sysv: don't call sb_bread() with pointers_lock held

stable inclusion
from stable-v6.6.27
commit 89e8524135a3902e7563a5a59b7b5ec1bf4904ac
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I9QGIO
CVE: CVE-2023-52699

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=89e8524135a3902e7563a5a59b7b5ec1bf4904ac



--------------------------------

syzbot is reporting sleep in atomic context in SysV filesystem [1], for
sb_bread() is called with rw_spinlock held.

A "write_lock(&pointers_lock) => read_lock(&pointers_lock) deadlock" bug
and a "sb_bread() with write_lock(&pointers_lock)" bug were introduced by
"Replace BKL for chain locking with sysvfs-private rwlock" in Linux 2.5.12.

Then, "[PATCH] err1-40: sysvfs locking fix" in Linux 2.6.8 fixed the
former bug by moving pointers_lock lock to the callers, but instead
introduced a "sb_bread() with read_lock(&pointers_lock)" bug (which made
this problem easier to hit).

Al Viro suggested that why not to do like get_branch()/get_block()/
find_shared() in Minix filesystem does. And doing like that is almost a
revert of "[PATCH] err1-40: sysvfs locking fix" except that get_branch()
 from with find_shared() is called without write_lock(&pointers_lock).

Reported-by: syzbot <syzbot+69b40dc5fd40f32c199f@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=69b40dc5fd40f32c199f


Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: https://lore.kernel.org/r/0d195f93-a22a-49a2-0020-103534d6f7f6@I-love.SAKURA.ne.jp


Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Yifan Qiao <qiaoyifan4@huawei.com>