diff mbox series

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

Message ID 20201102205320.1458656-6-ira.weiny@intel.com (mailing list archive)
State New
Headers show
Series PKS: Add Protection Keys Supervisor (PKS) support | expand

Commit Message

Ira Weiny Nov. 2, 2020, 8:53 p.m. UTC
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 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(-)

Comments

Greg KH Nov. 3, 2020, 6:50 a.m. UTC | #1
On Mon, Nov 02, 2020 at 12:53:15PM -0800, ira.weiny@intel.com wrote:
> 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 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);

So for panic-on-warn systems, this is ok to reboot the box?

Are you sure, that feels odd...

greg k-h
Ira Weiny Nov. 3, 2020, 5:53 p.m. UTC | #2
On Tue, Nov 03, 2020 at 07:50:24AM +0100, Greg KH wrote:
> On Mon, Nov 02, 2020 at 12:53:15PM -0800, ira.weiny@intel.com wrote:
> > From: Fenghua Yu <fenghua.yu@intel.com>
> > 

[snip]

> > 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);
> 
> So for panic-on-warn systems, this is ok to reboot the box?

I would not expect this to reboot the box no.  But it is a violation of the API
contract.  If pky_key_alloc() returns an error calling any of the other
functions is an error.

> 
> Are you sure, that feels odd...

It does feel odd and downright wrong...  But there are a lot of WARN_ON_ONCE's
out there to catch this type of internal programming error.  Is panic-on-warn
commonly used?

Ira
Greg KH Nov. 3, 2020, 6:14 p.m. UTC | #3
On Tue, Nov 03, 2020 at 09:53:36AM -0800, Ira Weiny wrote:
> On Tue, Nov 03, 2020 at 07:50:24AM +0100, Greg KH wrote:
> > On Mon, Nov 02, 2020 at 12:53:15PM -0800, ira.weiny@intel.com wrote:
> > > From: Fenghua Yu <fenghua.yu@intel.com>
> > > 
> 
> [snip]
> 
> > > 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);
> > 
> > So for panic-on-warn systems, this is ok to reboot the box?
> 
> I would not expect this to reboot the box no.  But it is a violation of the API
> contract.  If pky_key_alloc() returns an error calling any of the other
> functions is an error.
> 
> > 
> > Are you sure, that feels odd...
> 
> It does feel odd and downright wrong...  But there are a lot of WARN_ON_ONCE's
> out there to catch this type of internal programming error.  Is panic-on-warn
> commonly used?

Yes it is, and we are trying to recover from that as it is something
that you should recover from.  Properly handle the error and move on.

thanks,

greg k-h
Ira Weiny Nov. 3, 2020, 6:42 p.m. UTC | #4
On Tue, Nov 03, 2020 at 07:14:07PM +0100, Greg KH wrote:
> On Tue, Nov 03, 2020 at 09:53:36AM -0800, Ira Weiny wrote:
> > On Tue, Nov 03, 2020 at 07:50:24AM +0100, Greg KH wrote:
> > > On Mon, Nov 02, 2020 at 12:53:15PM -0800, ira.weiny@intel.com wrote:
> > > > From: Fenghua Yu <fenghua.yu@intel.com>
> > > > 
> > 
> > [snip]
> > 
> > > > 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);
> > > 
> > > So for panic-on-warn systems, this is ok to reboot the box?
> > 
> > I would not expect this to reboot the box no.  But it is a violation of the API
> > contract.  If pky_key_alloc() returns an error calling any of the other
> > functions is an error.
> > 
> > > 
> > > Are you sure, that feels odd...
> > 
> > It does feel odd and downright wrong...  But there are a lot of WARN_ON_ONCE's
> > out there to catch this type of internal programming error.  Is panic-on-warn
> > commonly used?
> 
> Yes it is, and we are trying to recover from that as it is something
> that you should recover from.  Properly handle the error and move on.

Sorry, I did not know that...  Ok I'll look at the series because I probably
have others I need to change.

Thanks,
Ira
diff mbox series

Patch

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 */