bpf: prevent out-of-bounds speculation

commit b2157399 upstream.

Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
memory accesses under a bounds check may be speculated even if the
bounds check fails, providing a primitive for building a side channel.

To avoid leaking kernel data round up array-based maps and mask the index
after bounds check, so speculated load with out of bounds index will load
either valid value from the array or zero from the padded area.

Unconditionally mask index for all array types even when max_entries
are not rounded to power of 2 for root user.
When map is created by unpriv user generate a sequence of bpf insns
that includes AND operation to make sure that JITed code includes
the same 'index & index_mask' operation.

If prog_array map is created by unpriv user replace
  bpf_tail_call(ctx, map, index);
with
  if (index >= max_entries) {
    index &= map->index_mask;
    bpf_tail_call(ctx, map, index);
  }
(along with roundup to power 2) to prevent out-of-bounds speculation.
There is secondary redundant 'if (index >= max_entries)' in the interpreter
and in all JITs, but they can be optimized later if necessary.

Other array-like maps (cpumap, devmap, sockmap, perf_event_array, cgroup_array)
cannot be used by unpriv, so no changes there.

That fixes bpf side of "Variant 1: bounds check bypass (CVE-2017-5753)" on
all architectures with and without JIT.

v2->v3:
Daniel noticed that attack potentially can be crafted via syscall commands
without loading the program, so add masking to those paths as well.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/bpf.h

@@ -37,6 +37,7 @@ struct bpf_map {
 	u32 value_size;
 	u32 max_entries;
 	u32 pages;
+	bool unpriv_array;
 	struct user_struct *user;
 	const struct bpf_map_ops *ops;
 	struct work_struct work;
@@ -141,6 +142,7 @@ struct bpf_prog_aux {
 struct bpf_array {
 	struct bpf_map map;
 	u32 elem_size;
+	u32 index_mask;
 	/* 'ownership' of prog_array is claimed by the first program that
 	 * is going to use this map or by the first program which FD is stored
 	 * in the map to make sure that all callers and callees have the same

kernel/bpf/arraymap.c

@@ -20,8 +20,9 @@
 /* Called from syscall */
 static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 {
+	u32 elem_size, array_size, index_mask, max_entries;
+	bool unpriv = !capable(CAP_SYS_ADMIN);
 	struct bpf_array *array;
-	u32 elem_size, array_size;
 
 	/* check sanity of attributes */
 	if (attr->max_entries == 0 || attr->key_size != 4 ||
@@ -36,12 +37,21 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 
 	elem_size = round_up(attr->value_size, 8);
 
+	max_entries = attr->max_entries;
+	index_mask = roundup_pow_of_two(max_entries) - 1;
+
+	if (unpriv)
+		/* round up array size to nearest power of 2,
+		 * since cpu will speculate within index_mask limits
+		 */
+		max_entries = index_mask + 1;
+
 	/* check round_up into zero and u32 overflow */
 	if (elem_size == 0 ||
-	    attr->max_entries > (U32_MAX - PAGE_SIZE - sizeof(*array)) / elem_size)
+	    max_entries > (U32_MAX - PAGE_SIZE - sizeof(*array)) / elem_size)
 		return ERR_PTR(-ENOMEM);
 
-	array_size = sizeof(*array) + attr->max_entries * elem_size;
+	array_size = sizeof(*array) + max_entries * elem_size;
 
 	/* allocate all map elements and zero-initialize them */
 	array = kzalloc(array_size, GFP_USER | __GFP_NOWARN);
@@ -50,6 +60,8 @@ static struct bpf_map *array_map_alloc(union bpf_attr *attr)
 		if (!array)
 			return ERR_PTR(-ENOMEM);
 	}
+	array->index_mask = index_mask;
+	array->map.unpriv_array = unpriv;
 
 	/* copy mandatory map attributes */
 	array->map.key_size = attr->key_size;
@@ -70,7 +82,7 @@ static void *array_map_lookup_elem(struct bpf_map *map, void *key)
 	if (index >= array->map.max_entries)
 		return NULL;
 
