Re: [PATCH v2 2/5] powerpc/lib: Initialize a temporary mm for code patching

When code patching a STRICT_KERNEL_RWX kernel the page containing the
address to be patched is temporarily mapped with permissive memory
protections. Currently, a per-cpu vmalloc patch area is used for this
purpose. While the patch area is per-cpu, the temporary page mapping is
inserted into the kernel page tables for the duration of the patching.
The mapping is exposed to CPUs other than the patching CPU - this is
undesirable from a hardening perspective.

Use the `poking_init` init hook to prepare a temporary mm and patching
address. Initialize the temporary mm by copying the init mm. Choose a
randomized patching address inside the temporary mm userspace address
portion. The next patch uses the temporary mm and patching address for
code patching.

Based on x86 implementation:

commit 4fc19708b165
("x86/alternatives: Initialize temporary mm for patching")

Signed-off-by: Christopher M. Riedl <redacted>
---
 arch/powerpc/lib/code-patching.c | 33 ++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c
index 0a051dfeb177..8ae1a9e5fe6e 100644
--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c

@@ -11,6 +11,8 @@
 #include <linux/cpuhotplug.h>
 #include <linux/slab.h>
 #include <linux/uaccess.h>
+#include <linux/sched/task.h>
+#include <linux/random.h>
 
 #include <asm/tlbflush.h>
 #include <asm/page.h>

@@ -44,6 +46,37 @@ int raw_patch_instruction(struct ppc_inst *addr, struct ppc_inst instr)
 }
 
 #ifdef CONFIG_STRICT_KERNEL_RWX
+
+static struct mm_struct *patching_mm __ro_after_init;
+static unsigned long patching_addr __ro_after_init;
+
+void __init poking_init(void)
+{
+	spinlock_t *ptl; /* for protecting pte table */
+	pte_t *ptep;
+
+	/*
+	 * Some parts of the kernel (static keys for example) depend on
+	 * successful code patching. Code patching under STRICT_KERNEL_RWX
+	 * requires this setup - otherwise we cannot patch at all. We use
+	 * BUG_ON() here and later since an early failure is preferred to
+	 * buggy behavior and/or strange crashes later.
+	 */
+	patching_mm = copy_init_mm();
+	BUG_ON(!patching_mm);
+
+	/*
+	 * In hash we cannot go above DEFAULT_MAP_WINDOW easily.
+	 * XXX: Do we want additional bits of entropy for radix?
+	 */
+	patching_addr = (get_random_long() & PAGE_MASK) %
+		(DEFAULT_MAP_WINDOW - PAGE_SIZE);