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Date:   Wed, 12 Jun 2019 09:13:46 -0400
From:   Neil Horman <nhorman@tuxdriver.com>
To:     Su Yanjun <suyj.fnst@cn.fujitsu.com>
Cc:     vyasevich@gmail.com, marcelo.leitner@gmail.com,
        davem@davemloft.net, linux-sctp@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sctp: Add rcu lock to protect dst entry in
 sctp_transport_route
Message-ID: <20190612131346.GA23166@hmswarspite.think-freely.org>
References: <1560136800-17961-1-git-send-email-suyj.fnst@cn.fujitsu.com>
 <20190610111209.GA15599@hmswarspite.think-freely.org>
 <08f76a75-9e5b-2434-9175-e5371fa9cb7e@cn.fujitsu.com>
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On Tue, Jun 11, 2019 at 10:33:17AM +0800, Su Yanjun wrote:
> 
> 在 2019/6/10 19:12, Neil Horman 写道:
> > On Mon, Jun 10, 2019 at 11:20:00AM +0800, Su Yanjun wrote:
> > > syzbot found a crash in rt_cache_valid. Problem is that when more
> > > threads release dst in sctp_transport_route, the route cache can
> > > be freed.
> > > 
> > > As follows,
> > > p1:
> > > sctp_transport_route
> > >    dst_release
> > >    get_dst
> > > 
> > > p2:
> > > sctp_transport_route
> > >    dst_release
> > >    get_dst
> > > ...
> > > 
> > > If enough threads calling dst_release will cause dst->refcnt==0
> > > then rcu softirq will reclaim the dst entry,get_dst then use
> > > the freed memory.
> > > 
> > > This patch adds rcu lock to protect the dst_entry here.
> > > 
> > > Fixes: 6e91b578bf3f("sctp: re-use sctp_transport_pmtu in
> > sctp_transport_route")
> > > Signed-off-by: Su Yanjun <suyj.fnst@cn.fujitsu.com>
> > > Reported-by: syzbot+a9e23ea2aa21044c2798@syzkaller.appspotmail.com
> > > ---
> > >   net/sctp/transport.c | 5 +++++
> > >   1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> > > index ad158d3..5ad7e20 100644
> > > --- a/net/sctp/transport.c
> > > +++ b/net/sctp/transport.c
> > > @@ -308,8 +308,13 @@ void sctp_transport_route(struct sctp_transport
> > *transport,
> > >   	struct sctp_association *asoc = transport->asoc;
> > >   	struct sctp_af *af = transport->af_specific;
> > > +	/* When dst entry is being released, route cache may be referred
> > > +	 * again. Add rcu lock here to protect dst entry.
> > > +	 */
> > > +	rcu_read_lock();
> > >   	sctp_transport_dst_release(transport);
> > >   	af->get_dst(transport, saddr, &transport->fl, sctp_opt2sk(opt));
> > > +	rcu_read_unlock();
> > What is the exact error that syzbot reported?  This doesn't seem like it
> > fixes
> BUG: KASAN: slab-out-of-bounds in rt_cache_valid+0x158/0x190
> net/ipv4/route.c:1556
> Read of size 2 at addr ffff8880654f3ac7 by task syz-executor.0/26603
> 
> CPU: 0 PID: 26603 Comm: syz-executor.0 Not tainted 5.2.0-rc2+ #9
> 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
>  print_address_description.cold+0x7c/0x20d mm/kasan/report.c:188
>  __kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
>  kasan_report+0x12/0x20 mm/kasan/common.c:614
>  __asan_report_load2_noabort+0x14/0x20 mm/kasan/generic_report.c:130
>  rt_cache_valid+0x158/0x190 net/ipv4/route.c:1556
>  __mkroute_output net/ipv4/route.c:2332 [inline]
>  ip_route_output_key_hash_rcu+0x819/0x2d50 net/ipv4/route.c:2564
>  ip_route_output_key_hash+0x1ef/0x360 net/ipv4/route.c:2393
>  __ip_route_output_key include/net/route.h:125 [inline]
>  ip_route_output_flow+0x28/0xc0 net/ipv4/route.c:2651
>  ip_route_output_key include/net/route.h:135 [inline]
>  sctp_v4_get_dst+0x467/0x1260 net/sctp/protocol.c:435
>  sctp_transport_route+0x12d/0x360 net/sctp/transport.c:297
>  sctp_assoc_add_peer+0x53e/0xfc0 net/sctp/associola.c:663
>  sctp_process_param net/sctp/sm_make_chunk.c:2531 [inline]
>  sctp_process_init+0x2491/0x2b10 net/sctp/sm_make_chunk.c:2344
>  sctp_cmd_process_init net/sctp/sm_sideeffect.c:667 [inline]
