linux-kernel - [PATCH v10 11/27] powerpc: introduce execute-only pkey

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Date:   Thu, 18 Jan 2018 17:50:32 -0800
From:   Ram Pai <linuxram@...ibm.com>
To:     mpe@...erman.id.au, mingo@...hat.com, akpm@...ux-foundation.org,
        corbet@....net, arnd@...db.de
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        dave.hansen@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, hbabu@...ibm.com, mhocko@...nel.org,
        bauerman@...ux.vnet.ibm.com, ebiederm@...ssion.com,
        linuxram@...ibm.com
Subject: [PATCH v10 11/27] powerpc: introduce execute-only pkey

This patch provides the implementation of execute-only pkey.
The architecture-independent layer expects the arch-dependent
layer, to support the ability to create and enable a special
key which has execute-only permission.

Acked-by: Balbir Singh <bsingharora@...il.com>
Signed-off-by: Ram Pai <linuxram@...ibm.com>
---
 arch/powerpc/include/asm/book3s/64/mmu.h |    1 +
 arch/powerpc/include/asm/pkeys.h         |    6 +++-
 arch/powerpc/mm/pkeys.c                  |   58 ++++++++++++++++++++++++++++++
 3 files changed, 64 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
index 37ef23c..0abeb0e 100644
--- a/arch/powerpc/include/asm/book3s/64/mmu.h
+++ b/arch/powerpc/include/asm/book3s/64/mmu.h
@@ -116,6 +116,7 @@ struct patb_entry {
 	 * bit unset -> key available for allocation
 	 */
 	u32 pkey_allocation_map;
+	s16 execute_only_pkey; /* key holding execute-only protection */
 #endif
 } mm_context_t;
 
diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 3def5af..2b5bb35 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h
@@ -128,9 +128,13 @@ static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
  * Try to dedicate one of the protection keys to be used as an
  * execute-only protection key.
  */
+extern int __execute_only_pkey(struct mm_struct *mm);
 static inline int execute_only_pkey(struct mm_struct *mm)
 {
-	return 0;
+	if (static_branch_likely(&pkey_disabled))
+		return -1;
+
+	return __execute_only_pkey(mm);
 }
 
 static inline int arch_override_mprotect_pkey(struct vm_area_struct *vma,
diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
index 7dfcf2d..b466a2c 100644
--- a/arch/powerpc/mm/pkeys.c
+++ b/arch/powerpc/mm/pkeys.c
@@ -96,6 +96,8 @@ void pkey_mm_init(struct mm_struct *mm)
 	if (static_branch_likely(&pkey_disabled))
 		return;
 	mm_pkey_allocation_map(mm) = initial_allocation_mask;
+	/* -1 means unallocated or invalid */
+	mm->context.execute_only_pkey = -1;
 }
 
 static inline u64 read_amr(void)
@@ -260,3 +262,59 @@ void thread_pkey_regs_init(struct thread_struct *thread)
 	write_iamr(read_iamr() & pkey_iamr_mask);
 	write_uamor(read_uamor() & pkey_amr_uamor_mask);
 }
+
+static inline bool pkey_allows_readwrite(int pkey)
+{
+	int pkey_shift = pkeyshift(pkey);
+
+	if (!is_pkey_enabled(pkey))
+		return true;
+
+	return !(read_amr() & ((AMR_RD_BIT|AMR_WR_BIT) << pkey_shift));
+}
+
+int __execute_only_pkey(struct mm_struct *mm)
+{
+	bool need_to_set_mm_pkey = false;
+	int execute_only_pkey = mm->context.execute_only_pkey;
+	int ret;
+
+	/* Do we need to assign a pkey for mm's execute-only maps? */
+	if (execute_only_pkey == -1) {
+		/* Go allocate one to use, which might fail */
+		execute_only_pkey = mm_pkey_alloc(mm);
+		if (execute_only_pkey < 0)
+			return -1;
+		need_to_set_mm_pkey = true;
+	}
+
+	/*
+	 * We do not want to go through the relatively costly dance to set AMR
+	 * if we do not need to. Check it first and assume that if the
+	 * execute-only pkey is readwrite-disabled than we do not have to set it
+	 * ourselves.
+	 */
+	if (!need_to_set_mm_pkey && !pkey_allows_readwrite(execute_only_pkey))
+		return execute_only_pkey;
+
+	/*
+	 * Set up AMR so that it denies access for everything other than
+	 * execution.
+	 */
+	ret = __arch_set_user_pkey_access(current, execute_only_pkey,
+					  PKEY_DISABLE_ACCESS |
+					  PKEY_DISABLE_WRITE);
+	/*
+	 * If the AMR-set operation failed somehow, just return 0 and
+	 * effectively disable execute-only support.
+	 */
+	if (ret) {
+		mm_pkey_free(mm, execute_only_pkey);
+		return -1;
+	}
+
+	/* We got one, store it and use it from here on out */
+	if (need_to_set_mm_pkey)
+		mm->context.execute_only_pkey = execute_only_pkey;
+	return execute_only_pkey;
+}
-- 
1.7.1