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 EFE281DA55 for ; Tue, 22 Aug 2023 16:02:19 +0000 (UTC) Received: from out-27.mta1.migadu.com (out-27.mta1.migadu.com [IPv6:2001:41d0:203:375::1b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 885C8BE for ; Tue, 22 Aug 2023 09:02:17 -0700 (PDT) Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1692720135; h=from:from:reply-to: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=+r5UfTX3lllvtiy0BgOAzFEWjPYUDYgCP542WvGJY24=; b=ZsR00vBbgQNB0nRfrHHGlO4kz0Pdeq9ZXQoEvQrfVJdJb3PHJNw66iakLps/jp6dWVPkNp QxUPAMo9yOGVRfJg4B8JcrN2T4NE757/yF/M0OjzLiUfDsWrQWAf7R4hRBedCJHcoUJjfP OcuEOJp5717sld4MHpS7zWJbNsIWJXI= Date: Tue, 22 Aug 2023 09:02:09 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Reply-To: yonghong.song@linux.dev Subject: Re: [PATCH v2 bpf-next 5/7] bpf: Consider non-owning refs to refcounted nodes RCU protected Content-Language: en-US To: David Marchevsky , 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> <21f00803-d20f-e584-6512-67e5107e3865@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <21f00803-d20f-e584-6512-67e5107e3865@linux.dev> Content-Type: text/plain; charset=UTF-8; format=flowed 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,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 10:47 PM, David Marchevsky wrote: > 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/ Okay, I see. Thanks for the pointer. If MEM_RCU is marked not in explicit rcu cs, it does make sense to clobber everything when bpf_spin_unlock is hit. But see below. > >> >> 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. I removed >>> + case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU: and >>> + if (rec->refcount_off >= 0) >>> + reg->type |= MEM_RCU; All 'refcount' selftests seem working fine. If this is indeed unnecessary for this patch set, maybe we can delay this RCU marking thing for a future patch set? > >>> + >>>       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; >>>                   }