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Date:   Tue, 23 Apr 2019 10:40:52 -0700
From:   Eric Dumazet <>
To:     David Ahern <>
Cc:     "David S . Miller" <>,
        netdev <>,
        Eric Dumazet <>,
        Wei Wang <>
Subject: Re: [PATCH net-next 1/3] ipv6: fib6_info_destroy_rcu() cleanup

On Tue, Apr 23, 2019 at 9:33 AM David Ahern <> wrote:
> On 4/22/19 7:35 PM, Eric Dumazet wrote:
> > We do not need to clear f6i->rt6i_exception_bucket right before
> > freeing f6i.
> >
> > Note that f6i->rt6i_exception_bucket is properly protected by
> > f6i->exception_bucket_flushed being set to one in rt6_flush_exceptions()
> > under the protection of rt6_exception_lock.
> >
> > Signed-off-by: Eric Dumazet <>
> > Cc: Wei Wang <>
> > ---
> >  net/ipv6/ip6_fib.c | 5 +----
> >  1 file changed, 1 insertion(+), 4 deletions(-)
> >
> Reviewed-by: David Ahern <>

Thanks David

I believe we have a problem with f6i->rt6i_pcpu rules :

It seems concurrent lookups can happily use it while
the call_rcu() is ongoing before fib6_info_destroy_rcu() is finally called.

We lack proper RCU rules here. Concurrent lookups should be warned
that the destroy operation is ongoing.

One solution could look at fib6_ref value : If it is 0, then we are
not allowed to mess with rt6i_pcpu.

This is why I was looking at this function :/

BUG: KASAN: user-memory-access in atomic_read
include/asm-generic/atomic-instrumented.h:26 [inline]
BUG: KASAN: user-memory-access in atomic_fetch_add_unless
include/linux/atomic-fallback.h:1086 [inline]
BUG: KASAN: user-memory-access in atomic_add_unless
include/linux/atomic-fallback.h:1111 [inline]
BUG: KASAN: user-memory-access in atomic_inc_not_zero
include/linux/atomic-fallback.h:1127 [inline]
BUG: KASAN: user-memory-access in dst_hold_safe include/net/dst.h:308 [inline]
BUG: KASAN: user-memory-access in ip6_hold_safe+0xb3/0x3a0 net/ipv6/route.c:1023
Read of size 4 at addr 00000000000191e0 by task syz-executor.0/28086

CPU: 0 PID: 28086 Comm: syz-executor.0 Not tainted 5.1.0-rc5+ #176
Hardware name: Google Google Compute Engine/Google Compute Engine,
BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x172/0x1f0 lib/dump_stack.c:113
 kasan_report.cold+0x5/0x40 mm/kasan/report.c:321
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x123/0x190 mm/kasan/generic.c:191
 kasan_check_read+0x11/0x20 mm/kasan/common.c:102
 atomic_read include/asm-generic/atomic-instrumented.h:26 [inline]
 atomic_fetch_add_unless include/linux/atomic-fallback.h:1086 [inline]
 atomic_add_unless include/linux/atomic-fallback.h:1111 [inline]
 atomic_inc_not_zero include/linux/atomic-fallback.h:1127 [inline]
 dst_hold_safe include/net/dst.h:308 [inline]
 ip6_hold_safe+0xb3/0x3a0 net/ipv6/route.c:1023
 rt6_get_pcpu_route net/ipv6/route.c:1245 [inline]
 ip6_pol_route+0x348/0x1040 net/ipv6/route.c:1906
 ip6_pol_route_output+0x54/0x70 net/ipv6/route.c:2082
 fib6_rule_lookup+0x269/0x560 net/ipv6/fib6_rules.c:122
 ip6_route_output_flags+0x2c4/0x350 net/ipv6/route.c:2111
 ip6_route_output include/net/ip6_route.h:88 [inline]
 icmpv6_xrlim_allow net/ipv6/icmp.c:213 [inline]
 icmp6_send+0x1150/0x21f0 net/ipv6/icmp.c:541
 icmpv6_send+0xee/0x230 net/ipv6/ip6_icmp.c:43
 __udp6_lib_rcv+0x1ad1/0x2cc0 net/ipv6/udp.c:939
 udpv6_rcv+0x22/0x30 net/ipv6/udp.c:1028
 ip6_protocol_deliver_rcu+0x303/0x16c0 net/ipv6/ip6_input.c:394
 ip6_input_finish+0x84/0x170 net/ipv6/ip6_input.c:434
 NF_HOOK include/linux/netfilter.h:289 [inline]
 NF_HOOK include/linux/netfilter.h:283 [inline]
 ip6_input+0xe4/0x3f0 net/ipv6/ip6_input.c:443
 dst_input include/net/dst.h:450 [inline]
 ip6_rcv_finish+0x1e7/0x320 net/ipv6/ip6_input.c:76
 NF_HOOK include/linux/netfilter.h:289 [inline]
 NF_HOOK include/linux/netfilter.h:283 [inline]
 ipv6_rcv+0x10e/0x420 net/ipv6/ip6_input.c:272
 __netif_receive_skb_one_core+0x115/0x1a0 net/core/dev.c:4987
 __netif_receive_skb+0x2c/0x1c0 net/core/dev.c:5099
 netif_receive_skb_internal+0x117/0x660 net/core/dev.c:5202
 napi_frags_finish net/core/dev.c:5769 [inline]
 napi_gro_frags+0xade/0xd10 net/core/dev.c:5843
 tun_get_user+0x2f24/0x3fb0 drivers/net/tun.c:1981
 tun_chr_write_iter+0xbd/0x156 drivers/net/tun.c:2027
 call_write_iter include/linux/fs.h:1866 [inline]
 do_iter_readv_writev+0x5e1/0x8e0 fs/read_write.c:681
 do_iter_write fs/read_write.c:957 [inline]
 do_iter_write+0x184/0x610 fs/read_write.c:938
 vfs_writev+0x1b3/0x2f0 fs/read_write.c:1002
 do_writev+0x15e/0x370 fs/read_write.c:1037
 __do_sys_writev fs/read_write.c:1110 [inline]
 __se_sys_writev fs/read_write.c:1107 [inline]
 __x64_sys_writev+0x75/0xb0 fs/read_write.c:1107

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