On Thu, Jun 13, 2019 at 07:35:44AM -0400, Neil Horman wrote:

On Thu, Jun 13, 2019 at 10:37:51AM +0800, Su Yanjun wrote:

quoted

在 2019/6/12 21:13, Neil Horman 写道:

quoted

On Tue, Jun 11, 2019 at 10:33:17AM +0800, Su Yanjun wrote:

quoted

在 2019/6/10 19:12, Neil Horman 写道:

quoted

On Mon, Jun 10, 2019 at 11:20:00AM +0800, Su Yanjun wrote:

quoted

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")

quoted

Signed-off-by: Su Yanjun <redacted>
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,

quoted

   	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

quoted

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
==================================================================

quoted

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).

Yes, you are right. __mkroute_output is impossible to cause dst entry to be
released.

quoted

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,

But dst entry may be  released somewhere.

Yes, thats how dst entries get released.

quoted

As syzbot reports,
HEAD commit:    9221dced Merge tag 'for-linus-20190601' of git://git.kerne..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=114cdc0ea00000
kernel config: https://syzkaller.appspot.com/x/.config?x=1fa7e451a5cac069
dashboard link: https://syzkaller.appspot.com/bug?extid=a9e23ea2aa21044c2798
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386

i searched dst_release in sctp code, sctp_transport_dst_release is a big
suspicion.
If multiple processes calling sctp_transport_dst_release would cause
refcnt==0,
dst entry will be reclaimed.

Yes, that too is clear, but dst_release drops the refcount, and if its zero,
calls call_rcu(..., dst_destroy_rcu), which only finally frees the dst entry
after an rcu grace period.

__mkroute_output is always called under the protection of rcu_read_lock (see
ip_route_output_key_hash), so any call to dst_release will have the actual free
until some time after rcu_read_unlock is called, meaning any access made to the
corresponding dst entry in __mkroute_output is safe.  And if__mkroute_output
selects a dst entry that is pending deletion from an already registered rcu
callback will get skipped, because its refcount is zero, and dst_hold_safe will
return zero (since it uses atomic_inc_not_zero to increment the refcount).

quoted

__mkroute_output in route.c:
prth = raw_cpu_ptr(nh->nh_pcpu_rth_output);
rth = rcu_dereference(*prth);
it uses *nh_pcpu_rth_output* to refer the route cache. If dst entry has been
released, no one
sets *nh_pcpu_rth_output* to null, only rt_cache_route updates it with a new
one.

anywhere release_dst and get_dst which may access the same dst concurrently
should
be under rcu lock protection.

Yes, and it is, via ip_route_output_key_hash, and that appears in both call
traces, so we should be under rcu lock protection.

quoted

I'm not familar with sctp. but looks like a problem  dst_release related.

Generally speaking, yes, this appears to be an issue in which the rcu callback
for dst_release is getting called while we are inside __mkroute_output with the
dst still findable via the ip_route_output_key path.

One other point is that this particular dst entry doesn't "belong" to
a transport or even the sctp subsystem. Dst entries are shared
throughout the system.

  Marcelo

No idea yet, how thats happening though.

Neil

quoted

quoted

which should never happen.  How we are getting into that situation though, I
have no idea yet.

Neil

quoted

quoted

Protecting the lookup/release operations with a read side rcu
lock
won't fix that.

Neil

quoted

   	if (saddr)
   		memcpy(&transport->saddr, saddr, sizeof(union sctp_addr));
-- 
2.7.4