Re: KASAN: use-after-free Read in p9_conn_cancel

[syzbot <syzbot+f0fdc967350bd580a80b@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following crash on:

HEAD commit:    1e4b044d2251 Linux 4.18-rc4
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10d5ba2c400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=25856fac4e580aa7
dashboard link: https://syzkaller.appspot.com/bug?extid=f0fdc967350bd580a80b
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
userspace arch: i386
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=179f4f48400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12aba452400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f0fdc967350bd580a80b@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: use-after-free in p9_conn_cancel+0x9de/0xd30  
net/9p/trans_fd.c:207
Read of size 4 at addr ffff8801b075c2e8 by task kworker/1:2/4574

CPU: 1 PID: 4574 Comm: kworker/1:2 Not tainted 4.18.0-rc4+ #41
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: events p9_poll_workfn
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
  print_address_description+0x6c/0x20b mm/kasan/report.c:256
  kasan_report_error mm/kasan/report.c:354 [inline]
  kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
  __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
  p9_conn_cancel+0x9de/0xd30 net/9p/trans_fd.c:207
  p9_poll_mux net/9p/trans_fd.c:620 [inline]
  p9_poll_workfn+0x4b2/0x6d0 net/9p/trans_fd.c:1107
  process_one_work+0xc73/0x1ba0 kernel/workqueue.c:2153
  worker_thread+0x189/0x13c0 kernel/workqueue.c:2296
  kthread+0x345/0x410 kernel/kthread.c:246
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412

Allocated by task 4689:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
  set_track mm/kasan/kasan.c:460 [inline]
  kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
  kmem_cache_alloc_trace+0x152/0x780 mm/slab.c:3620
  kmalloc include/linux/slab.h:513 [inline]
  kzalloc include/linux/slab.h:707 [inline]
  p9_fd_open net/9p/trans_fd.c:796 [inline]
  p9_fd_create+0x1a7/0x3f0 net/9p/trans_fd.c:1036
  p9_client_create+0x915/0x16c9 net/9p/client.c:1062
  v9fs_session_init+0x21a/0x1a80 fs/9p/v9fs.c:400
  v9fs_mount+0x7c/0x900 fs/9p/vfs_super.c:135
  mount_fs+0xae/0x328 fs/super.c:1277
  vfs_kern_mount.part.34+0xdc/0x4e0 fs/namespace.c:1037
  vfs_kern_mount fs/namespace.c:1027 [inline]
  do_new_mount fs/namespace.c:2518 [inline]
  do_mount+0x581/0x30e0 fs/namespace.c:2848
  __do_compat_sys_mount fs/compat.c:125 [inline]
  __se_compat_sys_mount fs/compat.c:92 [inline]
  __ia32_compat_sys_mount+0x5d5/0x860 fs/compat.c:92
  do_syscall_32_irqs_on arch/x86/entry/common.c:326 [inline]
  do_fast_syscall_32+0x34d/0xfb2 arch/x86/entry/common.c:397
  entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

Freed by task 4689:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
  set_track mm/kasan/kasan.c:460 [inline]
  __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
  kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
  __cache_free mm/slab.c:3498 [inline]
  kfree+0xd9/0x260 mm/slab.c:3813
  p9_fd_close+0x416/0x5b0 net/9p/trans_fd.c:893
  p9_client_create+0xac2/0x16c9 net/9p/client.c:1076
  v9fs_session_init+0x21a/0x1a80 fs/9p/v9fs.c:400
  v9fs_mount+0x7c/0x900 fs/9p/vfs_super.c:135
  mount_fs+0xae/0x328 fs/super.c:1277
  vfs_kern_mount.part.34+0xdc/0x4e0 fs/namespace.c:1037
  vfs_kern_mount fs/namespace.c:1027 [inline]
  do_new_mount fs/namespace.c:2518 [inline]
  do_mount+0x581/0x30e0 fs/namespace.c:2848
  __do_compat_sys_mount fs/compat.c:125 [inline]
  __se_compat_sys_mount fs/compat.c:92 [inline]
  __ia32_compat_sys_mount+0x5d5/0x860 fs/compat.c:92
  do_syscall_32_irqs_on arch/x86/entry/common.c:326 [inline]
  do_fast_syscall_32+0x34d/0xfb2 arch/x86/entry/common.c:397
  entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

The buggy address belongs to the object at ffff8801b075c2c0
  which belongs to the cache kmalloc-512 of size 512
The buggy address is located 40 bytes inside of
  512-byte region [ffff8801b075c2c0, ffff8801b075c4c0)
The buggy address belongs to the page:
page:ffffea0006c1d700 count:1 mapcount:0 mapping:ffff8801da800940 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffffea0006cc1c88 ffffea0006c48288 ffff8801da800940
raw: 0000000000000000 ffff8801b075c040 0000000100000006 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8801b075c180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff8801b075c200: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
> ffff8801b075c280: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
                                                           ^
  ffff8801b075c300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff8801b075c380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================