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

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: sysv: don't call sb_bread() with pointers_lock held 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).

References

• https://git.kernel.org/stable/c/13b33feb2ebddc2b1aa607f553566b18a4af1d76
• https://git.kernel.org/stable/c/1b4fe801b5bedec2b622ddb18e5c9bf26c63d79f
• https://git.kernel.org/stable/c/674c1c4229e743070e09db63a23442950ff000d1
• https://git.kernel.org/stable/c/fd203d2c671bdee9ab77090ff394d3b71b627927
• https://git.kernel.org/stable/c/53cb1e52c9db618c08335984d1ca80db220ccf09
• https://git.kernel.org/stable/c/89e8524135a3902e7563a5a59b7b5ec1bf4904ac
• https://git.kernel.org/stable/c/a69224223746ab96d43e5db9d22d136827b7e2d3
• https://git.kernel.org/stable/c/f123dc86388cb669c3d6322702dc441abc35c31e