9p: Transport error uninitialized

The p9_tag_alloc() does not initialize the transport error t_err field.
The struct p9_req_t *req is allocated and stored in a struct p9_client
variable. The field t_err is never initialized before p9_conn_cancel()
checks its value.

KUMSAN(KernelUninitializedMemorySantizer, a new error detection tool)
reports this bug.

==================================================================
BUG: KUMSAN: use of uninitialized memory in p9_conn_cancel+0x2d9/0x3b0
Read of size 4 at addr ffff88805f9b600c by task kworker/1:2/1216

CPU: 1 PID: 1216 Comm: kworker/1:2 Not tainted 5.2.0-rc4+ #28
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
Workqueue: events p9_write_work
Call Trace:
 dump_stack+0x75/0xae
 __kumsan_report+0x17c/0x3e6
 kumsan_report+0xe/0x20
 p9_conn_cancel+0x2d9/0x3b0
 p9_write_work+0x183/0x4a0
 process_one_work+0x4d1/0x8c0
 worker_thread+0x6e/0x780
 kthread+0x1ca/0x1f0
 ret_from_fork+0x35/0x40

Allocated by task 1979:
 save_stack+0x19/0x80
 __kumsan_kmalloc.constprop.3+0xbc/0x120
 kmem_cache_alloc+0xa7/0x170
 p9_client_prepare_req.part.9+0x3b/0x380
 p9_client_rpc+0x15e/0x880
 p9_client_create+0x3d0/0xac0
 v9fs_session_init+0x192/0xc80
 v9fs_mount+0x67/0x470
 legacy_get_tree+0x70/0xd0
 vfs_get_tree+0x4a/0x1c0
 do_mount+0xba9/0xf90
 ksys_mount+0xa8/0x120
 __x64_sys_mount+0x62/0x70
 do_syscall_64+0x6d/0x1e0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff88805f9b6008
 which belongs to the cache p9_req_t of size 144
The buggy address is located 4 bytes inside of
 144-byte region [ffff88805f9b6008, ffff88805f9b6098)
The buggy address belongs to the page:
page:ffffea00017e6d80 refcount:1 mapcount:0 mapping:ffff888068b63740 index:0xffff88805f9b7d90 compound_mapcount: 0
flags: 0x100000000010200(slab|head)
raw: 0100000000010200 ffff888068b66450 ffff888068b66450 ffff888068b63740
raw: ffff88805f9b7d90 0000000000100001 00000001ffffffff 0000000000000000
page dumped because: kumsan: bad access detected
==================================================================

Link: http://lkml.kernel.org/r/20190613070854.10434-1-shuaibinglu@126.com
Signed-off-by: Lu Shuaibing <shuaibinglu@126.com>
[dominique.martinet@cea.fr: grouped the added init with the others]
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>

net/9p/client.c

@@ -281,6 +281,7 @@ p9_tag_alloc(struct p9_client *c, int8_t type, unsigned int max_size)
 
 	p9pdu_reset(&req->tc);
 	p9pdu_reset(&req->rc);
+	req->t_err = 0;
 	req->status = REQ_STATUS_ALLOC;
 	init_waitqueue_head(&req->wq);
 	INIT_LIST_HEAD(&req->req_list);