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CVE Record

CVE-2025-39775: mm/mremap: fix WARN with uffd that has remap events disabled

In the Linux kernel, the following vulnerability has been resolved: mm/mremap: fix WARN with uffd that has remap events disabled Registering userfaultd on a VMA that spans at least one PMD and then mremap()'ing that VMA can trigger a WARN when recovering from a failed page table move due to a page table allocation error. The code ends up doing the right thing (recurse, avoiding moving actual page tables), but triggering that WARN is unpleasant: WARNING: CPU: 2 PID: 6133 at mm/mremap.c:357 move_normal_pmd mm/mremap.c:357 [inline] WARNING: CPU: 2 PID: 6133 at mm/mremap.c:357 move_pgt_entry mm/mremap.c:595 [inline] WARNING: CPU: 2 PID: 6133 at mm/mremap.c:357 move_page_tables+0x3832/0x44a0 mm/mremap.c:852 Modules linked in: CPU: 2 UID: 0 PID: 6133 Comm: syz.0.19 Not tainted 6.17.0-rc1-syzkaller-00004-g53e760d89498 #0 PREEMPT(full) Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 RIP: 0010:move_normal_pmd mm/mremap.c:357 [inline] RIP: 0010:move_pgt_entry mm/mremap.c:595 [inline] RIP: 0010:move_page_tables+0x3832/0x44a0 mm/mremap.c:852 Code: ... RSP: 0018:ffffc900037a76d8 EFLAGS: 00010293 RAX: 0000000000000000 RBX: 0000000032930007 RCX: ffffffff820c6645 RDX: ffff88802e56a440 RSI: ffffffff820c7201 RDI: 0000000000000007 RBP: ffff888037728fc0 R08: 0000000000000007 R09: 0000000000000000 R10: 0000000032930007 R11: 0000000000000000 R12: 0000000000000000 R13: ffffc900037a79a8 R14: 0000000000000001 R15: dffffc0000000000 FS: 000055556316a500(0000) GS:ffff8880d68bc000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000001b30863fff CR3: 0000000050171000 CR4: 0000000000352ef0 Call Trace: <TASK> copy_vma_and_data+0x468/0x790 mm/mremap.c:1215 move_vma+0x548/0x1780 mm/mremap.c:1282 mremap_to+0x1b7/0x450 mm/mremap.c:1406 do_mremap+0xfad/0x1f80 mm/mremap.c:1921 __do_sys_mremap+0x119/0x170 mm/mremap.c:1977 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xcd/0x4c0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f00d0b8ebe9 Code: ... RSP: 002b:00007ffe5ea5ee98 EFLAGS: 00000246 ORIG_RAX: 0000000000000019 RAX: ffffffffffffffda RBX: 00007f00d0db5fa0 RCX: 00007f00d0b8ebe9 RDX: 0000000000400000 RSI: 0000000000c00000 RDI: 0000200000000000 RBP: 00007ffe5ea5eef0 R08: 0000200000c00000 R09: 0000000000000000 R10: 0000000000000003 R11: 0000000000000246 R12: 0000000000000002 R13: 00007f00d0db5fa0 R14: 00007f00d0db5fa0 R15: 0000000000000005 </TASK> The underlying issue is that we recurse during the original page table move, but not during the recovery move. Fix it by checking for both VMAs and performing the check before the pmd_none() sanity check. Add a new helper where we perform+document that check for the PMD and PUD level. Thanks to Harry for bisecting.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysislow

Security readout for executives and security teams

Plain-English summary

This Linux kernel issue can trigger a kernel WARN during a specific memory remapping path involving userfaultd. The public record describes an unpleasant warning, not confirmed data theft, privilege escalation, or active exploitation. Business urgency depends on how quickly affected kernels can be updated, especially on hosts running untrusted local workloads.

Executive priority

Treat this as routine kernel maintenance unless your environment runs untrusted local code at scale. There is no cited evidence of active exploitation or severe impact, but kernel WARN issues should still be corrected through normal patch cycles.

Technical view

The bug is in mm/mremap recovery handling. A userfaultd-registered VMA spanning at least one PMD, followed by mremap after a failed page-table move allocation, can hit a WARN because recursion logic was not applied during recovery. The fix checks both VMAs earlier and documents PMD/PUD handling.

Likely exposure

Exposure appears limited to Linux systems running affected kernel versions in the CVE record. The described path is local kernel memory-management behavior involving userfaultd and mremap, not a remotely reachable service. The sources do not provide CVSS, CWE, distribution package status, or broad product mapping.

Exploitation context

The source bundle does not show active exploitation, and KEV is false. The trigger was demonstrated by syzkaller in a QEMU test kernel. Public evidence only supports a local warning condition under specific memory-management circumstances, not a weaponized exploit or confirmed privilege impact.

Researcher notes

Evidence is narrow: the CVE text describes a syzkaller-discovered WARN and the kernel fix rationale. Impact beyond warning behavior is not established in the supplied sources. Avoid assuming privilege escalation, denial of service, or affected distribution packages without vendor confirmation.

Mitigation direction

  • Upgrade to a kernel release containing the referenced stable fixes.
  • Confirm your distribution has backported one of the listed kernel commits.
  • Prioritize hosts that allow untrusted local workloads to exercise kernel interfaces.
  • If upgrade timing is unclear, follow your vendor's specific interim guidance.

Validation and detection

  • Inventory running Linux kernel versions across servers, container hosts, and desktops.
  • Compare versions against the CVE record's affected and unaffected entries.
  • Check vendor advisories for backported fixes matching the referenced stable commits.
  • Review kernel logs for matching mremap or move_page_tables WARN traces.
Prepared
Confidence
medium
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
0ADP providers
3Source links

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

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Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinux0cef0bb836e3cfe00f08f9606c72abd72fe78ca3, 0cef0bb836e3cfe00f08f9606c72abd72fe78ca3, 310ac886d68de661c3a334198d8604b722d7fdf8, 6.12.11unaffected
LinuxLinux6.13, 0, 6.16.4, 6.17affected
Weakness

CWE details

No CWE listed

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