net: filter: initialize A and X registers

exisiting BPF verifier allows uninitialized access to registers,
'ret A' is considered to be a valid filter.
So initialize A and X to zero to prevent leaking kernel memory
In the future BPF verifier will be rejecting such filters
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -122,6 +122,13 @@ noinline u64 __bpf_call_base(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
 	return 0;
 }
 
+/* Register mappings for user programs. */
+#define A_REG		0
+#define X_REG		7
+#define TMP_REG		8
+#define ARG2_REG	2
+#define ARG3_REG	3
+
 /**
  *	__sk_run_filter - run a filter on a given context
  *	@ctx: buffer to run the filter on
@@ -242,6 +249,8 @@ unsigned int __sk_run_filter(void *ctx, const struct sock_filter_int *insn)
 
 	regs[FP_REG]  = (u64) (unsigned long) &stack[ARRAY_SIZE(stack)];
 	regs[ARG1_REG] = (u64) (unsigned long) ctx;
+	regs[A_REG] = 0;
+	regs[X_REG] = 0;
 
 select_insn:
 	goto *jumptable[insn->code];
@@ -643,13 +652,6 @@ static u64 __get_raw_cpu_id(u64 ctx, u64 A, u64 X, u64 r4, u64 r5)
 	return raw_smp_processor_id();
 }
 
-/* Register mappings for user programs. */
-#define A_REG		0
-#define X_REG		7
-#define TMP_REG		8
-#define ARG2_REG	2
-#define ARG3_REG	3
-
 static bool convert_bpf_extensions(struct sock_filter *fp,
 				   struct sock_filter_int **insnp)
 {