CVE-2025-38554 (GCVE-0-2025-38554)

Vulnerability from cvelistv5 – Published: 2025-08-19 17:02 – Updated: 2025-09-29 05:53
VLAI?
Title
mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped
Summary
In the Linux kernel, the following vulnerability has been resolved: mm: fix a UAF when vma->mm is freed after vma->vm_refcnt got dropped By inducing delays in the right places, Jann Horn created a reproducer for a hard to hit UAF issue that became possible after VMAs were allowed to be recycled by adding SLAB_TYPESAFE_BY_RCU to their cache. Race description is borrowed from Jann's discovery report: lock_vma_under_rcu() looks up a VMA locklessly with mas_walk() under rcu_read_lock(). At that point, the VMA may be concurrently freed, and it can be recycled by another process. vma_start_read() then increments the vma->vm_refcnt (if it is in an acceptable range), and if this succeeds, vma_start_read() can return a recycled VMA. In this scenario where the VMA has been recycled, lock_vma_under_rcu() will then detect the mismatching ->vm_mm pointer and drop the VMA through vma_end_read(), which calls vma_refcount_put(). vma_refcount_put() drops the refcount and then calls rcuwait_wake_up() using a copy of vma->vm_mm. This is wrong: It implicitly assumes that the caller is keeping the VMA's mm alive, but in this scenario the caller has no relation to the VMA's mm, so the rcuwait_wake_up() can cause UAF. The diagram depicting the race: T1 T2 T3 == == == lock_vma_under_rcu mas_walk <VMA gets removed from mm> mmap <the same VMA is reallocated> vma_start_read __refcount_inc_not_zero_limited_acquire munmap __vma_enter_locked refcount_add_not_zero vma_end_read vma_refcount_put __refcount_dec_and_test rcuwait_wait_event <finish operation> rcuwait_wake_up [UAF] Note that rcuwait_wait_event() in T3 does not block because refcount was already dropped by T1. At this point T3 can exit and free the mm causing UAF in T1. To avoid this we move vma->vm_mm verification into vma_start_read() and grab vma->vm_mm to stabilize it before vma_refcount_put() operation. [surenb@google.com: v3]
Severity ?
No CVSS data available.
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 3104138517fc66aad21f4a2487bb572e9fc2e3ec , < 6e88fe54721dee17d3496bc998f0c7d243896348 (git)
Affected: 3104138517fc66aad21f4a2487bb572e9fc2e3ec , < 1bcd236a2536a451e385f8d6d2bb589689ec812f (git)
Affected: 3104138517fc66aad21f4a2487bb572e9fc2e3ec , < 9bbffee67ffd16360179327b57f3b1245579ef08 (git)
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    Linux Linux Affected: 6.15
Unaffected: 0 , < 6.15 (semver)
Unaffected: 6.15.10 , ≤ 6.15.* (semver)
Unaffected: 6.16.1 , ≤ 6.16.* (semver)
Unaffected: 6.17 , ≤ * (original_commit_for_fix)
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{
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          "programFiles": [
            "include/linux/mmap_lock.h",
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          "vendor": "Linux",
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              "cpeMatch": [
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                  "vulnerable": true
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                }
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              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nmm: fix a UAF when vma-\u003emm is freed after vma-\u003evm_refcnt got dropped\n\nBy inducing delays in the right places, Jann Horn created a reproducer for\na hard to hit UAF issue that became possible after VMAs were allowed to be\nrecycled by adding SLAB_TYPESAFE_BY_RCU to their cache.\n\nRace description is borrowed from Jann\u0027s discovery report:\nlock_vma_under_rcu() looks up a VMA locklessly with mas_walk() under\nrcu_read_lock().  At that point, the VMA may be concurrently freed, and it\ncan be recycled by another process.  vma_start_read() then increments the\nvma-\u003evm_refcnt (if it is in an acceptable range), and if this succeeds,\nvma_start_read() can return a recycled VMA.\n\nIn this scenario where the VMA has been recycled, lock_vma_under_rcu()\nwill then detect the mismatching -\u003evm_mm pointer and drop the VMA through\nvma_end_read(), which calls vma_refcount_put().  vma_refcount_put() drops\nthe refcount and then calls rcuwait_wake_up() using a copy of vma-\u003evm_mm. \nThis is wrong: It implicitly assumes that the caller is keeping the VMA\u0027s\nmm alive, but in this scenario the caller has no relation to the VMA\u0027s mm,\nso the rcuwait_wake_up() can cause UAF.\n\nThe diagram depicting the race:\nT1         T2         T3\n==         ==         ==\nlock_vma_under_rcu\n  mas_walk\n          \u003cVMA gets removed from mm\u003e\n                      mmap\n                        \u003cthe same VMA is reallocated\u003e\n  vma_start_read\n    __refcount_inc_not_zero_limited_acquire\n                      munmap\n                        __vma_enter_locked\n                          refcount_add_not_zero\n  vma_end_read\n    vma_refcount_put\n      __refcount_dec_and_test\n                          rcuwait_wait_event\n                            \u003cfinish operation\u003e\n      rcuwait_wake_up [UAF]\n\nNote that rcuwait_wait_event() in T3 does not block because refcount was\nalready dropped by T1.  At this point T3 can exit and free the mm causing\nUAF in T1.\n\nTo avoid this we move vma-\u003evm_mm verification into vma_start_read() and\ngrab vma-\u003evm_mm to stabilize it before vma_refcount_put() operation.\n\n[surenb@google.com: v3]"
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2025-09-29T05:53:41.103Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
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          "url": "https://git.kernel.org/stable/c/6e88fe54721dee17d3496bc998f0c7d243896348"
        },
        {
          "url": "https://git.kernel.org/stable/c/1bcd236a2536a451e385f8d6d2bb589689ec812f"
        },
        {
          "url": "https://git.kernel.org/stable/c/9bbffee67ffd16360179327b57f3b1245579ef08"
        }
      ],
      "title": "mm: fix a UAF when vma-\u003emm is freed after vma-\u003evm_refcnt got dropped",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2025-38554",
    "datePublished": "2025-08-19T17:02:33.315Z",
    "dateReserved": "2025-04-16T04:51:24.025Z",
    "dateUpdated": "2025-09-29T05:53:41.103Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.1"
}


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