bpf: Support variable offset stack access from helpers

Currently there is a difference in how verifier checks memory access for
helper arguments for PTR_TO_MAP_VALUE and PTR_TO_STACK with regard to
variable part of offset.

check_map_access, that is used for PTR_TO_MAP_VALUE, can handle variable
offsets just fine, so that BPF program can call a helper like this:

  some_helper(map_value_ptr + off, size);

, where offset is unknown at load time, but is checked by program to be
in a safe rage (off >= 0 && off + size < map_value_size).

But it's not the case for check_stack_boundary, that is used for
PTR_TO_STACK, and same code with pointer to stack is rejected by
verifier:

  some_helper(stack_value_ptr + off, size);

For example:
  0: (7a) *(u64 *)(r10 -16) = 0
  1: (7a) *(u64 *)(r10 -8) = 0
  2: (61) r2 = *(u32 *)(r1 +0)
  3: (57) r2 &= 4
  4: (17) r2 -= 16
  5: (0f) r2 += r10
  6: (18) r1 = 0xffff888111343a80
  8: (85) call bpf_map_lookup_elem#1
  invalid variable stack read R2 var_off=(0xfffffffffffffff0; 0x4)

Add support for variable offset access to check_stack_boundary so that
if offset is checked by program to be in a safe range it's accepted by
verifier.
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -2157,6 +2157,29 @@ static int check_xadd(struct bpf_verifier_env *env, int insn_idx, struct bpf_ins
 				BPF_SIZE(insn->code), BPF_WRITE, -1, true);
 }
 
+static int __check_stack_boundary(struct bpf_verifier_env *env, u32 regno,
+				  int off, int access_size,
+				  bool zero_size_allowed)
+{
+	struct bpf_reg_state *reg = reg_state(env, regno);
+
+	if (off >= 0 || off < -MAX_BPF_STACK || off + access_size > 0 ||
+	    access_size < 0 || (access_size == 0 && !zero_size_allowed)) {
+		if (tnum_is_const(reg->var_off)) {
+			verbose(env, "invalid stack type R%d off=%d access_size=%d\n",
+				regno, off, access_size);
+		} else {
+			char tn_buf[48];
+
+			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+			verbose(env, "invalid stack type R%d var_off=%s access_size=%d\n",
+				regno, tn_buf, access_size);
+		}
+		return -EACCES;
+	}
+	return 0;
+}
+
 /* when register 'regno' is passed into function that will read 'access_size'
  * bytes from that pointer, make sure that it's within stack boundary
  * and all elements of stack are initialized.
@@ -2169,7 +2192,7 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 {
 	struct bpf_reg_state *reg = reg_state(env, regno);
 	struct bpf_func_state *state = func(env, reg);
-	int off, i, slot, spi;
+	int err, min_off, max_off, i, slot, spi;
 
 	if (reg->type != PTR_TO_STACK) {
 		/* Allow zero-byte read from NULL, regardless of pointer type */
@@ -2183,21 +2206,23 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 		return -EACCES;
 	}
 
-	/* Only allow fixed-offset stack reads */
-	if (!tnum_is_const(reg->var_off)) {
-		char tn_buf[48];
-
-		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
-		verbose(env, "invalid variable stack read R%d var_off=%s\n",
-			regno, tn_buf);
-		return -EACCES;
-	}
-	off = reg->off + reg->var_off.value;
-	if (off >= 0 || off < -MAX_BPF_STACK || off + access_size > 0 ||
-	    access_size < 0 || (access_size == 0 && !zero_size_allowed)) {
-		verbose(env, "invalid stack type R%d off=%d access_size=%d\n",
-			regno, off, access_size);
-		return -EACCES;
+	if (tnum_is_const(reg->var_off)) {
+		min_off = max_off = reg->var_off.value + reg->off;
+		err = __check_stack_boundary(env, regno, min_off, access_size,
+					     zero_size_allowed);
+		if (err)
+			return err;
+	} else {
+		min_off = reg->smin_value + reg->off;
+		max_off = reg->umax_value + reg->off;
+		err = __check_stack_boundary(env, regno, min_off, access_size,
+					     zero_size_allowed);
+		if (err)
+			return err;
+		err = __check_stack_boundary(env, regno, max_off, access_size,
+					     zero_size_allowed);
+		if (err)
+			return err;
 	}
 
 	if (meta && meta->raw_mode) {
@@ -2206,10 +2231,10 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 		return 0;
 	}
 
-	for (i = 0; i < access_size; i++) {
+	for (i = min_off; i < max_off + access_size; i++) {
 		u8 *stype;
 
-		slot = -(off + i) - 1;
+		slot = -i - 1;
 		spi = slot / BPF_REG_SIZE;
 		if (state->allocated_stack <= slot)
 			goto err;
@@ -2222,8 +2247,16 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 			goto mark;
 		}
 err:
+		if (tnum_is_const(reg->var_off)) {
 			verbose(env, "invalid indirect read from stack off %d+%d size %d\n",
-			off, i, access_size);
+				min_off, i - min_off, access_size);
+		} else {
+			char tn_buf[48];
+
+			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+			verbose(env, "invalid indirect read from stack var_off %s+%d size %d\n",
+				tn_buf, i - min_off, access_size);
+		}
 		return -EACCES;
 mark:
 		/* reading any byte out of 8-byte 'spill_slot' will cause
@@ -2232,7 +2265,7 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno,
 		mark_reg_read(env, &state->stack[spi].spilled_ptr,
 			      state->stack[spi].spilled_ptr.parent);
 	}
-	return update_stack_depth(env, state, off);
+	return update_stack_depth(env, state, min_off);
 }
 
 static int check_helper_mem_access(struct bpf_verifier_env *env, int regno,