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SubjectRe: [PATCH 1/3] x86, pkeys: do not special case protection key 0
On Fri, Mar 16, 2018 at 02:46:56PM -0700, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> mm_pkey_is_allocated() treats pkey 0 as unallocated. That is
> inconsistent with the manpages, and also inconsistent with
> mm->context.pkey_allocation_map. Stop special casing it and only
> disallow values that are actually bad (< 0).
>
> The end-user visible effect of this is that you can now use
> mprotect_pkey() to set pkey=0.
>
> This is a bit nicer than what Ram proposed because it is simpler
> and removes special-casing for pkey 0. On the other hand, it does
> allow applciations to pkey_free() pkey-0, but that's just a silly
> thing to do, so we are not going to protect against it.

So your proposal
(a) allocates pkey 0 implicitly,
(b) does not stop anyone from freeing pkey-0
(c) and allows pkey-0 to be explicitly associated with any address range.
correct?

My proposal
(a) allocates pkey 0 implicitly,
(b) stops anyone from freeing pkey-0
(c) and allows pkey-0 to be explicitly associated with any address range.

So the difference between the two proposals is just the freeing part i.e (b).
Did I get this right?

Its a philosophical debate; allow the user
to shoot-in-the-feet or stop from not doing so. There is no
clear answer either way. I am fine either way.

So here is my

Reviewed-by: Ram Pai <linuxram@us.ibm.com>

I will write a corresponding patch for powerpc.

>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Ram Pai <linuxram@us.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Dave Hansen <dave.hansen@intel.com>
> Cc: Michael Ellermen <mpe@ellerman.id.au>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>p
> Cc: Shuah Khan <shuah@kernel.org>
> ---
>
> b/arch/x86/include/asm/mmu_context.h | 2 +-
> b/arch/x86/include/asm/pkeys.h | 6 +++---
> 2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff -puN arch/x86/include/asm/mmu_context.h~x86-pkey-0-default-allocated arch/x86/include/asm/mmu_context.h
> --- a/arch/x86/include/asm/mmu_context.h~x86-pkey-0-default-allocated 2018-03-16 14:46:39.023285476 -0700
> +++ b/arch/x86/include/asm/mmu_context.h 2018-03-16 14:46:39.028285476 -0700
> @@ -191,7 +191,7 @@ static inline int init_new_context(struc
>
> #ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
> if (cpu_feature_enabled(X86_FEATURE_OSPKE)) {
> - /* pkey 0 is the default and always allocated */
> + /* pkey 0 is the default and allocated implicitly */
> mm->context.pkey_allocation_map = 0x1;
> /* -1 means unallocated or invalid */
> mm->context.execute_only_pkey = -1;
> diff -puN arch/x86/include/asm/pkeys.h~x86-pkey-0-default-allocated arch/x86/include/asm/pkeys.h
> --- a/arch/x86/include/asm/pkeys.h~x86-pkey-0-default-allocated 2018-03-16 14:46:39.025285476 -0700
> +++ b/arch/x86/include/asm/pkeys.h 2018-03-16 14:46:39.028285476 -0700
> @@ -49,10 +49,10 @@ bool mm_pkey_is_allocated(struct mm_stru
> {
> /*
> * "Allocated" pkeys are those that have been returned
> - * from pkey_alloc(). pkey 0 is special, and never
> - * returned from pkey_alloc().
> + * from pkey_alloc() or pkey 0 which is allocated
> + * implicitly when the mm is created.
> */
> - if (pkey <= 0)
> + if (pkey < 0)
> return false;
> if (pkey >= arch_max_pkey())
> return false;
> _

--
Ram Pai

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