>  sctp_cmd_interpreter net/sctp/sm_sideeffect.c:1369 [inline]
>  sctp_side_effects net/sctp/sm_sideeffect.c:1179 [inline]
>  sctp_do_sm+0x3a30/0x50e0 net/sctp/sm_sideeffect.c:1150
>  sctp_assoc_bh_rcv+0x343/0x660 net/sctp/associola.c:1059
>  sctp_inq_push+0x1e4/0x280 net/sctp/inqueue.c:80
>  sctp_backlog_rcv+0x196/0xbe0 net/sctp/input.c:339
>  sk_backlog_rcv include/net/sock.h:945 [inline]
>  __release_sock+0x129/0x390 net/core/sock.c:2412
>  release_sock+0x59/0x1c0 net/core/sock.c:2928
>  sctp_wait_for_connect+0x316/0x540 net/sctp/socket.c:9039
>  __sctp_connect+0xab2/0xcd0 net/sctp/socket.c:1226
>  __sctp_setsockopt_connectx+0x133/0x1a0 net/sctp/socket.c:1334
>  sctp_setsockopt_connectx_old net/sctp/socket.c:1350 [inline]
>  sctp_setsockopt net/sctp/socket.c:4644 [inline]
>  sctp_setsockopt+0x22c0/0x6d10 net/sctp/socket.c:4608
>  compat_sock_common_setsockopt+0x106/0x140 net/core/sock.c:3137
>  __compat_sys_setsockopt+0x185/0x380 net/compat.c:383
>  __do_compat_sys_setsockopt net/compat.c:396 [inline]
>  __se_compat_sys_setsockopt net/compat.c:393 [inline]
>  __ia32_compat_sys_setsockopt+0xbd/0x150 net/compat.c:393
>  do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
>  do_fast_syscall_32+0x27b/0xd7d arch/x86/entry/common.c:408
>  entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
> RIP: 0023:0xf7ff5849
> Code: 85 d2 74 02 89 0a 5b 5d c3 8b 04 24 c3 8b 14 24 c3 8b 3c 24 c3 90 90
> 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90
> 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> RSP: 002b:00000000f5df10cc EFLAGS: 00000296 ORIG_RAX: 000000000000016e
> RAX: ffffffffffffffda RBX: 0000000000000007 RCX: 0000000000000084
> RDX: 000000000000006b RSI: 000000002055bfe4 RDI: 000000000000001c
> RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> 
> Allocated by task 480:
>  save_stack+0x23/0x90 mm/kasan/common.c:71
>  set_track mm/kasan/common.c:79 [inline]
>  __kasan_kmalloc mm/kasan/common.c:489 [inline]
>  __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:462
>  kasan_slab_alloc+0xf/0x20 mm/kasan/common.c:497
>  slab_post_alloc_hook mm/slab.h:437 [inline]
>  slab_alloc mm/slab.c:3326 [inline]
>  kmem_cache_alloc+0x11a/0x6f0 mm/slab.c:3488
>  dst_alloc+0x10e/0x200 net/core/dst.c:93
>  rt_dst_alloc+0x83/0x3f0 net/ipv4/route.c:1624
>  __mkroute_output net/ipv4/route.c:2337 [inline]
>  ip_route_output_key_hash_rcu+0x8f3/0x2d50 net/ipv4/route.c:2564
>  ip_route_output_key_hash+0x1ef/0x360 net/ipv4/route.c:2393
>  __ip_route_output_key include/net/route.h:125 [inline]
>  ip_route_output_flow+0x28/0xc0 net/ipv4/route.c:2651
>  ip_route_output_key include/net/route.h:135 [inline]
>  sctp_v4_get_dst+0x467/0x1260 net/sctp/protocol.c:435
>  sctp_transport_route+0x12d/0x360 net/sctp/transport.c:297
>  sctp_assoc_add_peer+0x53e/0xfc0 net/sctp/associola.c:663
>  sctp_process_param net/sctp/sm_make_chunk.c:2531 [inline]
>  sctp_process_init+0x2491/0x2b10 net/sctp/sm_make_chunk.c:2344
>  sctp_sf_do_unexpected_init net/sctp/sm_statefuns.c:1541 [inline]
>  sctp_sf_do_unexpected_init.isra.0+0x7cd/0x1350net/sctp/sm_statefuns.c:1441
>  sctp_sf_do_5_2_1_siminit+0x35/0x40 net/sctp/sm_statefuns.c:1670
>  sctp_do_sm+0x121/0x50e0 net/sctp/sm_sideeffect.c:1147
>  sctp_assoc_bh_rcv+0x343/0x660 net/sctp/associola.c:1059
>  sctp_inq_push+0x1e4/0x280 net/sctp/inqueue.c:80
>  sctp_backlog_rcv+0x196/0xbe0 net/sctp/input.c:339
>  sk_backlog_rcv include/net/sock.h:945 [inline]
>  __release_sock+0x129/0x390 net/core/sock.c:2412
>  release_sock+0x59/0x1c0 net/core/sock.c:2928
>  sctp_wait_for_connect+0x316/0x540 net/sctp/socket.c:9039
>  __sctp_connect+0xab2/0xcd0 net/sctp/socket.c:1226
>  sctp_connect net/sctp/socket.c:4846 [inline]
>  sctp_inet_connect+0x29c/0x340 net/sctp/socket.c:4862
