mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

Overview

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

mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

On arm64 server, we found folio that get from migration entry isn't locked in softleaf_to_folio(). This issue triggers when mTHP splitting and zap_nonpresent_ptes() races, and the root cause is lack of memory barrier in softleaf_to_folio(). The race is as follows:

CPU0                                             CPU1

deferred_split_scan() zap_nonpresent_ptes() lock folio split_folio() unmap_folio() change ptes to migration entries __split_folio_to_order() softleaf_to_folio() set flags(including PG_locked) for tail pages folio = pfn_folio(softleaf_to_pfn(entry)) smp_wmb() VM_WARN_ON_ONCE(!folio_test_locked(folio)) prep_compound_page() for tail pages

In __split_folio_to_order(), smp_wmb() guarantees page flags of tail pages are visible before the tail page becomes non-compound. smp_wmb() should be paired with smp_rmb() in softleaf_to_folio(), which is missed. As a result, if zap_nonpresent_ptes() accesses migration entry that stores tail pfn, softleaf_to_folio() may see the updated compound_head of tail page before page->flags.

This issue will trigger VM_WARN_ON_ONCE() in pfn_swap_entry_folio() because of the race between folio split and zap_nonpresent_ptes() leading to a folio incorrectly undergoing modification without a folio lock being held.

This is a BUG_ON() before commit 93976a20345b ("mm: eliminate further swapops predicates"), which in merged in v6.19-rc1.

To fix it, add missing smp_rmb() if the softleaf entry is migration entry in softleaf_to_folio() and softleaf_to_page().

[[email protected]: update function name and comments]

Affected

• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < 426ee10711586617da869c8bb798214965337617
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < f1acf5887c2bbaf998dc3fe32c72b7a8b84a3ddd
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < 722cfaf6b31d31123439e67b5deac6b1261a3dea
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < 7ddcf4a245c1c5a91fdd9698757e3d95179ffe41
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < b8c49ad888892ad7b77062b9c102b799a3e9b4f8
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < 7ad1997b9bc8032603df8f091761114479285769
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < 8bfb8414e9f2ce6f5f2f0e3d0da52f2d132128e7
• Linux >= e9b61f19858a5d6c42ce2298cf138279375d0d9b < 4c5e7f0fcd592801c9cc18f29f80fbee84eb8669
• Linux 4.5

Remediation

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