CVE-2023-52935

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

mm/khugepaged: fix ->anon_vma race

If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires it to be locked.

Page table traversal is allowed under any one of the mmap lock, the anon_vma lock (if the VMA is associated with an anon_vma), and the mapping lock (if the VMA is associated with a mapping); and so to be able to remove page tables, we must hold all three of them. retract_page_tables() bails out if an ->anon_vma is attached, but does this check before holding the mmap lock (as the comment above the check explains).

If we racily merged an existing ->anon_vma (shared with a child process) from a neighboring VMA, subsequent rmap traversals on pages belonging to the child will be able to see the page tables that we are concurrently removing while assuming that nothing else can access them.

Repeat the ->anon_vma check once we hold the mmap lock to ensure that there really is no concurrent page table access.

Hitting this bug causes a lockdep warning in collapse_and_free_pmd(), in the line lockdep_assert_held_write(&vma->anon_vma->root->rwsem). It can also lead to use-after-free access.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/023f47a8250c6bdb4aebe744db4bf7f73414028b
• https://git.kernel.org/stable/c/352fbf61ce776fef18dca6a68680a6cd943dac95
• https://git.kernel.org/stable/c/abdf3c33918185c3e8ffeb09ed3e334b3d7df47c
• https://git.kernel.org/stable/c/acb08187b5a83cdb9ac4112fae9e18cf983b0128
• https://git.kernel.org/stable/c/cee956ab1efbd858b4ca61c8b474af5aa24b29a6