linux-kernel - [PATCH 1/3] x86, pkeys: do not special case protection key 0

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Date:   Fri, 16 Mar 2018 14:46:56 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        linuxram@...ibm.com, tglx@...utronix.de, dave.hansen@...el.com,
        mpe@...erman.id.au, mingo@...nel.org, akpm@...ux-foundation.org,
        shuah@...nel.org
Subject: [PATCH 1/3] x86, pkeys: do not special case protection key 0


From: Dave Hansen <dave.hansen@...ux.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.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Ram Pai <linuxram@...ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: Michael Ellermen <mpe@...erman.id.au>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>p
Cc: Shuah Khan <shuah@...nel.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;
_