nfsd: make sure exp active before svc_export_show

The function `e_show` was called with protection from RCU. This only
ensures that `exp` will not be freed. Therefore, the reference count for
`exp` can drop to zero, which will trigger a refcount use-after-free
warning when `exp_get` is called. To resolve this issue, use
`cache_get_rcu` to ensure that `exp` remains active.

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 3 PID: 819 at lib/refcount.c:25
refcount_warn_saturate+0xb1/0x120
CPU: 3 UID: 0 PID: 819 Comm: cat Not tainted 6.12.0-rc3+ #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
1.16.1-2.fc37 04/01/2014
RIP: 0010:refcount_warn_saturate+0xb1/0x120
...
Call Trace:
 <TASK>
 e_show+0x20b/0x230 [nfsd]
 seq_read_iter+0x589/0x770
 seq_read+0x1e5/0x270
 vfs_read+0x125/0x530
 ksys_read+0xc1/0x160
 do_syscall_64+0x5f/0x170
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: bf18f16 ("NFSD: Using exp_get for export getting")
Cc: stable@vger.kernel.org # 4.20+
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: Yang Erkun

fs/nfsd/export.c

@@ -1425,9 +1425,12 @@ static int e_show(struct seq_file *m, void *p)
 		return 0;
 	}
 
-	exp_get(exp);
+	if (!cache_get_rcu(&exp->h))
+		return 0;
+
 	if (cache_check(cd, &exp->h, NULL))
 		return 0;
+
 	exp_put(exp);
 	return svc_export_show(m, cd, cp);
 }