bpf: Support bpf_trampoline on functions with IPMODIFY (e.g. livepatch)

When tracing a function with IPMODIFY ftrace_ops (livepatch), the bpf
trampoline must follow the instruction pointer saved on stack. This needs
extra handling for bpf trampolines with BPF_TRAMP_F_CALL_ORIG flag.

Implement bpf_tramp_ftrace_ops_func and use it for the ftrace_ops used
by BPF trampoline. This enables tracing functions with livepatch.

This also requires moving bpf trampoline to *_ftrace_direct_mult APIs.

Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/all/20220602193706.2607681-2-song@kernel.org/
Link: https://lore.kernel.org/bpf/20220720002126.803253-5-song@kernel.org

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 1d22df8bf306a9328345f9cc83819b1523fbb170..20c26aed78962e43ffff7e3d6ec0c8ff99507219 100644 (file)
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -47,6 +47,7 @@ struct kobject;
 struct mem_cgroup;
 struct module;
 struct bpf_func_state;
+struct ftrace_ops;
 
 extern struct idr btf_idr;
 extern spinlock_t btf_idr_lock;
@@ -756,6 +757,11 @@ struct btf_func_model {
  */
 #define BPF_TRAMP_F_ORIG_STACK         BIT(5)
 
+/* This trampoline is on a function with another ftrace_ops with IPMODIFY,
+ * e.g., a live patch. This flag is set and cleared by ftrace call backs,
+ */
+#define BPF_TRAMP_F_SHARE_IPMODIFY     BIT(6)
+
 /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
  * bytes on x86.
  */
@@ -838,9 +844,11 @@ struct bpf_tramp_image {
 struct bpf_trampoline {
        /* hlist for trampoline_table */
        struct hlist_node hlist;
+       struct ftrace_ops *fops;
        /* serializes access to fields of this trampoline */
        struct mutex mutex;
        refcount_t refcnt;
+       u32 flags;
        u64 key;
        struct {
                struct btf_func_model model;

diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
index 6691dbf9e467a48e39179f66bb6d7f7f12634429..42e387a12694515aa040cf95d4441860322bed92 100644 (file)
--- a/kernel/bpf/trampoline.c
+++ b/kernel/bpf/trampoline.c
@@ -13,6 +13,7 @@
 #include <linux/static_call.h>
 #include <linux/bpf_verifier.h>
 #include <linux/bpf_lsm.h>
+#include <linux/delay.h>
 
 /* dummy _ops. The verifier will operate on target program's ops. */
 const struct bpf_verifier_ops bpf_extension_verifier_ops = {
@@ -29,6 +30,81 @@ static struct hlist_head trampoline_table[TRAMPOLINE_TABLE_SIZE];
 /* serializes access to trampoline_table */
 static DEFINE_MUTEX(trampoline_mutex);
 
