CVE-2026-31466

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

mm/hugememory: fix folio isn't locked in softleafto_folio()

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

CPU0                                             CPU1

deferredsplitscan() zapnonpresentptes() lock folio splitfolio() unmapfolio() change ptes to migration entries _splitfoliotoorder() softleaftofolio() set flags(including PGlocked) for tail pages folio = pfnfolio(softleaftopfn(entry)) smpwmb() VMWARNONONCE(!foliotestlocked(folio)) prepcompoundpage() for tail pages

In _splitfoliotoorder(), smpwmb() guarantees page flags of tail pages are visible before the tail page becomes non-compound. smpwmb() should be paired with smprmb() in softleaftofolio(), which is missed. As a result, if zapnonpresentptes() accesses migration entry that stores tail pfn, softleaftofolio() may see the updated compoundhead of tail page before page->flags.

This issue will trigger VMWARNONONCE() in pfnswapentryfolio() because of the race between folio split and zapnonpresentptes() 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 smprmb() if the softleaf entry is migration entry in softleaftofolio() and softleafto_page().

[tujinjiang@huawei.com: update function name and comments]