Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

Once an address range is associated with an allocated pkey, it cannot be
reverted back to key-0. There is no valid reason for the above behavior.  On
the contrary applications need the ability to do so.

The patch relaxes the restriction.

Tested on powerpc and x86_64.

cc: Dave Hansen <redacted>
cc: Michael Ellermen <mpe@ellerman.id.au>
cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Ram Pai <redacted>
---
 arch/powerpc/include/asm/pkeys.h | 19 ++++++++++++++-----
 arch/x86/include/asm/pkeys.h     |  5 +++--
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 0409c80..3e8abe4 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h

@@ -101,10 +101,18 @@ static inline u16 pte_to_pkey_bits(u64 pteflags)
 
 static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
 {
-	/* A reserved key is never considered as 'explicitly allocated' */
-	return ((pkey < arch_max_pkey()) &&
-		!__mm_pkey_is_reserved(pkey) &&
-		__mm_pkey_is_allocated(mm, pkey));
+	/* pkey 0 is allocated by default. */
+	if (!pkey)
+	       return true;
+
+	if (pkey < 0 || pkey >= arch_max_pkey())
+	       return false;
+
+	/* reserved keys are never allocated. */
+	if (__mm_pkey_is_reserved(pkey))
+	       return false;