CVE-2025-39776
High · 7.8CVSS
7.8
EPSS
0.2%
Published
Sep 11, 2025
CISA due date
—
Cloudanix analysis
A Linux kernel test left behind old memory references that could confuse the system when new programs started, leading to crashes or errors on debug builds.
- Exploitation
- Local Privilege
- Attacker needs
- Low Privilege
- Complexity
- Low
- Impact
- Full Compromise
- Component
- OS kernel
- Class
- Use After Free
- Remediation
- Patch
- Effort
- Medium
- Patch risk
- Low
What it actually does
Failure to clear manually allocated page table entries in the debug_vm_pgtable test at destroy_args() leaves stale PGD/PUD entries; reuse of the same mm_struct slab address by a subsequent process triggers use-after-free style access during free_pud_range and related mm teardown paths.
Why it matters to the business
On debug kernels with the affected test enabled, kernel warnings, RSS counter bugs, and potential crashes can occur during process exit, disrupting development, CI, and testing workloads that rely on CONFIG_DEBUG_VM_PGTABLE.
How an attack unfolds
An attacker with local access compiles or loads a kernel module that triggers the debug_vm_pgtable test; after the test exits, a new process reuses the mm_struct address containing stale entries, causing invalid page table walks and negative pgtables_bytes during exit_mmap.
How to fix it
Upgrade to a kernel containing commit 7bf57a0709cd7c9088cea8de023d6f4fbf2518b0 which ensures page table entries are cleared with the appropriate *_clear functions in destroy_args().
How to confirm the fix worked
Confirm the fix is present by checking that destroy_args() now invokes the clear helpers before freeing the mm_struct; re-run the debug_vm_pgtable test and verify absence of negative pgtables_bytes warnings and bad RSS state on process exit.
Typical attack path
- Enable CONFIG_DEBUG_VM_PGTABLE and run the debug_vm_pgtable test module
- Test module allocates mm_struct and page table entries then calls destroy_args without clearing
- Attacker triggers process that reuses the freed mm_struct slab address
- Kernel performs exit_mmap on the reused mm, walking stale entries and corrupting counters
How to detect it
Search kernel logs for "BUG: Bad rss-counter state" and "BUG: non-zero pgtables_bytes on freeing mm" messages originating from exit_mmap or free_pud_range after debug_vm_pgtable activity; correlate with modprobe or test module load times.
What your security team should do
- Apply the referenced commit or later stable update on all debug/development kernels
- Disable CONFIG_DEBUG_VM_PGTABLE on production systems where feasible
- Monitor for mm teardown warnings in dmesg on systems running the test
The record
In the Linux kernel, the following vulnerability has been resolved: mm/debug_vm_pgtable: clear page table entries at destroy_args() The mm/debug_vm_pagetable test allocates manually page table entries for the tests it runs, using also its manually allocated mm_struct. That in itself is ok, but when it exits, at destroy_args() it fails to clear those entries with the *_clear functions. The problem is that leaves stale entries. If another process allocates an mm_struct with a pgd at the same address, it may end up running into the stale entry. This is happening in practice on a debug kernel with CONFIG_DEBUG_VM_PGTABLE=y, for example this is the output with some extra debugging I added (it prints a warning trace if pgtables_bytes