CVE-2025-71074

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

functionfs: fix the open/removal races

ffs_epfile_open() can race with removal, ending up with file->private_data

pointing to freed object.

There is a total count of opened files on functionfs (both ep0 and

dynamic ones) and when it hits zero, dynamic files get removed.

Unfortunately, that removal can happen while another thread is

in ffs_epfile_open(), but has not incremented the count yet.

In that case open will succeed, leaving us with UAF on any subsequent

read() or write().

The root cause is that ffs->opened is misused; atomic_dec_and_test() vs.

atomic_add_return() is not a good idea, when object remains visible all

along.

To untangle that

* serialize openers on ffs->mutex (both for ep0 and for dynamic files)

* have dynamic ones use atomic_inc_not_zero() and fail if we had

zero ->opened; in that case the file we are opening is doomed.

* have the inodes of dynamic files marked on removal (from the

callback

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References