CVE-2021-46988

Summary

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:

  1. 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.
  2. 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().
  3. Meanwhile, let's say another process filled up the tmpfs being used.
  4. 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.

Affected Software

VendorProductVersion RangeStatus
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < 319116227e52d49eee671f0aa278bac89b3c1b69affected
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < 07c9b834c97d0fa3402fb7f3f3b32df370a6ff1faffected
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < b3f1731c6d7fbc1ebe3ed8eff6d6bec56d76ff43affected
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < 140cfd9980124aecb6c03ef2e69c72d0548744deaffected
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < ad53127973034c63b5348715a1043d0e80ceb330affected
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < 2d59a0ed8b26b8f3638d8afc31f839e27759f1f6affected
LinuxLinuxcb658a453b9327ce96ce5222c24d162b5b65b564 < 7ed9d238c7dbb1fdb63ad96a6184985151b0171caffected
LinuxLinux4.11affected
LinuxLinux0 < 4.11unaffected
LinuxLinux4.14.233 <= 4.14.*unaffected
LinuxLinux4.19.191 <= 4.19.*unaffected
LinuxLinux5.4.120 <= 5.4.*unaffected
LinuxLinux5.10.38 <= 5.10.*unaffected
LinuxLinux5.11.22 <= 5.11.*unaffected
LinuxLinux5.12.5 <= 5.12.*unaffected
LinuxLinux5.13 <= *unaffected

Weaknesses

ADP Enrichment

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: partial

CVE Program Container

Additional References

References