filemap: replace pte_offset_map() with pte_offset_map_nolock()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: filemap: replace pte_offset_map() with pte_offset_map_nolock() The vmf->ptl in filemap_fault_recheck_pte_none() is still set from handle_pte_fault(). But at the same time, we did a pte_unmap(vmf->pte). After a pte_unmap(vmf->pte) unmap and rcu_read_unlock(), the page table may be racily changed and vmf->ptl maybe fails to protect the actual page table. Fix this by replacing pte_offset_map() with pte_offset_map_nolock(). As David said, the PTL pointer might be stale so if we continue to use it infilemap_fault_recheck_pte_none(), it might trigger UAF. Also, if the PTL fails, the issue fixed by commit 58f327f2ce80 ("filemap: avoid unnecessary major faults in filemap_fault()") might reappear.

References

• https://git.kernel.org/stable/c/6a6c2aec1a89506595801b4cf7e8eef035f33748
• https://git.kernel.org/stable/c/24be02a42181f0707be0498045c4c4b13273b16d