Re: [RFC v5 38/38] Documentation: PowerPC specific updates to memory protection keys

[Dave Hansen]

On 07/05/2017 02:22 PM, Ram Pai wrote:
> Add documentation updates that capture PowerPC specific changes.
> 
> Signed-off-by: Ram Pai <linuxram@xxxxxxxxxx>
> ---
>  Documentation/vm/protection-keys.txt |   85 ++++++++++++++++++++++++++--------
>  1 files changed, 65 insertions(+), 20 deletions(-)
> 
> diff --git a/Documentation/vm/protection-keys.txt b/Documentation/vm/protection-keys.txt
> index b643045..d50b6ab 100644
> --- a/Documentation/vm/protection-keys.txt
> +++ b/Documentation/vm/protection-keys.txt
> @@ -1,21 +1,46 @@
> -Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature
> -which will be found on future Intel CPUs.
> +Memory Protection Keys for Userspace (PKU aka PKEYs) is a CPU feature found in
> +new generation of intel CPUs and on PowerPC 7 and higher CPUs.

Please try not to change the wording here.  I really did mean to
literally put "future Intel CPUs."  Also, you broke my nice wrapping. :)

I'm also thinking that this needs to be more generic.  The ppc _CPU_
feature is *NOT* for userspace-only, right?

>  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.
> -
> -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.
> -
> -There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> -to the new register.  The feature is only available in 64-bit mode,
> -even though there is theoretically space in the PAE PTEs.  These
> -permissions are enforced on data access only and have no effect on
> +protections, but without requiring modification of the page tables when an
> +application changes protection domains.
> +
> +
> +On Intel:
> +
> +	It works by dedicating 4 previously ignored bits in each page table
> +	entry to a "protection key", giving 16 possible keys.
> +
> +	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.
> +
> +	There are two new instructions (RDPKRU/WRPKRU) for reading and writing
> +	to the new register.  The feature is only available in 64-bit mode,
> +	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.
> +
> +
> +On PowerPC:
> +
> +	It works by dedicating 5 page table entry bits to a "protection key",
> +	giving 32 possible keys.
> +
> +	There  is  a  user-accessible  register (AMR)  with  two separate bits;
> +	Access Disable and  Write  Disable, for  each key.  Being  a  CPU
> +	register,  AMR  is inherently  thread-local,  potentially  giving  each
> +	thread a different set of protections from every other thread.  NOTE:
> +	Disabling read permission does not disable write and vice-versa.
> +
> +	The feature is available on 64-bit HPTE mode only.
> +	'mtspr 0xd, mem' reads the AMR register
> +	'mfspr mem, 0xd' writes into the AMR register.

The whole "being a CPU register" bits seem pretty common.  Should it be
in the leading paragraph that is shared?

> +Permissions are enforced on data access only and have no effect on
>  instruction fetches.

Shouldn't we mention the ppc support for execute-disable here too?

Also, *does* this apply to ppc?  You have it both in this common area
and in the x86 portion.

>  =========================== Syscalls ===========================
> @@ -28,9 +53,9 @@ There are 3 system calls which directly interact with pkeys:
>  			  unsigned long prot, int pkey);
>  
>  Before a pkey can be used, it must first be allocated with
> -pkey_alloc().  An application calls the WRPKRU instruction
> +pkey_alloc().  An application calls the WRPKRU/AMR instruction
>  directly in order to change access permissions to memory covered
> -with a key.  In this example WRPKRU is wrapped by a C function
> +with a key.  In this example WRPKRU/AMR is wrapped by a C function
>  called pkey_set().
>  
>  	int real_prot = PROT_READ|PROT_WRITE;
> @@ -52,11 +77,11 @@ is no longer in use:
>  	munmap(ptr, PAGE_SIZE);
>  	pkey_free(pkey);
>  
> -(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
> +(Note: pkey_set() is a wrapper for the RDPKRU,WRPKRU or AMR instructions.
>   An example implementation can be found in
>   tools/testing/selftests/x86/protection_keys.c)
>  
> -=========================== Behavior ===========================
> +=========================== Behavior =================================
>  
>  The kernel attempts to make protection keys consistent with the
>  behavior of a plain mprotect().  For instance if you do this:
> @@ -83,3 +108,23 @@ 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.
> +
> +
> +====================================================================
> +		Semantic differences
> +
> +The following semantic differences exist between x86 and power.
> +
> +a) powerpc allows creation of a key with execute-disabled.  The following
> +	is allowed on powerpc.
> +	pkey = pkey_alloc(0, PKEY_DISABLE_WRITE | PKEY_DISABLE_ACCESS |
> +			PKEY_DISABLE_EXECUTE);
> +   x86 disallows PKEY_DISABLE_EXECUTE during key creation.

It isn't that powerpc supports *creation* of the key.  It doesn't
support setting PKEY_DISABLE_EXECUTE, period, which implies that you
can't set it at pkey_alloc().  That's a pretty important distinction, IMNHO.

> +b) changing the permission bits of a key from a signal handler does not
> +   persist on x86. The PKRU specific fpregs entry needs to be modified
> +   for it to persist.  On powerpc the permission bits of the key can be
> +   modified by programming the AMR register from the signal handler.
> +   The changes persists across signal boundaries.

^"changes persist", not "persists".