[PATCH V2 05/10] x86/pks: Add PKS kernel API

From: ira . weiny
Date: Mon Nov 02 2020 - 15:54:14 EST

From: Fenghua Yu <fenghua.yu@xxxxxxxxx>

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@xxxxxxxxx>
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>

---
Changes from V1
Per Dave Hansen
Add flags to pks_key_alloc() to help future proof the
interface if/when the key space is exhausted.

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 | 102 ++++++++++++++---
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 | 126 +++++++++++++++++++++
include/linux/pgtable.h | 4 +
include/linux/pkeys.h | 24 ++++
7 files changed, 265 insertions(+), 18 deletions(-)

diff --git a/Documentation/core-api/protection-keys.rst b/Documentation/core-api/protection-keys.rst
index ec575e72d0b2..c4e6c480562f 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 +109,58 @@ 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, int flags);
+ #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. 'flags' alter the allocation based on the users need. Currently
+they can request an exclusive key.
+
+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..f84351b4ac7c 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, int flags);
+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..0dc77409957a 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 +266,125 @@ 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().
+ * @flags: Flags to modify behavior: see pks_alloc_flags
+ *
+ * Returns: pkey if success
+ * -EOPNOTSUPP if pks is not supported or not enabled
+ * -ENOSPC if no keys are available (even for sharing)
+ */
+int pks_key_alloc(const char * const pkey_user, int flags)
+{
+ 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..0959a4c0ca64 100644
--- a/include/linux/pkeys.h
+++ b/include/linux/pkeys.h
@@ -50,4 +50,28 @@ static inline void copy_init_pkru_to_fpregs(void)

#endif /* ! CONFIG_ARCH_HAS_PKEYS */

+#define PKS_FLAG_EXCLUSIVE 0x00
+
+#ifndef CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
+static inline int pks_key_alloc(const char * const pkey_user, int flags)
+{
+ 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