[PATCH net] ipmr: Fix access to mfc_cache_list without lock held

[Breno Leitao]

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@xxxxxxxxxx>
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;
 	}
 

---
base-commit: 25d70702142ac2115e75e01a0a985c6ea1d78033
change-id: 20241107-ipmr_rcu-291d85400b16

Best regards,
-- 
Breno Leitao <leitao@xxxxxxxxxx>