>  __sys_connect+0x264/0x330 net/socket.c:1834
>  __do_sys_connect net/socket.c:1845 [inline]
>  __se_sys_connect net/socket.c:1842 [inline]
>  __ia32_sys_connect+0x72/0xb0 net/socket.c:1842
>  do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
>  do_fast_syscall_32+0x27b/0xd7d arch/x86/entry/common.c:408
>  entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
> 
> Freed by task 9:
>  save_stack+0x23/0x90 mm/kasan/common.c:71
>  set_track mm/kasan/common.c:79 [inline]
>  __kasan_slab_free+0x102/0x150 mm/kasan/common.c:451
>  kasan_slab_free+0xe/0x10 mm/kasan/common.c:459
>  __cache_free mm/slab.c:3432 [inline]
>  kmem_cache_free+0x86/0x260 mm/slab.c:3698
>  dst_destroy+0x29e/0x3c0 net/core/dst.c:129
>  dst_destroy_rcu+0x16/0x19 net/core/dst.c:142
>  __rcu_reclaim kernel/rcu/rcu.h:222 [inline]
>  rcu_do_batch kernel/rcu/tree.c:2092 [inline]
>  invoke_rcu_callbacks kernel/rcu/tree.c:2310 [inline]
>  rcu_core+0xba5/0x1500 kernel/rcu/tree.c:2291
>  __do_softirq+0x25c/0x94c kernel/softirq.c:293
> 
> The buggy address belongs to the object at ffff8880654f3a00
>  which belongs to the cache ip_dst_cache of size 176
> The buggy address is located 23 bytes to the right of
>  176-byte region [ffff8880654f3a00, ffff8880654f3ab0)
> The buggy address belongs to the page:
> page:ffffea0001953cc0 refcount:1 mapcount:0 mapping:ffff8880a76ad600
> index:0xffff8880654f3c00
> flags: 0x1fffc0000000200(slab)
> raw: 01fffc0000000200 ffffea00026be808 ffffea000181c088 ffff8880a76ad600
> raw: ffff8880654f3c00 ffff8880654f3000 0000000100000002 0000000000000000
> page dumped because: kasan: bad access detected
> 
> Memory state around the buggy address:
>  ffff8880654f3980: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
>  ffff8880654f3a00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > ffff8880654f3a80: 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc fc
>                                            ^
>  ffff8880654f3b00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>  ffff8880654f3b80: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
> ==================================================================
> > anything.  Based on what you've said above, we have multiple processes
> > looking
> > up and releasing routes in parallel (which IIRC should never happen, as
> > only one
> > process should traverse the sctp state machine for a given association at
> > any
> > one time).
> Looks like multiple process could run into sctp_transport_route.
Yeah, I'm sorry, my previous comment was a bit overstated, you can definately
have multiple process going through the state machine, but not with the same
packet.

That said, this fix still isn't right.  Looking at the code, It appears that we
are manipulating a route inside __mkroute_output that is in the process of being
destroyed.  But the destruction occurs from an rcu_callback, and the lookup
process in __mkroute_output is under the protection of the rcu_read_lock already
(as seen in ip_route_output_key_hash), so the destruction should be delayed
until that _mkroute_output call is complete, and the call in __mkroute_output
should skip any route that is in-flight to be destroyed, because the reference
count should be zero (causing dst_hold_safe to return 0).

Basically, it seems like somehow, __mkroute_output has found a route, and
started to dereference parts of it, while it is at the same time being freed,
which should never happen.  How we are getting into that situation though, I
have no idea yet.

Neil

> > Protecting the lookup/release operations with a read side rcu
> > lock
> > won't fix that.
> > 
> > Neil
> > 
> > >   	if (saddr)
> > >   		memcpy(&transport->saddr, saddr, sizeof(union sctp_addr));
> > > -- 
> > > 2.7.4
> > > 
> > > 
> > > 
> > > 
> > 
> 
>