powerpc/code-patching: Use dedicated memory routines for patching

The patching page set up as a writable alias may be in quadrant 0
(userspace) if the temporary mm path is used. This causes sanitiser
failures if so. Sanitiser failures also occur on the non-mm path
because the plain memset family is instrumented, and KASAN treats the
patching window as poisoned.

Introduce locally defined patch_* variants of memset that perform an
uninstrumented lower level set, as well as detecting write errors like
the original single patch variant does.

copy_to_user() is not correct here, as the PTE makes it a proper kernel
page (the EAA is privileged access only, RW). It just happens to be in
quadrant 0 because that's the hardware's mechanism for using the current
PID vs PID 0 in translations. Importantly, it's incorrect to allow user
page accesses.

Now that the patching memsets are used, we also propagate a failure up
to the caller as the single patch variant does.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240325052815.854044-2-bgray@linux.ibm.com

arch/powerpc/lib/code-patching.c

@@ -372,9 +372,32 @@ int patch_instruction(u32 *addr, ppc_inst_t instr)
 }
 NOKPROBE_SYMBOL(patch_instruction);
 
+static int patch_memset64(u64 *addr, u64 val, size_t count)
+{
+	for (u64 *end = addr + count; addr < end; addr++)
+		__put_kernel_nofault(addr, &val, u64, failed);
+
+	return 0;
+
+failed:
+	return -EPERM;
+}
+
+static int patch_memset32(u32 *addr, u32 val, size_t count)
+{
+	for (u32 *end = addr + count; addr < end; addr++)
+		__put_kernel_nofault(addr, &val, u32, failed);
+
+	return 0;
+
+failed:
+	return -EPERM;
+}
+
 static int __patch_instructions(u32 *patch_addr, u32 *code, size_t len, bool repeat_instr)
 {
 	unsigned long start = (unsigned long)patch_addr;
+	int err;
 
 	/* Repeat instruction */
 	if (repeat_instr) {
@@ -383,19 +406,19 @@ static int __patch_instructions(u32 *patch_addr, u32 *code, size_t len, bool rep
 		if (ppc_inst_prefixed(instr)) {
 			u64 val = ppc_inst_as_ulong(instr);
 
-			memset64((u64 *)patch_addr, val, len / 8);
+			err = patch_memset64((u64 *)patch_addr, val, len / 8);
 		} else {
 			u32 val = ppc_inst_val(instr);
 
-			memset32(patch_addr, val, len / 4);
+			err = patch_memset32(patch_addr, val, len / 4);
 		}
 	} else {
-		memcpy(patch_addr, code, len);
+		err = copy_to_kernel_nofault(patch_addr, code, len);
 	}
 
 	smp_wmb();	/* smp write barrier */
 	flush_icache_range(start, start + len);
-	return 0;
+	return err;
 }
 
 /*