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mutex);
+
+static int bpf_tramp_ftrace_ops_func(struct ftrace_ops *ops, enum ftrace_ops_cmd cmd)
+{
+       struct bpf_trampoline *tr = ops->private;
+       int ret = 0;
+
+       if (cmd == FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_SELF) {
+               /* This is called inside register_ftrace_direct_multi(), so
+                * tr->mutex is already locked.
+                */
+               lockdep_assert_held_once(&tr->mutex);
+
+               /* Instead of updating the trampoline here, we propagate
+                * -EAGAIN to register_ftrace_direct_multi(). Then we can
+                * retry register_ftrace_direct_multi() after updating the
+                * trampoline.
+                */
+               if ((tr->flags & BPF_TRAMP_F_CALL_ORIG) &&
+                   !(tr->flags & BPF_TRAMP_F_ORIG_STACK)) {
+                       if (WARN_ON_ONCE(tr->flags & BPF_TRAMP_F_SHARE_IPMODIFY))
+                               return -EBUSY;
+
+                       tr->flags |= BPF_TRAMP_F_SHARE_IPMODIFY;
+                       return -EAGAIN;
+               }
+
+               return 0;
+       }
+
+       /* The normal locking order is
+        *    tr->mutex => direct_mutex (ftrace.c) => ftrace_lock (ftrace.c)
+        *
+        * The following two commands are called from
+        *
+        *   prepare_direct_functions_for_ipmodify
+        *   cleanup_direct_functions_after_ipmodify
+        *
+        * In both cases, direct_mutex is already locked. Use
+        * mutex_trylock(&tr->mutex) to avoid deadlock in race condition
+        * (something else is making changes to this same trampoline).
+        */
+       if (!mutex_trylock(&tr->mutex)) {
+               /* sleep 1 ms to make sure whatever holding tr->mutex makes
+                * some progress.
+                */
+               msleep(1);
+               return -EAGAIN;
+       }
+
+       switch (cmd) {
+       case FTRACE_OPS_CMD_ENABLE_SHARE_IPMODIFY_PEER:
+               tr->flags |= BPF_TRAMP_F_SHARE_IPMODIFY;
+
+               if ((tr->flags & BPF_TRAMP_F_CALL_ORIG) &&
+                   !(tr->flags & BPF_TRAMP_F_ORIG_STACK))
+                       ret = bpf_trampoline_update(tr, false /* lock_direct_mutex */);
+               break;
+       case FTRACE_OPS_CMD_DISABLE_SHARE_IPMODIFY_PEER:
+               tr->flags &= ~BPF_TRAMP_F_SHARE_IPMODIFY;
+
+               if (tr->flags & BPF_TRAMP_F_ORIG_STACK)
+                       ret = bpf_trampoline_update(tr, false /* lock_direct_mutex */);
+               break;
+       default:
+               ret = -EINVAL;
+               break;
+       };
+
+       mutex_unlock(&tr->mutex);
+       return ret;
+}
+#endif
+
 bool bpf_prog_has_trampoline(const struct bpf_prog *prog)
 {
        enum bpf_attach_type eatype = prog->expected_attach_type;
@@ -89,6 +165,16 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
        tr = kzalloc(sizeof(*tr), GFP_KERNEL);
        if (!tr)
                goto out;
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+       tr->fops = kzalloc(sizeof(struct ftrace_ops), GFP_KERNEL);
+       if (!tr->fops) {
+               kfree(tr);
+               tr = NULL;
+               goto out;
+       }
+       tr->fops->private = tr;
+       tr->fops->ops_func = bpf_tramp_ftrace_ops_func;
+#endif
 
        tr->key = key;
        INIT_HLIST_NODE(&tr->hlist);
@@ -128,7 +214,7 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
        int ret;
 
        if (tr->func.ftrace_managed)
-               ret = unregister_ftrace_direct((long)ip, (long)old_addr);
+               ret = unregister_ftrace_direct_multi(tr->fops, (long)old_addr);
        else
                ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, NULL);
 
@@ -137,15 +223,20 @@ static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
        return ret;
 }
 
-static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_addr)
+static int modify_fentry(struct bpf_trampoline *tr, void *old_addr, void *new_addr,
+                        bool lock_direct_mutex)
 {
        void *ip = tr->func.addr;
        int ret;
 
-       if (tr->func.ftrace_managed)
-               ret = modify_ftrace_direct((long)ip, (long)old_addr, (long)new_addr);
-       else
+       if (tr->func.ftrace_managed) {
+               if (lock_direct_mutex)
+                       ret = modify_ftrace_direct_multi(tr->fops, (long)new_addr);
+               else
+                       ret = modify_ftrace_direct_multi_nolock(tr->fops, (long)new_addr);
+       } else {
                ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, old_addr, new_addr);
+       }
        return ret;
 }
 
@@ -163,10 +254,12 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
        if (bpf_trampoline_module_get(tr))
                return -ENOENT;
 
-       if (tr->func.ftrace_managed)
-               ret = register_ftrace_direct((long)ip, (long)new_addr);
-       else
+       if (tr->func.ftrace_managed) {
+               ftrace_set_filter_ip(tr->fops, (unsigned long)ip, 0, 0);
+               ret = register_ftrace_direct_multi(tr->fops, (long)new_addr);
+       } else {
                ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
+       }
 
        if (ret)
                bpf_trampoline_module_put(tr);
@@ -332,11 +425,11 @@ out:
        return ERR_PTR(err);
 }
 