-	return array->value + array->elem_size * index;
+	return array->value + array->elem_size * (index & array->index_mask);
 }
 
 /* Called from syscall */
@@ -111,7 +123,9 @@ static int array_map_update_elem(struct bpf_map *map, void *key, void *value,
 		/* all elements already exist */
 		return -EEXIST;
 
-	memcpy(array->value + array->elem_size * index, value, map->value_size);
+	memcpy(array->value +
+	       array->elem_size * (index & array->index_mask),
+	       value, map->value_size);
 	return 0;
 }
 

kernel/bpf/verifier.c

@@ -187,7 +187,10 @@ struct verifier_stack_elem {
 };
 
 struct bpf_insn_aux_data {
-	enum bpf_reg_type ptr_type;	/* pointer type for load/store insns */
+	union {
+		enum bpf_reg_type ptr_type;	/* pointer type for load/store insns */
+		struct bpf_map *map_ptr;	/* pointer for call insn into lookup_elem */
+	};
 };
 
 #define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */
@@ -950,7 +953,7 @@ static int check_map_func_compatibility(struct bpf_map *map, int func_id)
 	return -EINVAL;
 }
 
-static int check_call(struct verifier_env *env, int func_id)
+static int check_call(struct verifier_env *env, int func_id, int insn_idx)
 {
 	struct verifier_state *state = &env->cur_state;
 	const struct bpf_func_proto *fn = NULL;
@@ -986,6 +989,13 @@ static int check_call(struct verifier_env *env, int func_id)
 	err = check_func_arg(env, BPF_REG_2, fn->arg2_type, &map);
 	if (err)
 		return err;
+	if (func_id == BPF_FUNC_tail_call) {
+		if (map == NULL) {
+			verbose("verifier bug\n");
+			return -EINVAL;
+		}
+		env->insn_aux_data[insn_idx].map_ptr = map;
+	}
 	err = check_func_arg(env, BPF_REG_3, fn->arg3_type, &map);
 	if (err)
 		return err;
@@ -1911,7 +1921,7 @@ static int do_check(struct verifier_env *env)
 					return -EINVAL;
 				}
 
-				err = check_call(env, insn->imm);
+				err = check_call(env, insn->imm, insn_idx);
 				if (err)
 					return err;
 
@@ -2202,7 +2212,10 @@ static int fixup_bpf_calls(struct verifier_env *env)
 	struct bpf_insn *insn = prog->insnsi;
 	const struct bpf_func_proto *fn;
 	const int insn_cnt = prog->len;
-	int i;
+	struct bpf_insn insn_buf[16];
+	struct bpf_prog *new_prog;
+	struct bpf_map *map_ptr;
+	int i, cnt, delta = 0;
 
 	for (i = 0; i < insn_cnt; i++, insn++) {
 		if (insn->code != (BPF_JMP | BPF_CALL))
@@ -2220,6 +2233,31 @@ static int fixup_bpf_calls(struct verifier_env *env)
 			 */
 			insn->imm = 0;
 			insn->code |= BPF_X;
+
+			/* instead of changing every JIT dealing with tail_call
+			 * emit two extra insns:
+			 * if (index >= max_entries) goto out;
+			 * index &= array->index_mask;
+			 * to avoid out-of-bounds cpu speculation
+			 */
+			map_ptr = env->insn_aux_data[i + delta].map_ptr;
+			if (!map_ptr->unpriv_array)
+				continue;
+			insn_buf[0] = BPF_JMP_IMM(BPF_JGE, BPF_REG_3,
+						  map_ptr->max_entries, 2);
+			insn_buf[1] = BPF_ALU32_IMM(BPF_AND, BPF_REG_3,
+						    container_of(map_ptr,
+								 struct bpf_array,
+								 map)->index_mask);
+			insn_buf[2] = *insn;
+			cnt = 3;
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
+			if (!new_prog)
+				return -ENOMEM;
+
+			delta    += cnt - 1;
+			env->prog = prog = new_prog;
+			insn      = new_prog->insnsi + i + delta;
 			continue;
 		}