Describe CVE-2021-46927.
What is CVE-2021-46927?
In the Linux kernel, the following vulnerability has been resolved:

nitro_enclaves: Use get_user_pages_unlocked() call to handle mmap assert

After commit 5b78ed24e8ec ("mm/pagemap: add mmap_assert_locked()
annotations to find_vma*()"), the call to get_user_pages() will trigger
the mmap assert.

static inline void mmap_assert_locked(struct mm_struct *mm)
{
	lockdep_assert_held(&mm->mmap_lock);
	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
}

[   62.521410] kernel BUG at include/linux/mmap_lock.h:156!
...........................................................
[   62.538938] RIP: 0010:find_vma+0x32/0x80
...........................................................
[   62.605889] Call Trace:
[   62.608502]  <TASK>
[   62.610956]  ? lock_timer_base+0x61/0x80
[   62.614106]  find_extend_vma+0x19/0x80
[   62.617195]  __get_user_pages+0x9b/0x6a0
[   62.620356]  __gup_longterm_locked+0x42d/0x450
[   62.623721]  ? finish_wait+0x41/0x80
[   62.626748]  ? __kmalloc+0x178/0x2f0
[   62.629768]  ne_set_user_memory_region_ioctl.isra.0+0x225/0x6a0 [nitro_enclaves]
[   62.635776]  ne_enclave_ioctl+0x1cf/0x6d7 [nitro_enclaves]
[   62.639541]  __x64_sys_ioctl+0x82/0xb0
[   62.642620]  do_syscall_64+0x3b/0x90
[   62.645642]  entry_SYSCALL_64_after_hwframe+0x44/0xae

Use get_user_pages_unlocked() when setting the enclave memory regions.
That's a similar pattern as mmap_read_lock() used together with
get_user_pages()..