bpf: Free struct bpf_cpumask in call_rcu handler

The struct bpf_cpumask type uses the bpf_mem_cache_{alloc,free}() APIs
to allocate and free its cpumasks. The bpf_mem allocator may currently
immediately reuse some memory when its freed, without waiting for an RCU
read cycle to elapse. We want to be able to treat struct bpf_cpumask
objects as completely RCU safe.

This is necessary for two reasons:

1. bpf_cpumask_kptr_get() currently does an RCU-protected
   refcnt_inc_not_zero(). This of course assumes that the underlying
   memory is not reused, and is therefore unsafe in its current form.

2. We want to be able to get rid of bpf_cpumask_kptr_get() entirely, and
   intead use the superior kptr RCU semantics now afforded by the
   verifier.

This patch fixes (1), and enables (2), by making struct bpf_cpumask RCU
safe. A subsequent patch will update the verifier to allow struct
bpf_cpumask * pointers to be passed to KF_RCU kfuncs, and then a latter
patch will remove bpf_cpumask_kptr_get().

Fixes: 516f4d3397c9 ("bpf: Enable cpumasks to be queried and used as kptrs")
Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230316054028.88924-2-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
index b6587ec..98eea62 100644 (file)
--- a/kernel/bpf/cpumask.c
+++ b/kernel/bpf/cpumask.c
@@ -9,6 +9,7 @@
 /**
  * struct bpf_cpumask - refcounted BPF cpumask wrapper structure
  * @cpumask:   The actual cpumask embedded in the struct.
+ * @rcu:       The RCU head used to free the cpumask with RCU safety.
  * @usage:     Object reference counter. When the refcount goes to 0, the
  *             memory is released back to the BPF allocator, which provides
  *             RCU safety.
@@ -24,6 +25,7 @@
  */
 struct bpf_cpumask {
        cpumask_t cpumask;
+       struct rcu_head rcu;
        refcount_t usage;
 };
 
@@ -108,6 +110,16 @@ __bpf_kfunc struct bpf_cpumask *bpf_cpumask_kptr_get(struct bpf_cpumask **cpumas
        return cpumask;
 }
 
+static void cpumask_free_cb(struct rcu_head *head)
+{
+       struct bpf_cpumask *cpumask;
+
+       cpumask = container_of(head, struct bpf_cpumask, rcu);
+       migrate_disable();
+       bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
+       migrate_enable();
+}
+
 /**
  * bpf_cpumask_release() - Release a previously acquired BPF cpumask.
  * @cpumask: The cpumask being released.
@@ -121,11 +133,8 @@ __bpf_kfunc void bpf_cpumask_release(struct bpf_cpumask *cpumask)
        if (!cpumask)
                return;
 
-       if (refcount_dec_and_test(&cpumask->usage)) {
-               migrate_disable();
-               bpf_mem_cache_free(&bpf_cpumask_ma, cpumask);
-               migrate_enable();
-       }
+       if (refcount_dec_and_test(&cpumask->usage))
+               call_rcu(&cpumask->rcu, cpumask_free_cb);
 }
 
 /**