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CVE-2021-46988
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CVE-2021-46988
MITRE ATT&CK
Collection
Command and Control
Credential Access
Defense Evasion
Discovery
Execution
Exfiltration
Impact
Initial Access
Lateral Movement
Persistence
Privilege Escalation
Topic Tags
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:
- 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.
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