goes negative, in addition to the warning at check_mm() function): [ 2.539353] debug_vm_pgtable: [get_random_vaddr ]: random_vaddr is 0x7ea247140000 [ 2.539366] kmem_cache info [ 2.539374] kmem_cachep 0x000000002ce82385 - freelist 0x0000000000000000 - offset 0x508 [ 2.539447] debug_vm_pgtable: [init_args ]: args->mm is 0x000000002267cc9e (...) [ 2.552800] WARNING: CPU: 5 PID: 116 at include/linux/mm.h:2841 free_pud_range+0x8bc/0x8d0 [ 2.552816] Modules linked in: [ 2.552843] CPU: 5 UID: 0 PID: 116 Comm: modprobe Not tainted 6.12.0-105.debug_vm2.el10.ppc64le+debug #1 VOLUNTARY [ 2.552859] Hardware name: IBM,9009-41A POWER9 (architected) 0x4e0202 0xf000005 of:IBM,FW910.00 (VL910_062) hv:phyp pSeries [ 2.552872] NIP: c0000000007eef3c LR: c0000000007eef30 CTR: c0000000003d8c90 [ 2.552885] REGS: c0000000622e73b0 TRAP: 0700 Not tainted (6.12.0-105.debug_vm2.el10.ppc64le+debug) [ 2.552899] MSR: 800000000282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 24002822 XER: 0000000a [ 2.552954] CFAR: c0000000008f03f0 IRQMASK: 0 [ 2.552954] GPR00: c0000000007eef30 c0000000622e7650 c000000002b1ac00 0000000000000001 [ 2.552954] GPR04: 0000000000000008 0000000000000000 c0000000007eef30 ffffffffffffffff [ 2.552954] GPR08: 00000000ffff00f5 0000000000000001 0000000000000048 0000000000004000 [ 2.552954] GPR12: 00000003fa440000 c000000017ffa300 c0000000051d9f80 ffffffffffffffdb [ 2.552954] GPR16: 0000000000000000 0000000000000008 000000000000000a 60000000000000e0 [ 2.552954] GPR20: 4080000000000000 c0000000113af038 00007fffcf130000 0000700000000000 [ 2.552954] GPR24: c000000062a6a000 0000000000000001 8000000062a68000 0000000000000001 [ 2.552954] GPR28: 000000000000000a c000000062ebc600 0000000000002000 c000000062ebc760 [ 2.553170] NIP [c0000000007eef3c] free_pud_range+0x8bc/0x8d0 [ 2.553185] LR [c0000000007eef30] free_pud_range+0x8b0/0x8d0 [ 2.553199] Call Trace: [ 2.553207] [c0000000622e7650] [c0000000007eef30] free_pud_range+0x8b0/0x8d0 (unreliable) [ 2.553229] [c0000000622e7750] [c0000000007f40b4] free_pgd_range+0x284/0x3b0 [ 2.553248] [c0000000622e7800] [c0000000007f4630] free_pgtables+0x450/0x570 [ 2.553274] [c0000000622e78e0] [c0000000008161c0] exit_mmap+0x250/0x650 [ 2.553292] [c0000000622e7a30] [c0000000001b95b8] __mmput+0x98/0x290 [ 2.558344] [c0000000622e7a80] [c0000000001d1018] exit_mm+0x118/0x1b0 [ 2.558361] [c0000000622e7ac0] [c0000000001d141c] do_exit+0x2ec/0x870 [ 2.558376] [c0000000622e7b60] [c0000000001d1ca8] do_group_exit+0x88/0x150 [ 2.558391] [c0000000622e7bb0] [c0000000001d1db8] sys_exit_group+0x48/0x50 [ 2.558407] [c0000000622e7be0] [c00000000003d810] system_call_exception+0x1e0/0x4c0 [ 2.558423] [c0000000622e7e50] [c00000000000d05c] system_call_vectored_common+0x15c/0x2ec (...) [ 2.558892] ---[ end trace 0000000000000000 ]--- [ 2.559022] BUG: Bad rss-counter state mm:000000002267cc9e type:MM_ANONPAGES val:1 [ 2.559037] BUG: non-zero pgtables_bytes on freeing mm: -6144 Here the modprobe process ended up with an allocated mm_struct from the mm_struct slab that was used before by the debug_vm_pgtable test. That is not a problem, since the mm_stru ---truncated---
Affected packages
| Ecosystem | Package | Introduced | Fixed |
|---|---|---|---|
| Debian:12 | linux | 0 | No fix |
| Debian:13 | linux | 0 | No fix |
| Debian:14 | linux | 0 | No fix |
| Debian:14 | linux | 0 | 6.16.5-1 |
| Linux | Kernel | 5.15.0 | 5.15.190 |
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