powerpc/lib: Refactor __patch_instruction() to use __put_user_asm()

__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@russell.cc>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 587ff9788ab05287f8fcb36de1b1ddd5562e8b51..90c9d4a1e36fb43742b74ee5186ad1e8d8435f1a 100644 (file)
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c
@@ -25,9 +25,9 @@
 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;