CVE-2021-46988

Userspace issues a UFFD ioctl, which ends up calling into shmemmfillatomicpte(). We successfully account the blocks, we shmemallocpage(), but then the copyfrom_user() fails. We return -ENOENT. We don't release the page we allocated.
Our caller detects this error code, tries the copyfromuser() after dropping the mmaplock, and retries, calling back into shmemmfillatomicpte().
Meanwhile, let's say another process filled up the tmpfs being used.
So shmemmfillatomic_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.

References

Affected packages

Git /

Affected ranges

Database specific

unresolved_ranges

[
    {
        "events": [
            {
                "introduced": "4.11"
            },
            {
                "fixed": "4.14.233"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "4.15"
            },
            {
                "fixed": "4.19.191"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "4.20"
            },
            {
                "fixed": "5.4.120"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "5.5"
            },
            {
                "fixed": "5.10.38"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "5.11"
            },
            {
                "fixed": "5.11.22"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "5.12"
            },
            {
                "fixed": "5.12.5"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "0"
            },
            {
                "last_affected": "5.13-rc1"
            }
        ]
    }
]

source

"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2021-46988.json"