From: David Vernet void@manifault.com
[ Upstream commit 77473d1a962f3d4f7ba48324502b6d27b8ef2591 ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/cpumask.c | 19 ++++++++++++++----- 1 file changed, 14 insertions(+), 5 deletions(-)
diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c index 2b3fbbfebdc5f..c08132caca33d 100644 --- 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); }
/**