Bug 2461557 (CVE-2026-31575)

Summary: CVE-2026-31575 kernel: mm/userfaultfd: fix hugetlb fault mutex hash calculation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel. A mismatch in the calculation of page indexes for huge pages within the `mm/userfaultfd` component can lead to race conditions between threads. These race conditions can corrupt the reservation map, potentially causing a system crash and resulting in a Denial of Service (DoS).
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Description OSIDB Bzimport 2026-04-24 15:08:20 UTC 
In the Linux kernel, the following vulnerability has been resolved:

mm/userfaultfd: fix hugetlb fault mutex hash calculation

In mfill_atomic_hugetlb(), linear_page_index() is used to calculate the
page index for hugetlb_fault_mutex_hash().  However, linear_page_index()
returns the index in PAGE_SIZE units, while hugetlb_fault_mutex_hash()
expects the index in huge page units.  This mismatch means that different
addresses within the same huge page can produce different hash values,
leading to the use of different mutexes for the same huge page.  This can
cause races between faulting threads, which can corrupt the reservation
map and trigger the BUG_ON in resv_map_release().

Fix this by introducing hugetlb_linear_page_index(), which returns the
page index in huge page granularity, and using it in place of
linear_page_index().
Comment 1 Keith Grant 2026-04-24 18:14:53 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026042409-CVE-2026-31575-c1bf@gregkh/T