powerpc: Add a probe_user_read_inst() function

Introduce a probe_user_read_inst() function to use in cases where
probe_user_read() is used for getting an instruction. This will be
more useful for prefixed instructions.
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Alistair Popple <alistair@popple.id.au>
[mpe: Don't write to *inst on error, fold in __user annotations]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200506034050.24806-14-jniethe5@gmail.com

arch/powerpc/include/asm/inst.h

@@ -37,4 +37,7 @@ static inline bool ppc_inst_equal(struct ppc_inst x, struct ppc_inst y)
 	return ppc_inst_val(x) == ppc_inst_val(y);
 }
 
+int probe_user_read_inst(struct ppc_inst *inst,
+			 struct ppc_inst __user *nip);
+
 #endif /* _ASM_POWERPC_INST_H */

arch/powerpc/lib/Makefile

@@ -16,7 +16,7 @@ CFLAGS_code-patching.o += -DDISABLE_BRANCH_PROFILING
 CFLAGS_feature-fixups.o += -DDISABLE_BRANCH_PROFILING
 endif
 
-obj-y += alloc.o code-patching.o feature-fixups.o pmem.o
+obj-y += alloc.o code-patching.o feature-fixups.o pmem.o inst.o
 
 ifndef CONFIG_KASAN
 obj-y	+=	string.o memcmp_$(BITS).o

arch/powerpc/lib/inst.c

+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ *  Copyright 2020, IBM Corporation.
+ */
+
+#include <linux/uaccess.h>
+#include <asm/inst.h>
+
+int probe_user_read_inst(struct ppc_inst *inst,
+			 struct ppc_inst __user *nip)
+{
+	unsigned int val;
+	int err;
+
+	err = probe_user_read(&val, nip, sizeof(val));
+	if (!err)
+		*inst = ppc_inst(val);
+
+	return err;
+}

arch/powerpc/mm/fault.c

@@ -282,7 +282,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
 	 * expand to 1MB without further checks.
 	 */
 	if (address + 0x100000 < vma->vm_end) {
-		unsigned int __user *nip = (unsigned int __user *)regs->nip;
+		struct ppc_inst __user *nip = (struct ppc_inst __user *)regs->nip;
 		/* get user regs even if this fault is in kernel mode */
 		struct pt_regs *uregs = current->thread.regs;
 		if (uregs == NULL)
@@ -307,7 +307,7 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
 		    access_ok(nip, sizeof(*nip))) {
 			struct ppc_inst inst;
 
-			if (!probe_user_read(&inst, nip, sizeof(inst)))
+			if (!probe_user_read_inst(&inst, nip))
 				return !store_updates_sp(inst);
 			*must_retry = true;
 		}