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SubjectRe: KASAN: use-after-free Read in p9_fd_poll
From
syzbot has found a reproducer for the following crash on:

HEAD commit: d72e90f33aa4 Linux 4.18-rc6
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=161894c8400000
kernel config: https://syzkaller.appspot.com/x/.config?x=68af3495408deac5
dashboard link: https://syzkaller.appspot.com/bug?extid=0442e6e2f7e1e33b1037
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1569b51c400000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16e7a978400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+0442e6e2f7e1e33b1037@syzkaller.appspotmail.com

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KASAN: use-after-free in p9_fd_poll+0x280/0x2b0 net/9p/trans_fd.c:238
Read of size 8 at addr ffff8801d6ddf340 by task kworker/0:2/26

CPU: 0 PID: 26 Comm: kworker/0:2 Not tainted 4.18.0-rc6+ #160
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_load8_noabort+0x14/0x20 mm/kasan/report.c:433
p9_fd_poll+0x280/0x2b0 net/9p/trans_fd.c:238
p9_poll_mux net/9p/trans_fd.c:617 [inline]
p9_poll_workfn+0x463/0x6d0 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 4581:
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+0x8ed/0x1770 net/9p/client.c:1063
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
ksys_mount+0x12d/0x140 fs/namespace.c:3064
__do_sys_mount fs/namespace.c:3078 [inline]
__se_sys_mount fs/namespace.c:3075 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3075
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 4581:
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+0xa9a/0x1770 net/9p/client.c:1077
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
ksys_mount+0x12d/0x140 fs/namespace.c:3064
__do_sys_mount fs/namespace.c:3078 [inline]
__se_sys_mount fs/namespace.c:3075 [inline]
__x64_sys_mount+0xbe/0x150 fs/namespace.c:3075
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8801d6ddf340
which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
512-byte region [ffff8801d6ddf340, ffff8801d6ddf540)
The buggy address belongs to the page:
page:ffffea00075b77c0 count:1 mapcount:0 mapping:ffff8801da800940 index:0x0
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffffea0007398548 ffffea0006c59188 ffff8801da800940
raw: 0000000000000000 ffff8801d6ddf0c0 0000000100000006 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8801d6ddf200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d6ddf280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
> ffff8801d6ddf300: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
^
ffff8801d6ddf380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff8801d6ddf400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

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