On Sat, Jan 6, 2018 at 10:16 PM, Martin KaFai Lau <ka...@fb.com> wrote: > On Sat, Jan 06, 2018 at 05:41:28PM -0800, Wei Wang wrote: >> On Fri, Jan 5, 2018 at 11:42 PM, Martin KaFai Lau <ka...@fb.com> wrote: >> > On Fri, Jan 05, 2018 at 05:38:35PM -0800, Wei Wang wrote: >> >> From: Wei Wang <wei...@google.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 <syzkal...@googlegroups.com> >> >> Fixes: 66f5d6ce53e6 ("ipv6: replace rwlock with rcu and spinlock in >> >> fib6_table") >> >> Signed-off-by: Wei Wang <wei...@google.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? > Good point on the writer's side. I think we also need to modify > fib6_add_1() to expect fn->leaf == NULL. Also, agree that > RTN_RTINFO needs to be set/clear accordingly. We also need to > make changes to the CONFIG_IPV6_SUBTREES...... > It should be doable but requires more changes to break the writer's side > assumption. net-next may be a better fit for those changes. >
Understood. Yes. Agree. I will give it a try on net-next. > For this patch, I have one inline comment below: > >> >> >> } >> >> >> >> 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); > fib6_repair_tree() is responsible to ensure intermediate fn->leaf is > not NULL (or free the fn if possible). In this case, we want tb6_root.leaf > to be not NULL. Can we consolidate the tb6_root.leaf fixing codes to > fib6_repair_tree() also such that all tree repairing logic is > in one place? > Yes. Good idea. I will send out v2 for this. >> >> + } >> >> + } >> >> /* 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 >> >>