linux-kernel - Re: [PATCH net] ipmr: Fix access to mfc_cache

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Message-ID: <CANn89iL-L8iBwp=rq-YwAeeoUY2MTjr5akWm=S=k7ckpkaEy+Q@mail.gmail.com>
Date: Thu, 7 Nov 2024 14:13:14 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Breno Leitao <leitao@...ian.org>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	David Ahern <dsahern@...il.com>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel-team@...a.com
Subject: Re: [PATCH net] ipmr: Fix access to mfc_cache_list without lock held

On Thu, Nov 7, 2024 at 12:03 PM Breno Leitao <leitao@...ian.org> wrote:
>
> Accessing `mr_table->mfc_cache_list` is protected by an RCU lock. In the
> following code flow, the lock is not held, causing the following error
> when `RCU_PROVE` is not held.
>
>         6.12.0-rc5-kbuilder-01145-gbac17284bdcb #33 Tainted: G            E    N
>         -----------------------------
>         net/ipv4/ipmr_base.c:313 RCU-list traversed in non-reader section!!
>
>         rcu_scheduler_active = 2, debug_locks = 1
>                    2 locks held by RetransmitAggre/3519:
>                     #0: ffff88816188c6c0 (nlk_cb_mutex-ROUTE){+.+.}-{3:3}, at: __netlink_dump_start+0x8a/0x290
>                     #1: ffffffff83fcf7a8 (rtnl_mutex){+.+.}-{3:3}, at: rtnl_dumpit+0x6b/0x90
>
>         stack backtrace:
>                     lockdep_rcu_suspicious
>                     mr_table_dump
>                     ipmr_rtm_dumproute
>                     rtnl_dump_all
>                     rtnl_dumpit
>                     netlink_dump
>                     __netlink_dump_start
>                     rtnetlink_rcv_msg
>                     netlink_rcv_skb
>                     netlink_unicast
>                     netlink_sendmsg
>
> Fix accessing `mfc_cache_list` without holding the RCU read lock. Adds
> `rcu_read_lock()` and `rcu_read_unlock()` around `mr_table_dump()` to
> prevent RCU-list traversal in non-reader section.
>
> Since `mr_table_dump()` is the only function that touches the list, that
> might be the only critical section in `ipmr_rtm_dumproute()` that needs
> to be protected in ipmr_rtm_dumproute().
>
> Signed-off-by: Breno Leitao <leitao@...ian.org>
> Fixes: cb167893f41e ("net: Plumb support for filtering ipv4 and ipv6 multicast route dumps")
> ---
>  net/ipv4/ipmr.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
> index 089864c6a35eec146a1ba90c22d79245f8e48158..bb855f32f328024f384a2fa58f42fc227705206e 100644
> --- a/net/ipv4/ipmr.c
> +++ b/net/ipv4/ipmr.c
> @@ -2612,8 +2612,10 @@ static int ipmr_rtm_dumproute(struct sk_buff *skb, struct netlink_callback *cb)
>                         NL_SET_ERR_MSG(cb->extack, "ipv4: MR table does not exist");
>                         return -ENOENT;
>                 }
> +               rcu_read_lock();
>                 err = mr_table_dump(mrt, skb, cb, _ipmr_fill_mroute,
>                                     &mfc_unres_lock, &filter);
> +               rcu_read_unlock();
>                 return skb->len ? : err;
>         }
>
>

What about net/ipv6/ip6mr.c ip6mr_rtm_dumproute() ?

In my opinion, since we still hold RTNL in these paths, we should
change the lockdep annotation.

Then later we can remove RTNL from these dump operations.

diff --git a/net/ipv4/ipmr_base.c b/net/ipv4/ipmr_base.c
index 271dc03fc6dbd9b35db4d5782716679134f225e4..f0af12a2f70bcdf5ba54321bf7ebebe798318abb
100644
--- a/net/ipv4/ipmr_base.c
+++ b/net/ipv4/ipmr_base.c
@@ -310,7 +310,8 @@ int mr_table_dump(struct mr_table *mrt, struct sk_buff *skb,
        if (filter->filter_set)
                flags |= NLM_F_DUMP_FILTERED;

-       list_for_each_entry_rcu(mfc, &mrt->mfc_cache_list, list) {
+       list_for_each_entry_rcu(mfc, &mrt->mfc_cache_list, list,
+                               lockdep_rtnl_is_held()) {
                if (e < s_e)
                        goto next_entry;
                if (filter->dev &&