From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4A8681FA1 for ; Tue, 22 Aug 2023 05:47:13 +0000 (UTC) Received: from out-13.mta1.migadu.com (out-13.mta1.migadu.com [95.215.58.13]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DA085D7 for ; Mon, 21 Aug 2023 22:47:10 -0700 (PDT) Message-ID: <21f00803-d20f-e584-6512-67e5107e3865@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1692683227; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Xxr13pPfjkqiq/V6WW0yrGhbJUtOYjc/LI04ekneqUo=; b=KUbLoX9qg3tRiFX4spF5t6SUDTAyPjttJi1E8lQNNtVep6pw+GuqrwUhKyKns6mnYh+4sn +CCptdA+LwjnRPPQ/aXQpIGhAC7W6SoONIZjx6MydybvVHXXsJbUEnOn9JT8Kk1avxT6/E sM16JwgBbqYb9xtRa4XUAbTp0qohxA0= Date: Tue, 22 Aug 2023 01:47:01 -0400 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v2 bpf-next 5/7] bpf: Consider non-owning refs to refcounted nodes RCU protected Content-Language: en-US To: yonghong.song@linux.dev, Dave Marchevsky , bpf@vger.kernel.org Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Kernel Team References: <20230821193311.3290257-1-davemarchevsky@fb.com> <20230821193311.3290257-6-davemarchevsky@fb.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: David Marchevsky In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_BLOCKED, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net On 8/21/23 10:37 PM, Yonghong Song wrote: > > > On 8/21/23 12:33 PM, Dave Marchevsky wrote: >> An earlier patch in the series ensures that the underlying memory of >> nodes with bpf_refcount - which can have multiple owners - is not reused >> until RCU grace period has elapsed. This prevents >> use-after-free with non-owning references that may point to >> recently-freed memory. While RCU read lock is held, it's safe to >> dereference such a non-owning ref, as by definition RCU GP couldn't have >> elapsed and therefore underlying memory couldn't have been reused. >> >>  From the perspective of verifier "trustedness" non-owning refs to >> refcounted nodes are now trusted only in RCU CS and therefore should no >> longer pass is_trusted_reg, but rather is_rcu_reg. Let's mark them >> MEM_RCU in order to reflect this new state. >> >> Signed-off-by: Dave Marchevsky >> --- >>   include/linux/bpf.h   |  3 ++- >>   kernel/bpf/verifier.c | 13 ++++++++++++- >>   2 files changed, 14 insertions(+), 2 deletions(-) >> >> diff --git a/include/linux/bpf.h b/include/linux/bpf.h >> index eced6400f778..12596af59c00 100644 >> --- a/include/linux/bpf.h >> +++ b/include/linux/bpf.h >> @@ -653,7 +653,8 @@ enum bpf_type_flag { >>       MEM_RCU            = BIT(13 + BPF_BASE_TYPE_BITS), >>         /* Used to tag PTR_TO_BTF_ID | MEM_ALLOC references which are non-owning. >> -     * Currently only valid for linked-list and rbtree nodes. >> +     * Currently only valid for linked-list and rbtree nodes. If the nodes >> +     * have a bpf_refcount_field, they must be tagged MEM_RCU as well. >>        */ >>       NON_OWN_REF        = BIT(14 + BPF_BASE_TYPE_BITS), >>   diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> index 8db0afa5985c..55607ab30522 100644 >> --- a/kernel/bpf/verifier.c >> +++ b/kernel/bpf/verifier.c >> @@ -8013,6 +8013,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env, >>       case PTR_TO_BTF_ID | PTR_TRUSTED: >>       case PTR_TO_BTF_ID | MEM_RCU: >>       case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF: >> +    case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU: >>           /* When referenced PTR_TO_BTF_ID is passed to release function, >>            * its fixed offset must be 0. In the other cases, fixed offset >>            * can be non-zero. This was already checked above. So pass >> @@ -10479,6 +10480,7 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env, >>   static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state *reg) >>   { >>       struct bpf_verifier_state *state = env->cur_state; >> +    struct btf_record *rec = reg_btf_record(reg); >>         if (!state->active_lock.ptr) { >>           verbose(env, "verifier internal error: ref_set_non_owning w/o active lock\n"); >> @@ -10491,6 +10493,9 @@ static int ref_set_non_owning(struct bpf_verifier_env *env, struct bpf_reg_state >>       } >>         reg->type |= NON_OWN_REF; >> +    if (rec->refcount_off >= 0) >> +        reg->type |= MEM_RCU; > > Should the above MEM_RCU marking be done unless reg access is in > rcu critical section? I think it is fine, since non-owning references currently exist only within spin_lock CS. Based on Alexei's comments on v1 of this series [0], preemption disabled + spin_lock CS should imply RCU CS. [0]: https://lore.kernel.org/bpf/20230802230715.3ltalexaczbomvbu@MacBook-Pro-8.local/ > > I think we still have issues for state resetting > with bpf_spin_unlock() and bpf_rcu_read_unlock(), both of which > will try to convert the reg state to PTR_UNTRUSTED. > > Let us say reg state is >   PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU > > (1). If hitting bpf_spin_unlock(), since MEM_RCU is in > the reg state, the state should become >   PTR_TO_BTF_ID | MEM_ALLOC | MEM_RCU > some additional code might be needed so we wont have > verifier complaints about ref_obj_id == 0. > > (2). If hitting bpf_rcu_read_unlock(), the state should become >   PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF > since register access still in bpf_spin_lock() region. I agree w/ your comment in side reply stating that this case isn't possible since bpf_rcu_read_{lock,unlock} in spin_lock CS is currently not allowed. > > Does this make sense? > IIUC the specific reg state flow you're recommending is based on the convos we've had over the past few weeks re: getting rid of special non-owning ref lifetime rules, instead using RCU as much as possible. Specifically, this recommended change would remove non-owning ref clobbering, instead just removing NON_OWN_REF flag on bpf_spin_unlock so that such nodes can no longer be passed to collection kfuncs (refcount_acquire, etc). I agree that in general we'll be able to loosen the lifetime logic for non-owning refs, and your specific suggestion sounds reasonable. IMO it's better to ship that as a separate series, though, as this series was meant to be the minimum changes necessary to re-enable bpf_refcount_acquire, and it's expanded a bit past that already. Easier to reason about the rest of this series' changes without having to account for clobbering changes. >> + >>       return 0; >>   } >>   @@ -11328,6 +11333,11 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, >>           struct bpf_func_state *state; >>           struct bpf_reg_state *reg; >>   +        if (in_rbtree_lock_required_cb(env) && (rcu_lock || rcu_unlock)) { >> +            verbose(env, "Calling bpf_rcu_read_{lock,unlock} in unnecessary rbtree callback\n"); >> +            return -EACCES; >> +        } >> + >>           if (rcu_lock) { >>               verbose(env, "nested rcu read lock (kernel function %s)\n", func_name); >>               return -EINVAL; >> @@ -16689,7 +16699,8 @@ static int do_check(struct bpf_verifier_env *env) >>                       return -EINVAL; >>                   } >>   -                if (env->cur_state->active_rcu_lock) { >> +                if (env->cur_state->active_rcu_lock && >> +                    !in_rbtree_lock_required_cb(env)) { >>                       verbose(env, "bpf_rcu_read_unlock is missing\n"); >>                       return -EINVAL; >>                   }