[PATCH 5.12 295/296] neighbour: Prevent Race condition in neighbour subsytem

Greg Kroah-Hartman
2021-05-31T16:51:48Z

From: Chinmay Agarwal <chinagar@codeaurora.org>

commit eefb45eef5c4c425e87667af8f5e904fbdd47abf upstream.

Following Race Condition was detected:

<CPU A, t0>: Executing: __netif_receive_skb() ->__netif_receive_skb_core()
-> arp_rcv() -> arp_process().arp_process() calls __neigh_lookup() which
takes a reference on neighbour entry 'n'.
Moves further along, arp_process() and calls neigh_update()->
__neigh_update(). Neighbour entry is unlocked just before a call to
neigh_update_gc_list.

This unlocking paves way for another thread that may take a reference on
the same and mark it dead and remove it from gc_list.

<CPU B, t1> - neigh_flush_dev() is under execution and calls
neigh_mark_dead(n) marking the neighbour entry 'n' as dead. Also n will be
removed from gc_list.
Moves further along neigh_flush_dev() and calls
neigh_cleanup_and_release(n), but since reference count increased in t1,
'n' couldn't be destroyed.

<CPU A, t3>- Code hits neigh_update_gc_list, with neighbour entry
set as dead.

<CPU A, t4> - arp_process() finally calls neigh_release(n), destroying
the neighbour entry and we have a destroyed ntry still part of gc_list.

Fixes: eb4e8fac00d1("neighbour: Prevent a dead entry from updating gc_list")
Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/core/neighbour.c | 4 ++++
1 file changed, 4 insertions(+)

--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -131,6 +131,9 @@ static void neigh_update_gc_list(struct
write_lock_bh(&n->tbl->lock);
write_lock(&n->lock);

+ if (n->dead)
+ goto out;
+
/* remove from the gc list if new state is permanent or if neighbor
* is externally learned; otherwise entry should be on the gc list
*/
@@ -147,6 +150,7 @@ static void neigh_update_gc_list(struct
atomic_inc(&n->tbl->gc_entries);
}

+out:
write_unlock(&n->lock);
write_unlock_bh(&n->tbl->lock);
}