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Message-ID: <CAEA6p_DJXZ9Or7-CF8VC8yHEUHrSA48ty6U5ssA4WVoUSH-5Kg@mail.gmail.com>
Date: Sat, 6 Jan 2018 17:41:28 -0800
From: Wei Wang <weiwan@...gle.com>
To: Martin KaFai Lau <kafai@...com>
Cc: David Miller <davem@...emloft.net>,
Linux Kernel Network Developers <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] ipv6: remove null_entry before adding default route
On Fri, Jan 5, 2018 at 11:42 PM, Martin KaFai Lau <kafai@...com> wrote:
> On Fri, Jan 05, 2018 at 05:38:35PM -0800, Wei Wang wrote:
>> From: Wei Wang <weiwan@...gle.com>
>>
>> In the current code, when creating a new fib6 table, tb6_root.leaf gets
>> initialized to net->ipv6.ip6_null_entry.
>> If a default route is being added with rt->rt6i_metric = 0xffffffff,
>> fib6_add() will add this route after net->ipv6.ip6_null_entry. As
>> null_entry is shared, it could cause problem.
>>
>> In order to fix it, set fn->leaf to NULL before calling
>> fib6_add_rt2node() when trying to add the first default route.
>> And reset fn->leaf to null_entry when adding fails or when deleting the
>> last default route.
>>
>> syzkaller reported the following issue which is fixed by this commit:
>> =============================
>> WARNING: suspicious RCU usage
>> 4.15.0-rc5+ #171 Not tainted
>> -----------------------------
>> net/ipv6/ip6_fib.c:1702 suspicious rcu_dereference_protected() usage!
>>
>> other info that might help us debug this:
>>
>> rcu_scheduler_active = 2, debug_locks = 1
>> 4 locks held by swapper/0/0:
>> #0: ((&net->ipv6.ip6_fib_timer)){+.-.}, at: [<00000000d43f631b>] lockdep_copy_map include/linux/lockdep.h:178 [inline]
>> #0: ((&net->ipv6.ip6_fib_timer)){+.-.}, at: [<00000000d43f631b>] call_timer_fn+0x1c6/0x820 kernel/time/timer.c:1310
>> #1: (&(&net->ipv6.fib6_gc_lock)->rlock){+.-.}, at: [<000000002ff9d65c>] spin_lock_bh include/linux/spinlock.h:315 [inline]
>> #1: (&(&net->ipv6.fib6_gc_lock)->rlock){+.-.}, at: [<000000002ff9d65c>] fib6_run_gc+0x9d/0x3c0 net/ipv6/ip6_fib.c:2007
>> #2: (rcu_read_lock){....}, at: [<0000000091db762d>] __fib6_clean_all+0x0/0x3a0 net/ipv6/ip6_fib.c:1560
>> #3: (&(&tb->tb6_lock)->rlock){+.-.}, at: [<000000009e503581>] spin_lock_bh include/linux/spinlock.h:315 [inline]
>> #3: (&(&tb->tb6_lock)->rlock){+.-.}, at: [<000000009e503581>] __fib6_clean_all+0x1d0/0x3a0 net/ipv6/ip6_fib.c:1948
>>
>> stack backtrace:
>> CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.15.0-rc5+ #171
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> Call Trace:
>> <IRQ>
>> __dump_stack lib/dump_stack.c:17 [inline]
>> dump_stack+0x194/0x257 lib/dump_stack.c:53
>> lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4585
>> fib6_del+0xcaa/0x11b0 net/ipv6/ip6_fib.c:1701
>> fib6_clean_node+0x3aa/0x4f0 net/ipv6/ip6_fib.c:1892
>> fib6_walk_continue+0x46c/0x8a0 net/ipv6/ip6_fib.c:1815
>> fib6_walk+0x91/0xf0 net/ipv6/ip6_fib.c:1863
>> fib6_clean_tree+0x1e6/0x340 net/ipv6/ip6_fib.c:1933
>> __fib6_clean_all+0x1f4/0x3a0 net/ipv6/ip6_fib.c:1949
>> fib6_clean_all net/ipv6/ip6_fib.c:1960 [inline]
>> fib6_run_gc+0x16b/0x3c0 net/ipv6/ip6_fib.c:2016
>> fib6_gc_timer_cb+0x20/0x30 net/ipv6/ip6_fib.c:2033
>> call_timer_fn+0x228/0x820 kernel/time/timer.c:1320
>> expire_timers kernel/time/timer.c:1357 [inline]
>> __run_timers+0x7ee/0xb70 kernel/time/timer.c:1660
>> run_timer_softirq+0x4c/0xb0 kernel/time/timer.c:1686
>> __do_softirq+0x2d7/0xb85 kernel/softirq.c:285
>> invoke_softirq kernel/softirq.c:365 [inline]
>> irq_exit+0x1cc/0x200 kernel/softirq.c:405
>> exiting_irq arch/x86/include/asm/apic.h:540 [inline]
>> smp_apic_timer_interrupt+0x16b/0x700 arch/x86/kernel/apic/apic.c:1052
>> apic_timer_interrupt+0xa9/0xb0 arch/x86/entry/entry_64.S:904
>> </IRQ>
>>
>> Reported-by: syzbot <syzkaller@...glegroups.com>
