CVE-2024-50220 fork: do not invoke uffd on fork if error occurs

What is CVE-2024-50220?

CVE-2024-50220 is a medium-severity vulnerability in Linux kernel, published on 9 Nov 2024. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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

fork: do not invoke uffd on fork if error occurs

Patch series "fork: do not expose incomplete mm on fork".

During fork we may place the virtual memory address space into an inconsistent state before the fork operation is complete.

In addition, we may encounter an error during the fork operation that indicates that the virtual memory address space is invalidated.

As a result, we should not be exposing it in any way to external machinery that might interact with the mm or VMAs, machinery that is not designed to deal with incomplete state.

We specifically update the fork logic to defer khugepaged and ksm to the end of the operation and only to be invoked if no error arose, and disallow uffd from observing fork events should an error have occurred.

This patch (of 2):

Currently on fork we expose the virtual address space of a process to userland unconditionally if uffd is registered in VMAs, regardless of whether an error arose in the fork.

This is performed in dup_userfaultfd_complete() which is invoked unconditionally, and performs two duties - invoking registered handlers for the UFFD_EVENT_FORK event via dup_fctx(), and clearing down userfaultfd_fork_ctx objects established in dup_userfaultfd().

This is problematic, because the virtual address space may not yet be correctly initialised if an error arose.

The change in commit d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()") makes this more pertinent as we may be in a state where entries in the maple tree are not yet consistent.

We address this by, on fork error, ensuring that we roll back state that we would otherwise expect to clean up through the event being handled by userland and perform the memory freeing duty otherwise performed by dup_userfaultfd_complete().

We do this by implementing a new function, dup_userfaultfd_fail(), which performs the same loop, only decrementing reference counts.

Note that we perform mmgrab() on the parent and child mm's, however userfaultfd_ctx_put() will mmdrop() this once the reference count drops to zero, so we will avoid memory leaks correctly here.

Weakness: CWE-367

Which versions are affected by CVE-2024-50220?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2024-50220?

Apply the vendor's patch.

Is CVE-2024-50220 being exploited?

There is no known exploitation of CVE-2024-50220 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2024-50220, DEBIAN-CVE-2024-50220, GHSA-cx44-8c8j-5rr2, UBUNTU-CVE-2024-50220.