MIPS: Guard against any partial write attempt with PTRACE_SETREGSET

Complement commit d614fd58a283 ("mips/ptrace: Preserve previous
registers for short regset write") and ensure that no partial register
write attempt is made with PTRACE_SETREGSET, as we do not preinitialize
any temporaries used to hold incoming register data and consequently
random data could be written.

It is the responsibility of the caller, such as `ptrace_regset', to
arrange for writes to span whole registers only, so here we only assert
that it has indeed happened.

Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Fixes: 72b22bbad1e7 ("MIPS: Don't assume 64-bit FP registers for FP regset")
Cc: James Hogan <james.hogan@mips.com>
Cc: Paul Burton <Paul.Burton@mips.com>
Cc: Alex Smith <alex@alex-smith.me.uk>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org # v3.15+
Patchwork: https://patchwork.linux-mips.org/patch/17926/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/ptrace.c

@@ -516,7 +516,15 @@ static int fpr_set_msa(struct task_struct *target,
 	return 0;
 }
 
-/* Copy the supplied NT_PRFPREG buffer to the floating-point context.  */
+/*
+ * Copy the supplied NT_PRFPREG buffer to the floating-point context.
+ *
+ * We optimize for the case where `count % sizeof(elf_fpreg_t) == 0',
+ * which is supposed to have been guaranteed by the kernel before
+ * calling us, e.g. in `ptrace_regset'.  We enforce that requirement,
+ * so that we can safely avoid preinitializing temporaries for
+ * partial register writes.
+ */
 static int fpr_set(struct task_struct *target,
 		   const struct user_regset *regset,
 		   unsigned int pos, unsigned int count,
@@ -524,6 +532,8 @@ static int fpr_set(struct task_struct *target,
 {
 	int err;
 
+	BUG_ON(count % sizeof(elf_fpreg_t));
+
 	/* XXX fcr31  */
 
 	init_fp_ctx(target);