Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

ipv6: use RCU iterator to dump route exceptions

rt6_nh_dump_exceptions() uses hlist_for_each_entry() to iterate over RCU-protected exception lists. The caller holds rcu_read_lock(), but does not hold rt6_exception_lock, so rt6_insert_exception() can concurrently add an entry with hlist_add_head_rcu().

KCSAN reports this race (irrelevant details omitted):

================================================================== BUG: KCSAN: data-race in rt6_insert_exception / rt6_nh_dump_exceptions

write (marked) to 0xffff8a7c44c59620 of 8 bytes by interrupt on cpu 5: rt6_insert_exception+0x3bb/0x760 __ip6_rt_update_pmtu+0x4fe/0x750 ip6_sk_update_pmtu+0x19a/0x3b0 udpv6_err+0x3ff/0x800 icmpv6_notify+0x1e1/0x440 icmpv6_rcv+0x8c0/0xab0 ip6_protocol_deliver_rcu+0x616/0x840 ip6_input_finish+0xb9/0x160 ... entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff8a7c44c59620 of 8 bytes by task 549 on cpu 14: rt6_nh_dump_exceptions+0xb3/0x260 rt6_dump_route+0x53e/0x5f0 fib6_dump_node+0x6d/0xf0 fib6_walk_continue+0x290/0x2d0 fib6_dump_table+0x28d/0x360 inet6_dump_fib+0x37d/0x620 rtnl_dumpit+0x7b/0xd0 netlink_dump+0x3ae/0x7e0 ... entry_SYSCALL_64_after_hwframe+0x77/0x7f

4 locks held by dumper/549: ... #1: (rcu_read_lock){....}-{1:3}, at: inet6_dump_fib+0x88/0x620 #2: (&tb->tb6_lock){+.-.}-{3:3}, at: fib6_dump_table+0x1e9…