Re: [PATCH 13/25] powerpc: implementation for arch_override_mprotect_pkey()

On Wed, Oct 18, 2017 at 03:36:35PM +1100, Balbir Singh wrote:

On Fri,  8 Sep 2017 15:45:01 -0700
Ram Pai [off-list ref] wrote:

quoted

arch independent code calls arch_override_mprotect_pkey()
to return a pkey that best matches the requested protection.

This patch provides the implementation.

Signed-off-by: Ram Pai <redacted>
---
 arch/powerpc/include/asm/mmu_context.h |    5 +++
 arch/powerpc/include/asm/pkeys.h       |   17 ++++++++++-
 arch/powerpc/mm/pkeys.c                |   47 ++++++++++++++++++++++++++++++++
 3 files changed, 67 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
index c705a5d..8e5a87e 100644
--- a/arch/powerpc/include/asm/mmu_context.h
+++ b/arch/powerpc/include/asm/mmu_context.h

@@ -145,6 +145,11 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
 #ifndef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
 #define pkey_initialize()
 #define pkey_mm_init(mm)
+
+static inline int vma_pkey(struct vm_area_struct *vma)
+{
+	return 0;
+}
 #endif /* CONFIG_PPC64_MEMORY_PROTECTION_KEYS */
 
 #endif /* __KERNEL__ */

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index f13e913..d2fffef 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h

@@ -41,6 +41,16 @@ static inline u64 pkey_to_vmflag_bits(u16 pkey)
 		((pkey & 0x10UL) ? VM_PKEY_BIT4 : 0x0UL));
 }
 
+#define ARCH_VM_PKEY_FLAGS (VM_PKEY_BIT0 | VM_PKEY_BIT1 | VM_PKEY_BIT2 | \
+				VM_PKEY_BIT3 | VM_PKEY_BIT4)
+
+static inline int vma_pkey(struct vm_area_struct *vma)
+{
+	if (!pkey_inited)
+		return 0;

We don't want pkey_inited to be present in all functions, why do we need
a conditional branch for all functions. Even if we do, it should be a jump
label. I would rather we just removed !pkey_inited unless really really
required.

No. we really really need it.  For example when we build a kernel with
PROTECTION_KEYS config enabled and run that kernel on a older processor
or on a system where the key feature is not enabled in the device tree,
we have fail all the calls that get called-in by the arch-neutral code.

Hence we need this check.

BTW: jump labels are awkward IMHO, unless absolutely needed.

quoted

+	return (vma->vm_flags & ARCH_VM_PKEY_FLAGS) >> VM_PKEY_SHIFT;
+}
+
 #define arch_max_pkey()  pkeys_total
 #define AMR_RD_BIT 0x1UL
 #define AMR_WR_BIT 0x2UL

@@ -142,11 +152,14 @@ static inline int execute_only_pkey(struct mm_struct *mm)
 	return __execute_only_pkey(mm);
 }
 
-
+extern int __arch_override_mprotect_pkey(struct vm_area_struct *vma,
+		int prot, int pkey);
 static inline int arch_override_mprotect_pkey(struct vm_area_struct *vma,
 		int prot, int pkey)
 {
-	return 0;
+	if (!pkey_inited)
+		return 0;
+	return __arch_override_mprotect_pkey(vma, prot, pkey);
 }
 
 extern int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,

diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
index 8a24983..fb1a76a 100644
--- a/arch/powerpc/mm/pkeys.c
+++ b/arch/powerpc/mm/pkeys.c

@@ -245,3 +245,50 @@ int __execute_only_pkey(struct mm_struct *mm)
 		mm->context.execute_only_pkey = execute_only_pkey;
 	return execute_only_pkey;
 }
+
+static inline bool vma_is_pkey_exec_only(struct vm_area_struct *vma)
+{
+	/* Do this check first since the vm_flags should be hot */
+	if ((vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)) != VM_EXEC)
+		return false;
+
+	return (vma_pkey(vma) == vma->vm_mm->context.execute_only_pkey);
+}
+
+/*
+ * This should only be called for *plain* mprotect calls.

What's a plain mprotect call?

there is sys_mprotect() and now there is a sys_pkey_mprotect() call.
The 'plain' one is the former.

quoted

+ */
+int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot,
+		int pkey)
+{
+	/*
+	 * Is this an mprotect_pkey() call?  If so, never
+	 * override the value that came from the user.
+	 */
+	if (pkey != -1)
+		return pkey;

If the user specified a key, we always use it? Presumably the user
got it from pkey_alloc(), in other cases, the user was lazy and used
-1 in the mprotect call?

in the plain sys_mprotect() key is not specified. In that case this
function gets called with a -1.

quoted

+
+	/*
+	 * If the currently associated pkey is execute-only,
+	 * but the requested protection requires read or write,
+	 * move it back to the default pkey.
+	 */
+	if (vma_is_pkey_exec_only(vma) &&
+	    (prot & (PROT_READ|PROT_WRITE)))
+		return 0;
+
+	/*
+	 * the requested protection is execute-only. Hence
+	 * lets use a execute-only pkey.
+	 */
+	if (prot == PROT_EXEC) {
+		pkey = execute_only_pkey(vma->vm_mm);
+		if (pkey > 0)
+			return pkey;
+	}
+
+	/*
+	 * nothing to override.
+	 */
+	return vma_pkey(vma);
+}

Balbir Singh.

-- 
Ram Pai