[syzbot] [integrity?] [lsm?] possible deadlock in process_measurement (4)

[Shu Han]

> ======================================================
> WARNING: possible circular locking dependency detected
> 6.11.0-syzkaller-10045-g97d8894b6f4c #0 Not tainted
> ------------------------------------------------------
> syz-executor369/5231 is trying to acquire lock:
> ffff888072852370 (&sb->s_type->i_mutex_key#12){+.+.}-{3:3}, at: inode_lock include/linux/fs.h:815 [inline]
> ffff888072852370 (&sb->s_type->i_mutex_key#12){+.+.}-{3:3}, at: process_measurement+0x439/0x1fb0 security/integrity/ima/ima_main.c:250
>
> but task is already holding lock:
> ffff88807ac9a798 (&mm->mmap_lock){++++}-{3:3}, at: mmap_write_lock_killable include/linux/mmap_lock.h:122 [inline]
> ffff88807ac9a798 (&mm->mmap_lock){++++}-{3:3}, at: __do_sys_remap_file_pages mm/mmap.c:1649 [inline]
> ffff88807ac9a798 (&mm->mmap_lock){++++}-{3:3}, at: __se_sys_remap_file_pages+0x22d/0xa50 mm/mmap.c:1624
>
> which lock already depends on the new lock.

This issue (if not a false positive?) is due to the possible `prot`
change caused by the processing logic for READ_IMPLIES_EXEC in do_mmap(),
so the remap_file_pages() must perform LSM check before calling do_mmap(),
this is what the previous commit want to do.

The LSM check is required to know what the `prot` is, but `prot` must be
obtained after holding the `mmap_write_lock`.

If the `mmap_write_lock` is released after getting the `prot` and before
the LSM call in remap_file_pages(), it may cause TOCTOU.

So, possible solutions may include:
1. Remove the security check by removing the the possibility of the `prot`
   change:
1.1. move the the processing logic for READ_IMPLIES_EXEC out of the
     do_mmap(). This also ensures that such missing checks which the
     previous commit fixes will not occur again(suggested).
     See the RFC PATCH
     https://lore.kernel.org/all/20240928180044.50-1-ebpqwerty472123@gmail.com/
1.2. Replace do_mmap() in remap_file_pages() to mmap_region(), which do
     the actually memory mapping without the respect to READ_IMPLIES_EXEC.
     But this requires other checks in do_mmap() is performed in
     remap_file_pages(), such as the `file_mmap_ok`(may complex).
2. Perform operations similar to updating a value by CAS(may slow):
for (;;) {
    mmap_write_lock();
    prot = get_prot();
    mmap_write_unlock();
    if (!call_lsm(prot)) return;
    mmap_write_lock();
    if (prot != get_prot()) continue;
    do_mmap();
    mmap_write_unlock();
}