In the Linux kernel, the following vulnerability has been resolved: x86/mm/pat: Acquire init_mm read lock on attribute changes to avoid UAF A previous commit protected against races between ptdump and CPA collapse, however one still ex…

Overview

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

x86/mm/pat: Acquire init_mm read lock on attribute changes to avoid UAF

A previous commit protected against races between ptdump and CPA collapse, however one still exists between attribute changes and collapse as reported by Denis V. Lunev (linked).

When an attribute change arises, a lockless page table walker obtains a PTE entry, which is later written to via set_pte_atomic():

... -> change_page_attr_set_clr() -> __change_page_attr_set_clr() -> __change_page_attr() -> _lookup_address_cpa() -> lookup_address_in_pgd_attr() -> [ lockless page table walker ] -> set_pte_atomic()

There is nothing preventing a concurrent CPA collapse which can free the PTE that was retrieved here, resulting in a use-after-free.

With the mmap write lock taken on init_mm over CPA collapse, resolve this race by acquiring an mmap read lock on init_mm over __change_page_attr_set_clr().

This locks across the whole operation over which the walk and the PTE entry write occurs, solving the race.

It is safe to do this here, as no spinlocks are held upon entry to __change_page_attr_set_clr().

However, the lock must not be held over an allocation, as allocation can trigger reclaim and shrinkers may call into CPA recursively, making deadlocks possible (init_mm -> ... -> fs_reclaim -> init_mm).

A page table is allocated when a huge page needs to be split:

-> change_page_attr_set_clr() -> __change_page_attr_set_clr() -> __change_page_attr() -> split_large_page() [ pagetable_alloc() ] -> __split_large_page()

Avoid deadlocks by dropping the mmap lock across pagetable_alloc() in split_large_page() and track whether this is needed by adding a new 'init_mm_read_locked' flag to struct cpa_data.

This is safe as __split_large_page() (called with locks re-established) revalidates that the page table entry is the same as it was prior to the locks being dropped and __change_page_attr() repeats the entire page table walk whenever a split occurs, so concurrent split and collapse are accounted for.

Concurrent ptdump is also safe as the lock is only dropped over page table allocation during which time the page table has not yet been modified.

The CPA_COLLAPSE flag is only set by set_memory_rox(), which exclusively operates upon vmalloc ranges, and on x86 only within the module mapping space.

This is important, because some callers directly invoke __change_page_attr_set_clr(), bypassing this lock. However, none of these operate within the module mapping space.

• cpa_process_alias() - a recursive helper called by __change_page_attr_set_clr().
• __set_memory_enc_pgtable() - operates on the direct mapping and (via __vmbus_establish_gpadl()) the vmalloc mapping space.
• set_pages[n]p() - called by set_direct_map[invalid, default, valid]_noflush(), __kernel_map_pages() - operates on the direct map.
• kernel_[un]map_pages_in_pgd() - operates on EFI ranges.

This work is based upon Denis V. Lunev's excellent analysis of the bug with gratitude.

[ dhansen: move to imperative voice in changelog ]

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.