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[209.85.217.42]) by smtp.gmail.com with ESMTPSA id n27sm269840vsl.26.2020.10.02.08.29.03 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 02 Oct 2020 08:29:04 -0700 (PDT) Received: by mail-vs1-f42.google.com with SMTP id x1so829595vsf.1 for ; Fri, 02 Oct 2020 08:29:03 -0700 (PDT) X-Received: by 2002:a67:87c4:: with SMTP id j187mr1640749vsd.34.1601652543334; Fri, 02 Oct 2020 08:29:03 -0700 (PDT) MIME-Version: 1.0 References: <20201002141531.7081-1-manivannan.sadhasivam@linaro.org> In-Reply-To: <20201002141531.7081-1-manivannan.sadhasivam@linaro.org> From: Doug Anderson Date: Fri, 2 Oct 2020 08:28:51 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] net: qrtr: ns: Fix the incorrect usage of rcu_read_lock() To: Manivannan Sadhasivam Cc: David Miller , Jakub Kicinski , Bjorn Andersson , netdev , LKML , Alex Elder Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Fri, Oct 2, 2020 at 7:15 AM Manivannan Sadhasivam wrote: > > The rcu_read_lock() is not supposed to lock the kernel_sendmsg() API > since it has the lock_sock() in qrtr_sendmsg() which will sleep. Hence, > fix it by excluding the locking for kernel_sendmsg(). > > Fixes: a7809ff90ce6 ("net: qrtr: ns: Protect radix_tree_deref_slot() using rcu read locks") > Reported-by: Doug Anderson > Tested-by: Alex Elder > Signed-off-by: Manivannan Sadhasivam > --- > net/qrtr/ns.c | 20 ++++++++++++++------ > 1 file changed, 14 insertions(+), 6 deletions(-) > > diff --git a/net/qrtr/ns.c b/net/qrtr/ns.c > index 934999b56d60..0515433de922 100644 > --- a/net/qrtr/ns.c > +++ b/net/qrtr/ns.c > @@ -203,15 +203,17 @@ static int announce_servers(struct sockaddr_qrtr *sq) > /* Announce the list of servers registered in this node */ > radix_tree_for_each_slot(slot, &node->servers, &iter, 0) { > srv = radix_tree_deref_slot(slot); > + rcu_read_unlock(); My RCU-fu is mediocre at best and my radix-tree knowledge is non-existent. However: => Reading through radix_tree_deref_slot() it says that if you are only holding the read lock that you need to be calling radix_tree_deref_retry(). Why don't I see that here? => Without any real knowledge, it seems super sketchy to drop the lock while iterating over the tree. Somehow that feels unsafe. Hrm, there seems to be a function radix_tree_iter_resume() that might be exactly what you want, but I'm not totally sure. The only user I can see in-tree (other than radix tree regression testing) is btrfs-tests.c but it's using it together with radix_tree_deref_slot_protected(). In any case, my totally untested and totally knowedge-free proposal would look something like this: rcu_read_lock(); /* Announce the list of servers registered in this node */ radix_tree_for_each_slot(slot, &node->servers, &iter, 0) { srv = radix_tree_deref_slot(slot); if (!srv) continue; if (radix_tree_deref_retry(srv)) { slot = radix_tree_iter_retry(&iter); continue; } slot = radix_tree_iter_resume(slot, &iter); rcu_read_unlock(); ret = service_announce_new(sq, srv); if (ret < 0) { pr_err("failed to announce new service\n"); return ret; } rcu_read_lock(); } rcu_read_unlock(); What a beast! Given that this doesn't seem to be what anyone else in the kernel is doing exactly, it makes me suspect that there's a more fundamental design issue here, though... -Doug