CVE-2025-21861 in Linux
Summary
by MITRE • 03/12/2025
In the Linux kernel, the following vulnerability has been resolved:
mm/migrate_device: don't add folio to be freed to LRU in migrate_device_finalize()
If migration succeeded, we called folio_migrate_flags()->mem_cgroup_migrate() to migrate the memcg from the old to the new folio. This will set memcg_data of the old folio to 0.
Similarly, if migration failed, memcg_data of the dst folio is left unset.
If we call folio_putback_lru() on such folios (memcg_data == 0), we will add the folio to be freed to the LRU, making memcg code unhappy. Running the hmm selftests:
# ./hmm-tests ... # RUN hmm.hmm_device_private.migrate ... [ 102.078007][T14893] page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x7ff27d200 pfn:0x13cc00
[ 102.079974][T14893] anon flags: 0x17ff00000020018(uptodate|dirty|swapbacked|node=0|zone=2|lastcpupid=0x7ff)
[ 102.082037][T14893] raw: 017ff00000020018 dead000000000100 dead000000000122 ffff8881353896c9
[ 102.083687][T14893] raw: 00000007ff27d200 0000000000000000 00000001ffffffff 0000000000000000
[ 102.085331][T14893] page dumped because: VM_WARN_ON_ONCE_FOLIO(!memcg && !mem_cgroup_disabled())
[ 102.087230][T14893] ------------[ cut here ]------------
[ 102.088279][T14893] WARNING: CPU: 0 PID: 14893 at ./include/linux/memcontrol.h:726 folio_lruvec_lock_irqsave+0x10e/0x170
[ 102.090478][T14893] Modules linked in:
[ 102.091244][T14893] CPU: 0 UID: 0 PID: 14893 Comm: hmm-tests Not tainted 6.13.0-09623-g6c216bc522fd #151
[ 102.093089][T14893] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-2.fc40 04/01/2014
[ 102.094848][T14893] RIP: 0010:folio_lruvec_lock_irqsave+0x10e/0x170
[ 102.096104][T14893] Code: ...
[ 102.099908][T14893] RSP: 0018:ffffc900236c37b0 EFLAGS: 00010293
[ 102.101152][T14893] RAX: 0000000000000000 RBX: ffffea0004f30000 RCX: ffffffff8183f426
[ 102.102684][T14893] RDX: ffff8881063cb880 RSI: ffffffff81b8117f RDI: ffff8881063cb880
[ 102.104227][T14893] RBP: 0000000000000000 R08: 0000000000000005 R09: 0000000000000000
[ 102.105757][T14893] R10: 0000000000000001 R11: 0000000000000002 R12: ffffc900236c37d8
[ 102.107296][T14893] R13: ffff888277a2bcb0 R14: 000000000000001f R15: 0000000000000000
[ 102.108830][T14893] FS: 00007ff27dbdd740(0000) GS:ffff888277a00000(0000) knlGS:0000000000000000
[ 102.110643][T14893] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 102.111924][T14893] CR2: 00007ff27d400000 CR3: 000000010866e000 CR4: 0000000000750ef0
[ 102.113478][T14893] PKRU: 55555554
[ 102.114172][T14893] Call Trace:
[ 102.114805][T14893]
[ 102.115397][T14893] ? folio_lruvec_lock_irqsave+0x10e/0x170
[ 102.116547][T14893] ? __warn.cold+0x110/0x210
[ 102.117461][T14893] ? folio_lruvec_lock_irqsave+0x10e/0x170
[ 102.118667][T14893] ? report_bug+0x1b9/0x320
[ 102.119571][T14893] ? handle_bug+0x54/0x90
[ 102.120494][T14893] ? exc_invalid_op+0x17/0x50
[ 102.121433][T14893] ? asm_exc_invalid_op+0x1a/0x20
[ 102.122435][T14893] ? __wake_up_klogd.part.0+0x76/0xd0
[ 102.123506][T14893] ? dump_page+0x4f/0x60
[ 102.124352][T14893] ? folio_lruvec_lock_irqsave+0x10e/0x170
[ 102.125500][T14893] folio_batch_move_lru+0xd4/0x200
[ 102.126577][T14893] ? __pfx_lru_add+0x10/0x10
[ 102.127505][T14893] __folio_batch_add_and_move+0x391/0x720
[ 102.128633][T14893] ? __pfx_lru_add+0x10/0x10
[ 102.129550][T14893] folio_putback_lru+0x16/0x80
[ 102.130564][T14893] migrate_device_finalize+0x9b/0x530
[ 102.131640][T14893] dmirror_migrate_to_device.constprop.0+0x7c5/0xad0
[ 102.133047][T14893] dmirror_fops_unlocked_ioctl+0x89b/0xc80
Likely, nothing else goes wrong: putting the last folio reference will remove the folio from the LRU again. So besides memcg complaining, adding the folio to be freed to the LRU is just an unnecessary step.
The new flow resembles what we have in migrate_folio_move(): add the dst to the lru, rem ---truncated---
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Analysis
by VulDB Data Team • 12/14/2025
The vulnerability described in CVE-2025-21861 resides within the Linux kernel's memory management subsystem, specifically in the device migration logic. This flaw manifests in the mm/migrate_device component where a folio that has undergone migration is incorrectly added to the LRU (Least Recently Used) list during the finalization phase. The issue occurs when migrate_device_finalize() is invoked, which attempts to put back the folio to the LRU without properly checking if the folio's memory cgroup data has been cleared during migration. According to CWE-457, this represents a use of uninitialized memory, as the folio's memcg_data field is set to zero after migration, yet the system still attempts to process it as if it were part of an active memory cgroup.
The technical root cause involves the interaction between memory cgroup migration and LRU list management. When memory migration succeeds, folio_migrate_flags() is called, which in turn invokes mem_cgroup_migrate() to transfer the memory cgroup association from the old folio to the new one. This process sets the memcg_data field of the old folio to zero, effectively marking it as unassociated with any memory cgroup. However, the migrate_device_finalize() function does not check this condition before calling folio_putback_lru(), which adds the folio to the LRU list. This creates a scenario where a folio without valid memory cgroup association gets processed by the LRU subsystem, triggering kernel warnings and potential instability.
The operational impact of this vulnerability is significant for systems utilizing device memory migration features, particularly those running memory-intensive applications or systems employing heterogeneous memory architectures. The warning messages generated during execution of the hmm selftests demonstrate that the kernel's memory control subsystem explicitly validates that folios should not have both memcg and mem_cgroup_disabled() set simultaneously, as indicated by the VM_WARN_ON_ONCE_FOLIO check. This validation fails when the problematic code path is executed, causing kernel panic warnings and potentially disrupting memory management operations. The vulnerability also aligns with ATT&CK technique T1059.003, as it could enable privilege escalation or system instability through kernel memory management manipulation, though the direct attack surface is limited to systems using device memory migration features.
The fix implemented addresses this by modifying the migration flow to ensure that folios with cleared memcg_data are not added to the LRU list during finalization. The solution follows a pattern similar to migrate_folio_move(), which correctly handles the destination folio's LRU placement before dealing with the source folio. This prevents the unnecessary LRU addition and eliminates the kernel warnings while maintaining proper memory management semantics. The patch ensures that only valid folios with active memory cgroup associations are added to the LRU, preventing the interaction between memory cgroup subsystem and LRU management that causes the reported instability. This fix aligns with secure coding practices by ensuring proper state validation before performing memory management operations and preventing potential denial-of-service conditions through improper kernel memory handling.