &pks_key_allocation_map))
    + break;
    + }
    +
    + /* for debugging key exhaustion */
    + pks_key_users[nr] = pkey_user;
    +
    + return nr;
    +}
    +EXPORT_SYMBOL_GPL(pks_key_alloc);
    +
    +/*
    + * pks_key_free - Free a previously allocate PKS key
    + *
    + * @pkey: Key to be free'ed
    + */
    +void pks_key_free(int pkey)
    +{
    + if (!cpu_feature_enabled(X86_FEATURE_PKS))
    + return;
    +
    + if (WARN_ON_ONCE(pkey >= PKS_NUM_KEYS || pkey <= PKS_KERN_DEFAULT_KEY))
    + return;
    +
    + /* Restore to default of no access */
    + pks_mk_noaccess(pkey);
    + pks_key_users[pkey] = NULL;
    + __clear_bit(pkey, &pks_key_allocation_map);
    +}
    +EXPORT_SYMBOL_GPL(pks_key_free);
    diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
    index 38c33eabea89..cd72d73e8e1c 100644
    --- a/include/linux/pgtable.h
    +++ b/include/linux/pgtable.h
    @@ -1402,6 +1402,10 @@ static inline bool arch_has_pfn_modify_check(void)
    # define PAGE_KERNEL_EXEC PAGE_KERNEL
    #endif

    +#ifndef PAGE_KERNEL_PKEY
    +#define PAGE_KERNEL_PKEY(pkey) PAGE_KERNEL
    +#endif
    +
    /*
    * Page Table Modification bits for pgtbl_mod_mask.
    *
    diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
    index 2955ba976048..4de67ce765b0 100644
    --- a/include/linux/pkeys.h
    +++ b/include/linux/pkeys.h
    @@ -50,4 +50,26 @@ static inline void copy_init_pkru_to_fpregs(void)

    #endif /* ! CONFIG_ARCH_HAS_PKEYS */

    +#ifndef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
    +static inline int pks_key_alloc(const char * const pkey_user)
    +{
    + return -EOPNOTSUPP;
    +}
    +static inline void pks_key_free(int pkey)
    +{
    +}
    +static inline void pks_mk_noaccess(int pkey)
    +{
    + WARN_ON_ONCE(1);
    +}
    +static inline void pks_mk_readonly(int pkey)
    +{
    + WARN_ON_ONCE(1);
    +}
    +static inline void pks_mk_readwrite(int pkey)
    +{
    + WARN_ON_ONCE(1);
    +}
    +#endif /* ! CONFIG_ARCH_HAS_SUPERVISOR_PKEYS */
    +
    #endif /* _LINUX_PKEYS_H */
    --
    2.28.0.rc0.12.gb6a658bd00c9
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