bpf/verifier: track signed and unsigned min/max values

Allows us to, sometimes, combine information from a signed check of one
 bound and an unsigned check of the other.
We now track the full range of possible values, rather than restricting
 ourselves to [0, 1<<30) and considering anything beyond that as
 unknown.  While this is probably not necessary, it makes the code more
 straightforward and symmetrical between signed and unsigned bounds.
Signed-off-by: NEdward Cree <ecree@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/bpf_verifier.h

@@ -11,11 +11,15 @@
 #include <linux/filter.h> /* for MAX_BPF_STACK */
 #include <linux/tnum.h>
 
- /* 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 -1
+/* Maximum variable offset umax_value permitted when resolving memory accesses.
+ * In practice this is far bigger than any realistic pointer offset; this limit
+ * ensures that umax_value + (int)off + (int)size cannot overflow a u64.
+ */
+#define BPF_MAX_VAR_OFF	(1ULL << 31)
+/* Maximum variable size permitted for ARG_CONST_SIZE[_OR_ZERO].  This ensures
+ * that converting umax_value to int cannot overflow.
+ */
+#define BPF_MAX_VAR_SIZ	INT_MAX
 
 struct bpf_reg_state {
 	enum bpf_reg_type type;
@@ -36,7 +40,7 @@ struct bpf_reg_state {
 	 * came from, when one is tested for != NULL.
 	 */
 	u32 id;
-	/* These three fields must be last.  See states_equal() */
+	/* These five fields must be last.  See states_equal() */
 	/* For scalar types (SCALAR_VALUE), this represents our knowledge of
 	 * the actual value.
 	 * For pointer types, this represents the variable part of the offset
@@ -49,9 +53,10 @@ struct bpf_reg_state {
 	 * These refer to the same value as var_off, not necessarily the actual
 	 * contents of the register.
 	 */
-	s64 min_value;
-	u64 max_value;
-	bool value_from_signed;
+	s64 smin_value; /* minimum possible (s64)value */
+	s64 smax_value; /* maximum possible (s64)value */
+	u64 umin_value; /* minimum possible (u64)value */
+	u64 umax_value; /* maximum possible (u64)value */
 };
 
 enum bpf_stack_slot_type {

include/linux/tnum.h

@@ -17,6 +17,8 @@ struct tnum {
 struct tnum tnum_const(u64 value);
 /* A completely unknown value */
 extern const struct tnum tnum_unknown;
+/* A value that's unknown except that @min <= value <= @max */
+struct tnum tnum_range(u64 min, u64 max);
 
 /* Arithmetic and logical ops */
 /* Shift a tnum left (by a fixed shift) */

kernel/bpf/tnum.c

@@ -17,6 +17,22 @@ struct tnum tnum_const(u64 value)
 	return TNUM(value, 0);
 }
 
+struct tnum tnum_range(u64 min, u64 max)
+{
+	u64 chi = min ^ max, delta;
+	u8 bits = fls64(chi);
+
+	/* special case, needed because 1ULL << 64 is undefined */
+	if (bits > 63)
+		return tnum_unknown;
+	/* e.g. if chi = 4, bits = 3, delta = (1<<3) - 1 = 7.
+	 * if chi = 0, bits = 0, delta = (1<<0) - 1 = 0, so we return
+	 *  constant min (since min == max).
+	 */
+	delta = (1ULL << bits) - 1;
+	return TNUM(min & ~delta, delta);
+}
+
 struct tnum tnum_lshift(struct tnum a, u8 shift)
 {
 	return TNUM(a.value << shift, a.mask << shift);