cifs: Prevent NULL pointer dereference caused by cifs_sb->rsize is 0

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/IBO3FG


CVE: NA

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When cifs_sb->rsize is 0, cifs_send_async_read() fails to allocate the
pages array using iov_iter_get_pages_alloc(), resulting in data->pages
being NULL. This eventually triggers a NULL pointer dereference in the
callback function uncached_fill_pages().

cifs_sb->rsize is set to the minimum value between SMB3_DEFAULT_IOSIZE
and server->max_read. SMB3_DEFAULT_IOSIZE is 4MB.

There are two reasons that cause cifs_sb->rsize to be 0:

1. During the negotiation process, if the server returns an
   rsp->MaxReadSize of 0, server->max_read is assigned 0.
   And the calculated value of cifs_sb->rsize becomes 0.
2. In the cifsd thread, if cifs_reconnect() is invoked, server->max_read
   is set to 0. Prior to the completion of the reconnection process, the
   calculated value of cifs_sb->rsize becomes 0.

It is important to note that cifs_sb->rsize is only set to 0 during the
mount phase. Therefore, a straightforward approach to prevent potential
NULL pointer dereferences in the future is to implement a check for
cifs_sb->rsize during the mount stage. If the value is found to be 0, the
mount process should exit early to avoid any subsequent NULL pointer
dereferences.

Fixes: aa24d1e9 ("CIFS: Process reconnects for SMB2 shares")
Signed-off-by: Wang Zhaolong <wangzhaolong1@huawei.com>