[PATCH] powerpc/pkeys: preallocate execute_only key only if the key is available.

[Ram Pai]

On Tue, Jun 19, 2018 at 10:34:22PM -0700, Ram Pai wrote:
> Key 2 is preallocated and reserved for execute-only key. In rare
> cases if key-2 is unavailable, mprotect(PROT_EXEC) will behave
> incorrectly. NOTE: mprotect(PROT_EXEC) uses execute-only key.
> 
> Ensure key 2 is available for preallocation before reserving it for
> execute_only purpose.
> 
> CC: Michael Ellerman <mpe at ellerman.id.au>
> CC: Thiago Jung Bauermann <bauerman at linux.ibm.com>
> Signed-off-by: Ram Pai <linuxram at us.ibm.com>
> ---
>  arch/powerpc/mm/pkeys.c |   14 +++++++++-----
>  1 files changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
> index cec990c..2013ef0 100644
> --- a/arch/powerpc/mm/pkeys.c
> +++ b/arch/powerpc/mm/pkeys.c
> @@ -19,6 +19,7 @@
>  u64  pkey_amr_mask;		/* Bits in AMR not to be touched */
>  u64  pkey_iamr_mask;		/* Bits in AMR not to be touched */
>  u64  pkey_uamor_mask;		/* Bits in UMOR not to be touched */
> +u32  execute_only_key = 2;

this cannot be unsigned, since it can get set to -1 if key 2 is not available.  :(
Please ignore this patch.  Will resend a fixed version.

> 
>  #define AMR_BITS_PER_PKEY 2
>  #define AMR_RD_BIT 0x1UL
> @@ -26,7 +27,6 @@
>  #define IAMR_EX_BIT 0x1UL
>  #define PKEY_REG_BITS (sizeof(u64)*8)
>  #define pkeyshift(pkey) (PKEY_REG_BITS - ((pkey+1) * AMR_BITS_PER_PKEY))
> -#define EXECUTE_ONLY_KEY 2
> 
>  static void scan_pkey_feature(void)
>  {
> @@ -122,8 +122,12 @@ int pkey_initialize(void)
>  #else
>  	os_reserved = 0;
>  #endif
> +
> +	if ((pkeys_total - os_reserved) <= execute_only_key)
> +		execute_only_key = -1;
> +


.snip..

RP