bpf: remove __rcu annotations from bpf_prog_array

Drop __rcu annotations and rcu read sections from bpf_prog_array
helper functions. They are not needed since all existing callers
call those helpers from the rcu update side while holding a mutex.
This guarantees that use-after-free could not happen.

In the next patches I'll fix the callers with missing
rcu_dereference_protected to make sparse/lockdep happy, the proper
way to use these helpers is:

	struct bpf_prog_array __rcu *progs = ...;
	struct bpf_prog_array *p;

	mutex_lock(&mtx);
	p = rcu_dereference_protected(progs, lockdep_is_held(&mtx));
	bpf_prog_array_length(p);
	bpf_prog_array_copy_to_user(p, ...);
	bpf_prog_array_delete_safe(p, ...);
	bpf_prog_array_copy_info(p, ...);
	bpf_prog_array_copy(p, ...);
	bpf_prog_array_free(p);
	mutex_unlock(&mtx);

No functional changes! rcu_dereference_protected with lockdep_is_held
should catch any cases where we update prog array without a mutex
(I've looked at existing call sites and I think we hold a mutex
everywhere).

Motivation is to fix sparse warnings:
kernel/bpf/core.c:1803:9: warning: incorrect type in argument 1 (different address spaces)
kernel/bpf/core.c:1803:9:    expected struct callback_head *head
kernel/bpf/core.c:1803:9:    got struct callback_head [noderef] <asn:4> *
kernel/bpf/core.c:1877:44: warning: incorrect type in initializer (different address spaces)
kernel/bpf/core.c:1877:44:    expected struct bpf_prog_array_item *item
kernel/bpf/core.c:1877:44:    got struct bpf_prog_array_item [noderef] <asn:4> *
kernel/bpf/core.c:1901:26: warning: incorrect type in assignment (different address spaces)
kernel/bpf/core.c:1901:26:    expected struct bpf_prog_array_item *existing
kernel/bpf/core.c:1901:26:    got struct bpf_prog_array_item [noderef] <asn:4> *
kernel/bpf/core.c:1935:26: warning: incorrect type in assignment (different address spaces)
kernel/bpf/core.c:1935:26:    expected struct bpf_prog_array_item *[assigned] existing
kernel/bpf/core.c:1935:26:    got struct bpf_prog_array_item [noderef] <asn:4> *

v2:
* remove comment about potential race; that can't happen
  because all callers are in rcu-update section

Cc: Roman Gushchin <guro@fb.com>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index d98141e..ff3e00f 100644 (file)
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -514,17 +514,17 @@ struct bpf_prog_array {
 };
 
 struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags);
-void bpf_prog_array_free(struct bpf_prog_array __rcu *progs);
-int bpf_prog_array_length(struct bpf_prog_array __rcu *progs);
-int bpf_prog_array_copy_to_user(struct bpf_prog_array __rcu *progs,
+void bpf_prog_array_free(struct bpf_prog_array *progs);
+int bpf_prog_array_length(struct bpf_prog_array *progs);
+int bpf_prog_array_copy_to_user(struct bpf_prog_array *progs,
                                __u32 __user *prog_ids, u32 cnt);
 
-void bpf_prog_array_delete_safe(struct bpf_prog_array __rcu *progs,
+void bpf_prog_array_delete_safe(struct bpf_prog_array *progs,
                                struct bpf_prog *old_prog);
-int bpf_prog_array_copy_info(struct bpf_prog_array __rcu *array,
+int bpf_prog_array_copy_info(struct bpf_prog_array *array,
                             u32 *prog_ids, u32 request_cnt,
                             u32 *prog_cnt);
-int bpf_prog_array_copy(struct bpf_prog_array __rcu *old_array,
+int bpf_prog_array_copy(struct bpf_prog_array *old_array,
                        struct bpf_prog *exclude_prog,
                        struct bpf_prog *include_prog,
                        struct bpf_prog_array **new_array);

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 3675b19..33fb292 100644 (file)
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -1795,38 +1795,33 @@ struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags)
        return &empty_prog_array.hdr;
 }
 
