bpf: allow access into map value arrays

Suppose you have a map array value that is something like this

struct foo {
	unsigned iter;
	int array[SOME_CONSTANT];
};

You can easily insert this into an array, but you cannot modify the contents of
foo->array[] after the fact.  This is because we have no way to verify we won't
go off the end of the array at verification time.  This patch provides a start
for this work.  We accomplish this by keeping track of a minimum and maximum
value a register could be while we're checking the code.  Then at the time we
try to do an access into a MAP_VALUE we verify that the maximum offset into that
region is a valid access into that memory region.  So in practice, code such as
this

unsigned index = 0;

if (foo->iter >= SOME_CONSTANT)
	foo->iter = index;
else
	index = foo->iter++;
foo->array[index] = bar;

would be allowed, as we can verify that index will always be between 0 and
SOME_CONSTANT-1.  If you wish to use signed values you'll have to have an extra
check to make sure the index isn't less than 0, or do something like index %=
SOME_CONSTANT.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpf_verifier.h

@@ -10,8 +10,19 @@
 #include <linux/bpf.h> /* for enum bpf_reg_type */
 #include <linux/filter.h> /* for MAX_BPF_STACK */
 
+ /* Just some arbitrary values so we can safely do math without overflowing and
+  * are obviously wrong for any sort of memory access.
+  */
+#define BPF_REGISTER_MAX_RANGE (1024 * 1024 * 1024)
+#define BPF_REGISTER_MIN_RANGE -(1024 * 1024 * 1024)
+
 struct bpf_reg_state {
 	enum bpf_reg_type type;
+	/*
+	 * Used to determine if any memory access using this register will
+	 * result in a bad access.
+	 */
+	u64 min_value, max_value;
 	union {
 		/* valid when type == CONST_IMM | PTR_TO_STACK | UNKNOWN_VALUE */
 		s64 imm;
@@ -81,6 +92,7 @@ struct bpf_verifier_env {
 	u32 id_gen;			/* used to generate unique reg IDs */
 	bool allow_ptr_leaks;
 	bool seen_direct_write;
+	bool varlen_map_value_access;
 	struct bpf_insn_aux_data *insn_aux_data; /* array of per-insn state */
 };