CVE-2021-46988

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.

• linux linux kernel (>= 4.11 < 4.14.233) • linux linux kernel (>= 4.15 < 4.19.191) • linux linux kernel (>= 4.20 < 5.4.120) • linux linux kernel (>= 5.5 < 5.10.38) • linux linux kernel (>= 5.11 < 5.11.22) • linux linux kernel (>= 5.12 < 5.12.5) • linux linux kernel 5.13