-void bpf_prog_array_free(struct bpf_prog_array __rcu *progs)
+void bpf_prog_array_free(struct bpf_prog_array *progs)
 {
-       if (!progs ||
-           progs == (struct bpf_prog_array __rcu *)&empty_prog_array.hdr)
+       if (!progs || progs == &empty_prog_array.hdr)
                return;
        kfree_rcu(progs, rcu);
 }
 
-int bpf_prog_array_length(struct bpf_prog_array __rcu *array)
+int bpf_prog_array_length(struct bpf_prog_array *array)
 {
        struct bpf_prog_array_item *item;
        u32 cnt = 0;
 
-       rcu_read_lock();
-       item = rcu_dereference(array)->items;
-       for (; item->prog; item++)
+       for (item = array->items; item->prog; item++)
                if (item->prog != &dummy_bpf_prog.prog)
                        cnt++;
-       rcu_read_unlock();
        return cnt;
 }
 
 
-static bool bpf_prog_array_copy_core(struct bpf_prog_array __rcu *array,
+static bool bpf_prog_array_copy_core(struct bpf_prog_array *array,
                                     u32 *prog_ids,
                                     u32 request_cnt)
 {
        struct bpf_prog_array_item *item;
        int i = 0;
 
-       item = rcu_dereference_check(array, 1)->items;
-       for (; item->prog; item++) {
+       for (item = array->items; item->prog; item++) {
                if (item->prog == &dummy_bpf_prog.prog)
                        continue;
                prog_ids[i] = item->prog->aux->id;
@@ -1839,7 +1834,7 @@ static bool bpf_prog_array_copy_core(struct bpf_prog_array __rcu *array,
        return !!(item->prog);
 }
 
-int bpf_prog_array_copy_to_user(struct bpf_prog_array __rcu *array,
+int bpf_prog_array_copy_to_user(struct bpf_prog_array *array,
                                __u32 __user *prog_ids, u32 cnt)
 {
        unsigned long err = 0;
@@ -1850,18 +1845,12 @@ int bpf_prog_array_copy_to_user(struct bpf_prog_array __rcu *array,
         * cnt = bpf_prog_array_length();
         * if (cnt > 0)
         *     bpf_prog_array_copy_to_user(..., cnt);
-        * so below kcalloc doesn't need extra cnt > 0 check, but
-        * bpf_prog_array_length() releases rcu lock and
-        * prog array could have been swapped with empty or larger array,
-        * so always copy 'cnt' prog_ids to the user.
-        * In a rare race the user will see zero prog_ids
+        * so below kcalloc doesn't need extra cnt > 0 check.
         */
        ids = kcalloc(cnt, sizeof(u32), GFP_USER | __GFP_NOWARN);
        if (!ids)
                return -ENOMEM;
-       rcu_read_lock();
        nospc = bpf_prog_array_copy_core(array, ids, cnt);
-       rcu_read_unlock();
        err = copy_to_user(prog_ids, ids, cnt * sizeof(u32));
        kfree(ids);
        if (err)
@@ -1871,19 +1860,19 @@ int bpf_prog_array_copy_to_user(struct bpf_prog_array __rcu *array,
        return 0;
 }
 
-void bpf_prog_array_delete_safe(struct bpf_prog_array __rcu *array,
+void bpf_prog_array_delete_safe(struct bpf_prog_array *array,
                                struct bpf_prog *old_prog)
 {
-       struct bpf_prog_array_item *item = array->items;
+       struct bpf_prog_array_item *item;
 
-       for (; item->prog; item++)
+       for (item = array->items; item->prog; item++)
                if (item->prog == old_prog) {
                        WRITE_ONCE(item->prog, &dummy_bpf_prog.prog);
                        break;
                }
 }
 
-int bpf_prog_array_copy(struct bpf_prog_array __rcu *old_array,
+int bpf_prog_array_copy(struct bpf_prog_array *old_array,
                        struct bpf_prog *exclude_prog,
                        struct bpf_prog *include_prog,
                        struct bpf_prog_array **new_array)
@@ -1947,7 +1936,7 @@ int bpf_prog_array_copy(struct bpf_prog_array __rcu *old_array,
        return 0;
 }
 
-int bpf_prog_array_copy_info(struct bpf_prog_array __rcu *array,
+int bpf_prog_array_copy_info(struct bpf_prog_array *array,
                             u32 *prog_ids, u32 request_cnt,
                             u32 *prog_cnt)
 {