migrate: correct lock ordering for hugetlb file folios

Overview

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

migrate: correct lock ordering for hugetlb file folios

Syzbot has found a deadlock (analyzed by Lance Yang):

1. Task (5749): Holds folio_lock, then tries to acquire i_mmap_rwsem(read lock).
2. Task (5754): Holds i_mmap_rwsem(write lock), then tries to acquire folio_lock.

migrate_pages() -> migrate_hugetlbs() -> unmap_and_move_huge_page() <- Takes folio_lock! -> remove_migration_ptes() -> __rmap_walk_file() -> i_mmap_lock_read() <- Waits for i_mmap_rwsem(read lock)!

hugetlbfs_fallocate() -> hugetlbfs_punch_hole() <- Takes i_mmap_rwsem(write lock)! -> hugetlbfs_zero_partial_page() -> filemap_lock_hugetlb_folio() -> filemap_lock_folio() -> __filemap_get_folio <- Waits for folio_lock!

The migration path is the one taking locks in the wrong order according to the documentation at the top of mm/rmap.c. So expand the scope of the existing i_mmap_lock to cover the calls to remove_migration_ptes() too.

This is (mostly) how it used to be after commit c0d0381ade79. That was removed by 336bf30eb765 for both file & anon hugetlb pages when it should only have been removed for anon hugetlb pages.

Affected

• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < e7396d23f9d5739f56cf9ab430c3a169f5508394
• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < ad97b9a55246eb940a26ac977f80892a395cabf9
• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < 5edb9854f8df5428b40990a1c7d60507da5bd330
• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < 526394af4e8ade89cacd1a9ce2b97712712fcc34
• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < b75070823b89009f5123fd0e05a8e0c3d39937c1
• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < 1b68efce6dd483d22f50d0d3800c4cfda14b1305
• Linux >= 336bf30eb76580b579dc711ded5d599d905c0217 < b7880cb166ab62c2409046b2347261abf701530e
• Linux ef792d6ce0db6a56e56743b1de1716a982c3b851
• Linux >= 5.9.9 < 5.10
• Linux 5.10

Remediation

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