In the Linux kernel, the following vulnerability has been resolved: ipv6: use RCU iterator to dump route exceptions rt6_nh_dump_exceptions

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+0x1e9/0x360
#3: (rcu_read_lock){....}-{1:3}, at: rt6_dump_route+0x483/0x5f0

value changed: 0xffff8a7c44e05700 -> 0xffff8a7c45d60100

Reported by Kernel Concurrency Sanitizer on:
CPU: 14 UID: 0 PID: 549 Comm: dumper Not tainted
7.2.0-rc7-virtme #38 PREEMPT(lazy)
...

Use hlist_for_each_entry_rcu() to safely iterate over the exception list.