CVE-2021-46988 Information

Feb 29, 2024 cve

Description

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.

Reference

https://git.kernel.org/stable/c/319116227e52d49eee671f0aa278bac89b3c1b69 https://git.kernel.org/stable/c/07c9b834c97d0fa3402fb7f3f3b32df370a6ff1f https://git.kernel.org/stable/c/b3f1731c6d7fbc1ebe3ed8eff6d6bec56d76ff43 https://git.kernel.org/stable/c/140cfd9980124aecb6c03ef2e69c72d0548744de https://git.kernel.org/stable/c/ad53127973034c63b5348715a1043d0e80ceb330 https://git.kernel.org/stable/c/2d59a0ed8b26b8f3638d8afc31f839e27759f1f6 https://git.kernel.org/stable/c/7ed9d238c7dbb1fdb63ad96a6184985151b0171c

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