ext4: fix possible UAF when remounting r/o a mmp-protected file system

In the Linux kernel, the following vulnerability has been resolved:

ext4: fix possible UAF when remounting r/o a mmp-protected file system

After commit 618f003199c6 ("ext4: fix memory leak in

ext4_fill_super"), after the file system is remounted read-only, there

is a race where the kmmpd thread can exit, causing sbi->s_mmp_tsk to

point at freed memory, which the call to ext4_stop_mmpd() can trip

over.

Fix this by only allowing kmmpd() to exit when it is stopped via

ext4_stop_mmpd().

Bug-Report-Link: <[email protected]>