bpf: support deferring bpf_link dealloc to after RCU grace period

BPF link for some program types is passed as a "context" which can be
used by those BPF programs to look up additional information. E.g., for
multi-kprobes and multi-uprobes, link is used to fetch BPF cookie values.

Because of this runtime dependency, when bpf_link refcnt drops to zero
there could still be active BPF programs running accessing link data.

This patch adds generic support to defer bpf_link dealloc callback to
after RCU GP, if requested. This is done by exposing two different
deallocation callbacks, one synchronous and one deferred. If deferred
one is provided, bpf_link_free() will schedule dealloc_deferred()
callback to happen after RCU GP.

BPF is using two flavors of RCU: "classic" non-sleepable one and RCU
tasks trace one. The latter is used when sleepable BPF programs are
used. bpf_link_free() accommodates that by checking underlying BPF
program's sleepable flag, and goes either through normal RCU GP only for
non-sleepable, or through RCU tasks trace GP *and* then normal RCU GP
(taking into account rcu_trace_implies_rcu_gp() optimization), if BPF
program is sleepable.

We use this for multi-kprobe and multi-uprobe links, which dereference
link during program run. We also preventively switch raw_tp link to use
deferred dealloc callback, as upcoming changes in bpf-next tree expose
raw_tp link data (specifically, cookie value) to BPF program at runtime
as well.

Fixes: 0dcac2725406 ("bpf: Add multi kprobe link")
Fixes: 89ae89f53d20 ("bpf: Add multi uprobe link")
Reported-by: syzbot+981935d9485a560bfbcb@syzkaller.appspotmail.com
Reported-by: syzbot+2cb5a6c573e98db598cc@syzkaller.appspotmail.com
Reported-by: syzbot+62d8b26793e8a2bd0516@syzkaller.appspotmail.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20240328052426.3042617-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4f20f62f9d63da87800af4ac21cbc7c92dae5fb9..890e152d553ea3cc8d16b2a058f2dfd657aba1f8 100644 (file)
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1574,12 +1574,26 @@ struct bpf_link {
        enum bpf_link_type type;
        const struct bpf_link_ops *ops;
        struct bpf_prog *prog;
-       struct work_struct work;
+       /* rcu is used before freeing, work can be used to schedule that
+        * RCU-based freeing before that, so they never overlap
+        */
+       union {
+               struct rcu_head rcu;
+               struct work_struct work;
+       };
 };
 
 struct bpf_link_ops {
        void (*release)(struct bpf_link *link);
+       /* deallocate link resources callback, called without RCU grace period
+        * waiting
+        */
        void (*dealloc)(struct bpf_link *link);
+       /* deallocate link resources callback, called after RCU grace period;
+        * if underlying BPF program is sleepable we go through tasks trace
+        * RCU GP and then "classic" RCU GP
+        */
+       void (*dealloc_deferred)(struct bpf_link *link);
        int (*detach)(struct bpf_link *link);
        int (*update_prog)(struct bpf_link *link, struct bpf_prog *new_prog,
                           struct bpf_prog *old_prog);

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ae2ff73bde7e79aa905f060f753bfc8261451972..c287925471f68ef989ffcd3022942397e9d1465c 100644 (file)
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -3024,17 +3024,46 @@ void bpf_link_inc(struct bpf_link *link)
        atomic64_inc(&link->refcnt);
 }
 
+static void bpf_link_defer_dealloc_rcu_gp(struct rcu_head *rcu)
+{
+       struct bpf_link *link = container_of(rcu, struct bpf_link, rcu);
+
+       /* free bpf_link and its containing memory */
+       link->ops->dealloc_deferred(link);
+}
+
+static void bpf_link_defer_dealloc_mult_rcu_gp(struct rcu_head *rcu)
+{
+       if (rcu_trace_implies_rcu_gp())
+               bpf_link_defer_dealloc_rcu_gp(rcu);
+       else
+               call_rcu(rcu, bpf_link_defer_dealloc_rcu_gp);
+}
+
 /* bpf_link_free is guaranteed to be called from process context */
 static void bpf_link_free(struct bpf_link *link)
 {
+       bool sleepable = false;
+
        bpf_link_free_id(link->id);
        if (link->prog) {
+               sleepable = link->prog->sleepable;
                /* detach BPF program, clean up used resources */
                link->ops->release(link);
                bpf_prog_put(link->prog);
        }
-       /* free bpf_link and its containing memory */
-       link->ops->dealloc(link);
+       if (link->ops->dealloc_deferred) {
+               /* schedule BPF link deallocation; if underlying BPF program
+                * is sleepable, we need to first wait for RCU tasks trace
+                * sync, then go through "classic" RCU grace period
+                */
+               if (sleepable)
+                       call_rcu_tasks_trace(&link->rcu, bpf_link_defer_dealloc_mult_rcu_gp);
+               else
+                       call_rcu(&link->rcu, bpf_link_defer_dealloc_rcu_gp);
+       }
+       if (link->ops->dealloc)
+               link->ops->dealloc(link);
 }
 
 static void bpf_link_put_deferred(struct work_struct *work)
@@ -3544,7 +3573,7 @@ static int bpf_raw_tp_link_fill_link_info(const struct bpf_link *link,
 
 static const struct bpf_link_ops bpf_raw_tp_link_lops = {
        .release = bpf_raw_tp_link_release,
-       .dealloc = bpf_raw_tp_link_dealloc,
+       .dealloc_deferred = bpf_raw_tp_link_dealloc,
        .show_fdinfo = bpf_raw_tp_link_show_fdinfo,
        .fill_link_info = bpf_raw_tp_link_fill_link_info,
 };

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 0b73fe5f7206bf779cf2743c116b49fdec7e7fdc..9dc605f08a23141866ae2340e74d9a1f9212e658 100644 (file)
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2728,7 +2728,7 @@ static int bpf_kprobe_multi_link_fill_link_info(const struct bpf_link *link,
 
 static const struct bpf_link_ops bpf_kprobe_multi_link_lops = {
        .release = bpf_kprobe_multi_link_release,
-       .dealloc = bpf_kprobe_multi_link_dealloc,
+       .dealloc_deferred = bpf_kprobe_multi_link_dealloc,
        .fill_link_info = bpf_kprobe_multi_link_fill_link_info,
 };
 
@@ -3242,7 +3242,7 @@ static int bpf_uprobe_multi_link_fill_link_info(const struct bpf_link *link,
 
 static const struct bpf_link_ops bpf_uprobe_multi_link_lops = {
        .release = bpf_uprobe_multi_link_release,
-       .dealloc = bpf_uprobe_multi_link_dealloc,
+       .dealloc_deferred = bpf_uprobe_multi_link_dealloc,
        .fill_link_info = bpf_uprobe_multi_link_fill_link_info,
 };