CVE-2021-46988 userfaultfd: release page in error path to avoid BUG_ON

2024-02-2808:13:15

Linux

github.com

linux kernel vulnerability

userfaultfd

release page

error path

bug_on

shmem_mfill_atomic_pte()

copy_from_user()

-enoent

mmap_lock

tmpfs

fix

accounting

dangling page

6.6 Medium

AI Score

Confidence

Low

0.0004 Low

EPSS

Percentile

10.4%

JSON

In the Linux kernel, the following vulnerability has been resolved:

userfaultfd: release page in error path to avoid BUG_ON

Consider the following sequence of events:

Userspace issues a UFFD ioctl, which ends up calling into
shmem_mfill_atomic_pte(). We successfully account the blocks, we
shmem_alloc_page(), but then the copy_from_user() fails. We return
-ENOENT. We don’t release the page we allocated.
Our caller detects this error code, tries the copy_from_user() after
dropping the mmap_lock, and retries, calling back into
shmem_mfill_atomic_pte().
Meanwhile, let’s say another process filled up the tmpfs being used.
So shmem_mfill_atomic_pte() fails to account blocks this time, and
immediately returns - without releasing the page.

This triggers a BUG_ON in our caller, which asserts that the page
should always be consumed, unless -ENOENT is returned.

To fix this, detect if we have such a “dangling” page when accounting
fails, and if so, release it before returning.

CNA Affected

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "mm/shmem.c"
    ],
    "versions": [
      {
        "version": "cb658a453b93",
        "lessThan": "319116227e52",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "cb658a453b93",
        "lessThan": "07c9b834c97d",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "cb658a453b93",
        "lessThan": "b3f1731c6d7f",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "cb658a453b93",
        "lessThan": "140cfd998012",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "cb658a453b93",
        "lessThan": "ad5312797303",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "cb658a453b93",
        "lessThan": "2d59a0ed8b26",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "cb658a453b93",
        "lessThan": "7ed9d238c7db",
        "status": "affected",
        "versionType": "git"
      }
    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "affected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "mm/shmem.c"
    ],
    "versions": [
      {
        "version": "4.11",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "4.11",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "4.14.233",
        "lessThanOrEqual": "4.14.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "4.19.191",
        "lessThanOrEqual": "4.19.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.4.120",
        "lessThanOrEqual": "5.4.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.10.38",
        "lessThanOrEqual": "5.10.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.11.22",
        "lessThanOrEqual": "5.11.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.12.5",
        "lessThanOrEqual": "5.12.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.13",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]