-static int bpf_trampoline_update(struct bpf_trampoline *tr)
+static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mutex)
 {
        struct bpf_tramp_image *im;
        struct bpf_tramp_links *tlinks;
-       u32 flags = BPF_TRAMP_F_RESTORE_REGS;
+       u32 orig_flags = tr->flags;
        bool ip_arg = false;
        int err, total;
 
@@ -358,18 +451,31 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
                goto out;
        }
 
+       /* clear all bits except SHARE_IPMODIFY */
+       tr->flags &= BPF_TRAMP_F_SHARE_IPMODIFY;
+
        if (tlinks[BPF_TRAMP_FEXIT].nr_links ||
-           tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links)
+           tlinks[BPF_TRAMP_MODIFY_RETURN].nr_links) {
                /* NOTE: BPF_TRAMP_F_RESTORE_REGS and BPF_TRAMP_F_SKIP_FRAME
                 * should not be set together.
                 */
-               flags = BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;
+               tr->flags |= BPF_TRAMP_F_CALL_ORIG | BPF_TRAMP_F_SKIP_FRAME;
+       } else {
+               tr->flags |= BPF_TRAMP_F_RESTORE_REGS;
+       }
 
        if (ip_arg)
-               flags |= BPF_TRAMP_F_IP_ARG;
+               tr->flags |= BPF_TRAMP_F_IP_ARG;
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+again:
+       if ((tr->flags & BPF_TRAMP_F_SHARE_IPMODIFY) &&
+           (tr->flags & BPF_TRAMP_F_CALL_ORIG))
+               tr->flags |= BPF_TRAMP_F_ORIG_STACK;
+#endif
 
        err = arch_prepare_bpf_trampoline(im, im->image, im->image + PAGE_SIZE,
-                                         &tr->func.model, flags, tlinks,
+                                         &tr->func.model, tr->flags, tlinks,
                                          tr->func.addr);
        if (err < 0)
                goto out;
@@ -378,17 +484,34 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr)
        WARN_ON(!tr->cur_image && tr->selector);
        if (tr->cur_image)
                /* progs already running at this address */
-               err = modify_fentry(tr, tr->cur_image->image, im->image);
+               err = modify_fentry(tr, tr->cur_image->image, im->image, lock_direct_mutex);
        else
                /* first time registering */
                err = register_fentry(tr, im->image);
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+       if (err == -EAGAIN) {
+               /* -EAGAIN from bpf_tramp_ftrace_ops_func. Now
+                * BPF_TRAMP_F_SHARE_IPMODIFY is set, we can generate the
+                * trampoline again, and retry register.
+                */
+               /* reset fops->func and fops->trampoline for re-register */
+               tr->fops->func = NULL;
+               tr->fops->trampoline = 0;
+               goto again;
+       }
+#endif
        if (err)
                goto out;
+
        if (tr->cur_image)
                bpf_tramp_image_put(tr->cur_image);
        tr->cur_image = im;
        tr->selector++;
 out:
+       /* If any error happens, restore previous flags */
+       if (err)
+               tr->flags = orig_flags;
        kfree(tlinks);
        return err;
 }
@@ -454,7 +577,7 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link, struct bpf_tr
 
        hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
        tr->progs_cnt[kind]++;
-       err = bpf_trampoline_update(tr);
+       err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
        if (err) {
                hlist_del_init(&link->tramp_hlist);
                tr->progs_cnt[kind]--;
@@ -487,7 +610,7 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link, struct bpf_
        }
        hlist_del_init(&link->tramp_hlist);
        tr->progs_cnt[kind]--;
-       return bpf_trampoline_update(tr);
+       return bpf_trampoline_update(tr, true /* lock_direct_mutex */);
 }
 
 /* bpf_trampoline_unlink_prog() should never fail. */
@@ -715,6 +838,7 @@ void bpf_trampoline_put(struct bpf_trampoline *tr)
         * multiple rcu callbacks.
         */
        hlist_del(&tr->hlist);
+       kfree(tr->fops);
        kfree(tr);
 out:
        mutex_unlock(&trampoline_mutex);