Re: [PATCH 10/25] powerpc: store and restore the pkey state across context switches

On Wed, Oct 18, 2017 at 02:49:14PM +1100, Balbir Singh wrote:

On Fri,  8 Sep 2017 15:44:58 -0700
Ram Pai [off-list ref] wrote:

quoted

Store and restore the AMR, IAMR and UAMOR register state of the task
before scheduling out and after scheduling in, respectively.

Signed-off-by: Ram Pai <redacted>
---
 arch/powerpc/include/asm/pkeys.h     |    4 +++
 arch/powerpc/include/asm/processor.h |    5 ++++
 arch/powerpc/kernel/process.c        |   10 ++++++++
 arch/powerpc/mm/pkeys.c              |   39 ++++++++++++++++++++++++++++++++++
 4 files changed, 58 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 7fd48a4..78c5362 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h

@@ -143,5 +143,9 @@ static inline void pkey_mm_init(struct mm_struct *mm)
 	mm_pkey_allocation_map(mm) = initial_allocation_mask;
 }
 
+extern void thread_pkey_regs_save(struct thread_struct *thread);
+extern void thread_pkey_regs_restore(struct thread_struct *new_thread,
+			struct thread_struct *old_thread);
+extern void thread_pkey_regs_init(struct thread_struct *thread);
 extern void pkey_initialize(void);
 #endif /*_ASM_PPC64_PKEYS_H */

diff --git a/arch/powerpc/include/asm/processor.h b/arch/powerpc/include/asm/processor.h
index fab7ff8..de9d9ba 100644
--- a/arch/powerpc/include/asm/processor.h
+++ b/arch/powerpc/include/asm/processor.h

@@ -309,6 +309,11 @@ struct thread_struct {
 	struct thread_vr_state ckvr_state; /* Checkpointed VR state */
 	unsigned long	ckvrsave; /* Checkpointed VRSAVE */
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
+#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
+	unsigned long	amr;
+	unsigned long	iamr;
+	unsigned long	uamor;
+#endif
 #ifdef CONFIG_KVM_BOOK3S_32_HANDLER
 	void*		kvm_shadow_vcpu; /* KVM internal data */
 #endif /* CONFIG_KVM_BOOK3S_32_HANDLER */

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index a0c74bb..ba80002 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c

@@ -42,6 +42,7 @@
 #include <linux/hw_breakpoint.h>
 #include <linux/uaccess.h>
 #include <linux/elf-randomize.h>
+#include <linux/pkeys.h>
 
 #include <asm/pgtable.h>
 #include <asm/io.h>

@@ -1085,6 +1086,9 @@ static inline void save_sprs(struct thread_struct *t)
 		t->tar = mfspr(SPRN_TAR);
 	}
 #endif
+#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
+	thread_pkey_regs_save(t);
+#endif

Just define two variants of thread_pkey_regs_save() based on
CONFIG_PPC64_MEMORY_PROTECTION_KEYS and remove the #ifdefs from process.c
Ditto for the lines below

ok.

quoted

 }
 
 static inline void restore_sprs(struct thread_struct *old_thread,

@@ -1120,6 +1124,9 @@ static inline void restore_sprs(struct thread_struct *old_thread,
 			mtspr(SPRN_TAR, new_thread->tar);
 	}
 #endif
+#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
+	thread_pkey_regs_restore(new_thread, old_thread);
+#endif

ok.

quoted

 }
 
 #ifdef CONFIG_PPC_BOOK3S_64

@@ -1705,6 +1712,9 @@ void start_thread(struct pt_regs *regs, unsigned long start, unsigned long sp)
 	current->thread.tm_tfiar = 0;
 	current->thread.load_tm = 0;
 #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
+#ifdef CONFIG_PPC64_MEMORY_PROTECTION_KEYS
+	thread_pkey_regs_init(&current->thread);
+#endif /* CONFIG_PPC64_MEMORY_PROTECTION_KEYS */
 }
 EXPORT_SYMBOL(start_thread);
 

diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
index 2282864..7cd1be4 100644
--- a/arch/powerpc/mm/pkeys.c
+++ b/arch/powerpc/mm/pkeys.c

@@ -149,3 +149,42 @@ int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
 	init_amr(pkey, new_amr_bits);
 	return 0;
 }
+
+void thread_pkey_regs_save(struct thread_struct *thread)
+{
+	if (!pkey_inited)
+		return;
+
+	/* @TODO skip saving any registers if the thread
+	 * has not used any keys yet.
+	 */

Comment style is broken

ok. this time i will fix them. It misses by radar screen because
checkpatch.pl does not complain. 

quoted

+
+	thread->amr = read_amr();
+	thread->iamr = read_iamr();
+	thread->uamor = read_uamor();
+}
+
+void thread_pkey_regs_restore(struct thread_struct *new_thread,
+			struct thread_struct *old_thread)
+{
+	if (!pkey_inited)
+		return;
+
+	/* @TODO just reset uamor to zero if the new_thread
+	 * has not used any keys yet.
+	 */

Comment style is broken.

quoted

+
+	if (old_thread->amr != new_thread->amr)
+		write_amr(new_thread->amr);
+	if (old_thread->iamr != new_thread->iamr)
+		write_iamr(new_thread->iamr);
+	if (old_thread->uamor != new_thread->uamor)
+		write_uamor(new_thread->uamor);

Is this order correct? Ideally, You want to write the uamor first
but since we are in supervisor state, I think we can get away
with this order. 

we could be in hypervisor state too, as is the case when we run
a powernv kernel.

But..does it matter in which order they are written? if
the thread is in the kernel, it cannot execute any instructions
in userspace. So it wont see a intermediate state. right?
or am i getting this wrong?

Do we want to expose the uamor to user space
for it to modify the AMR directly?

sorry I did not understand the comment. UAMOR cannot
be accessed from usespace. and there are no system calls
currently to help userspace to program the UAMOR on its
behalf.

quoted

+}
+
+void thread_pkey_regs_init(struct thread_struct *thread)
+{
+	write_amr(0x0ul);
+	write_iamr(0x0ul);
+	write_uamor(0x0ul);

This is not correct, reserved keys should not be set to 0's

ok. makes sense. best to not touch reserved key bits here.

Balbir Singh.

-- 
Ram Pai