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SubjectKASAN: use-after-free Read in rxrpc_send_keepalive

syzbot found the following crash on:

HEAD commit: b678c568 Merge tag 'nfs-for-5.3-2' of git://git.linux-nfs...
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:

BUG: KASAN: use-after-free in rxrpc_send_keepalive+0x8a2/0x940
Read of size 8 at addr ffff888064219698 by task kworker/0:3/11077

CPU: 0 PID: 11077 Comm: kworker/0:3 Not tainted 5.3.0-rc3+ #96
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: krxrpcd rxrpc_peer_keepalive_worker
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x172/0x1f0 lib/dump_stack.c:113
print_address_description.cold+0xd4/0x306 mm/kasan/report.c:351
__kasan_report.cold+0x1b/0x36 mm/kasan/report.c:482
kasan_report+0x12/0x17 mm/kasan/common.c:612
__asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
rxrpc_send_keepalive+0x8a2/0x940 net/rxrpc/output.c:635
rxrpc_peer_keepalive_dispatch net/rxrpc/peer_event.c:369 [inline]
rxrpc_peer_keepalive_worker+0x7be/0xd02 net/rxrpc/peer_event.c:430
process_one_work+0x9af/0x1740 kernel/workqueue.c:2269
worker_thread+0x98/0xe40 kernel/workqueue.c:2415
kthread+0x361/0x430 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Allocated by task 20465:
save_stack+0x23/0x90 mm/kasan/common.c:69
set_track mm/kasan/common.c:77 [inline]
__kasan_kmalloc mm/kasan/common.c:487 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:460
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:501
kmem_cache_alloc_trace+0x158/0x790 mm/slab.c:3550
kmalloc include/linux/slab.h:552 [inline]
kzalloc include/linux/slab.h:748 [inline]
rxrpc_alloc_local net/rxrpc/local_object.c:79 [inline]
rxrpc_lookup_local+0x64c/0x1b70 net/rxrpc/local_object.c:279
rxrpc_sendmsg+0x379/0x5f0 net/rxrpc/af_rxrpc.c:566
sock_sendmsg_nosec net/socket.c:637 [inline]
sock_sendmsg+0xd7/0x130 net/socket.c:657
___sys_sendmsg+0x3e2/0x920 net/socket.c:2311
__sys_sendmmsg+0x1bf/0x4d0 net/socket.c:2413
__do_sys_sendmmsg net/socket.c:2442 [inline]
__se_sys_sendmmsg net/socket.c:2439 [inline]
__x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2439
do_syscall_64+0xfd/0x6a0 arch/x86/entry/common.c:296

Freed by task 0:
save_stack+0x23/0x90 mm/kasan/common.c:69
set_track mm/kasan/common.c:77 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:449
kasan_slab_free+0xe/0x10 mm/kasan/common.c:457
__cache_free mm/slab.c:3425 [inline]
kfree+0x10a/0x2c0 mm/slab.c:3756
rxrpc_local_rcu+0x62/0x80 net/rxrpc/local_object.c:471
__rcu_reclaim kernel/rcu/rcu.h:222 [inline]
rcu_do_batch kernel/rcu/tree.c:2114 [inline]
rcu_core+0x67f/0x1580 kernel/rcu/tree.c:2314
rcu_core_si+0x9/0x10 kernel/rcu/tree.c:2323
__do_softirq+0x262/0x98c kernel/softirq.c:292

The buggy address belongs to the object at ffff888064219680
which belongs to the cache kmalloc-1k of size 1024
The buggy address is located 24 bytes inside of
1024-byte region [ffff888064219680, ffff888064219a80)
The buggy address belongs to the page:
page:ffffea0001908600 refcount:1 mapcount:0 mapping:ffff8880aa400c40
index:0xffff888064218480 compound_mapcount: 0
flags: 0x1fffc0000010200(slab|head)
raw: 01fffc0000010200 ffffea00025f5a08 ffffea00028fca08 ffff8880aa400c40
raw: ffff888064218480 ffff888064218000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff888064219580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888064219600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff888064219680: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888064219700: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff888064219780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

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