>> Fixes: 66f5d6ce53e6 ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
>> Signed-off-by: Wei Wang <weiwan@...gle.com>
>> ---
>> net/ipv6/ip6_fib.c | 45 +++++++++++++++++++++++++++++++++++----------
>> 1 file changed, 35 insertions(+), 10 deletions(-)
>>
>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
>> index d11a5578e4f8..37cb4ad1ea29 100644
>> --- a/net/ipv6/ip6_fib.c
>> +++ b/net/ipv6/ip6_fib.c
>> @@ -640,6 +640,11 @@ static struct fib6_node *fib6_add_1(struct net *net,
>> if (!(fn->fn_flags & RTN_RTINFO)) {
>> RCU_INIT_POINTER(fn->leaf, NULL);
>> rt6_release(leaf);
>> + /* remove null_entry in the root node */
>> + } else if (fn->fn_flags & RTN_TL_ROOT &&
>> + rcu_access_pointer(fn->leaf) ==
>> + net->ipv6.ip6_null_entry) {
>> + RCU_INIT_POINTER(fn->leaf, NULL);
> It seems the reader side could see tb6_root.leaf == NULL after
> this change and I think it should be fine? If it is, instead
> of switching betwen NULL and ip6_null_entry, would it be simpler
> to always set tb6_root.leaf to NULL until a legit default route
> is added?
>
Agree that reader side could see tb6_root.leaf == NULL and it should
be fine cause the lookup code path takes care of NULL case.
If we init tb6_root to NULL, it does seem simpler. The only place we
need to take care is that fib6_del() should not try to expunge fn if
is root node if it does not contain any route. But does it seem weird
that tb6_root.fn_flags is marked with RTN_RTINFO without containing
any route?
>> }
>>
>> return fn;
>> @@ -1270,14 +1275,27 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt,
>> return err;
>>
>> failure:
>> - /* fn->leaf could be NULL if fn is an intermediate node and we
>> - * failed to add the new route to it in both subtree creation
>> - * failure and fib6_add_rt2node() failure case.
>> - * In both cases, fib6_repair_tree() should be called to fix
>> + /* fn->leaf could be NULL if:
>> + * 1. fn is the root node in the table and we fail to add the default
>> + * route to it.
>> + * In this case, we put fn->leaf back to net->ipv6.ip6_null_entry as
>> + * the way the table was created.
>> + * 2. fn is an intermediate node and we failed to add the new
>> + * route to it in both subtree creation failure and fib6_add_rt2node()
>> + * failure case.
>> + * In this case, fib6_repair_tree() should be called to fix
>> * fn->leaf.
>> */
>> - if (fn && !(fn->fn_flags & (RTN_RTINFO|RTN_ROOT)))
>> - fib6_repair_tree(info->nl_net, table, fn);
>> + if (fn) {
>> + if (fn->fn_flags & RTN_TL_ROOT) {
>> + if (!rcu_access_pointer(fn->leaf)) {
>> + rcu_assign_pointer(fn->leaf,
>> + info->nl_net->ipv6.ip6_null_entry);
>> + }
>> + } else if (!(fn->fn_flags & (RTN_RTINFO|RTN_ROOT))) {
>> + fib6_repair_tree(info->nl_net, table, fn);
>> + }
>> + }
>> /* Always release dst as dst->__refcnt is guaranteed
>> * to be taken before entering this function
>> */
>> @@ -1685,11 +1703,18 @@ static void fib6_del_route(struct fib6_table *table, struct fib6_node *fn,
>> }
>> read_unlock(&net->ipv6.fib6_walker_lock);
>>
>> - /* If it was last route, expunge its radix tree node */
>> + /* If it was last route:
>> + * 1. For root node, put back null_entry as how the table was created.
>> + * 2. For other nodes, expunge its radix tree node.
>> + */
>> if (!rcu_access_pointer(fn->leaf)) {
>> - fn->fn_flags &= ~RTN_RTINFO;
>> - net->ipv6.rt6_stats->fib_route_nodes--;
>> - fn = fib6_repair_tree(net, table, fn);
>> + if (fn->fn_flags & RTN_TL_ROOT) {
>> + rcu_assign_pointer(fn->leaf, net->ipv6.ip6_null_entry);
>> + } else {
>> + fn->fn_flags &= ~RTN_RTINFO;
>> + net->ipv6.rt6_stats->fib_route_nodes--;
>> + fn = fib6_repair_tree(net, table, fn);
>> + }
>> }
>>
>> fib6_purge_rt(rt, fn, net);
>> --
>> 2.16.0.rc0.223.g4a4ac83678-goog
>>
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