[PATCH v5 08/10] powerpc/lib: Refactor __patch_instruction() to use __put_user_asm()

[Michael Ellerman]

From: Russell Currey <ruscur at russell.cc>

__patch_instruction() is called in early boot, and uses
__put_user_size(), which includes the allow/prevent calls to enforce
KUAP, which could either be called too early, or in the Radix case,
forced to use "early_" versions of functions just to safely handle
this one case.

__put_user_asm() does not do this, and thus is safe to use both in
early boot, and later on since in this case it should only ever be
touching kernel memory.

__patch_instruction() was previously refactored to use
__put_user_size() in order to be able to return -EFAULT, which would
allow the kernel to patch instructions in userspace, which should
never happen. This has the functional change of causing faults on
userspace addresses if KUAP is turned on, which should never happen in
practice.

A future enhancement could be to double check the patch address is
definitely allowed to be tampered with by the kernel.

Signed-off-by: Russell Currey <ruscur at russell.cc>
Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
---
v5: Unchanged.

 arch/powerpc/lib/code-patching.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 506413a2c25e..42fdadac6587 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -26,9 +26,9 @@
 static int __patch_instruction(unsigned int *exec_addr, unsigned int instr,
 			       unsigned int *patch_addr)
 {
-	int err;
+	int err = 0;
 
-	__put_user_size(instr, patch_addr, 4, err);
+	__put_user_asm(instr, patch_addr, err, "stw");
 	if (err)
 		return err;
 
-- 
2.20.1