arm64: ptrace: Avoid setting compat FP[SC]R to garbage if get_user fails

If get_user() fails when reading the new FPSCR value from userspace
in compat_vfp_get(), then garbage* will be written to the task's
FPSR and FPCR registers.

This patch prevents this by checking the return from get_user()
first.

[*] Actually, zero, due to the behaviour of get_user() on error, but
that's still not what userspace expects.

Fixes: 478fcb2c ("arm64: Debugging support")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/ptrace.c

@@ -947,9 +947,11 @@ static int compat_vfp_set(struct task_struct *target,
 
 	if (count && !ret) {
 		ret = get_user(fpscr, (compat_ulong_t *)ubuf);
+		if (!ret) {
 			uregs->fpsr = fpscr & VFP_FPSCR_STAT_MASK;
 			uregs->fpcr = fpscr & VFP_FPSCR_CTRL_MASK;
 		}
+	}
 
 	fpsimd_flush_task_state(target);
 	return ret;