CVE-2021-47607 bpf: Fix kernel address leakage in atomic cmpxchg's r0 aux reg

Medium 5.5 Published 19 Jun 2024 Updated 11 May 2026

An unprivileged user can leak kernel pointers via BPF atomic cmpxchg operations, potentially aiding further attacks.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2021-47607?

CVE-2021-47607 is a medium-severity vulnerability in Linux kernel, published on 19 Jun 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:

bpf: Fix kernel address leakage in atomic cmpxchg's r0 aux reg

The implementation of BPF_CMPXCHG on a high level has the following parameters:

.-[old-val] .-[new-val] BPF_R0 = cmpxchg{32,64}(DST_REG + insn->off, BPF_R0, SRC_REG) `-[mem-loc] `-[old-val]

Given a BPF insn can only have two registers (dst, src), the R0 is fixed and used as an auxilliary register for input (old value) as well as output (returning old value from memory location). While the verifier performs a number of safety checks, it misses to reject unprivileged programs where R0 contains a pointer as old value.

Through brute-forcing it takes about ~16sec on my machine to leak a kernel pointer with BPF_CMPXCHG. The PoC is basically probing for kernel addresses by storing the guessed address into the map slot as a scalar, and using the map value pointer as R0 while SRC_REG has a canary value to detect a matching address.

Fix it by checking R0 for pointers, and reject if that's the case for unprivileged programs.

Which versions are affected by CVE-2021-47607?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2021-47607?

Apply the vendor's patch.

Is CVE-2021-47607 being exploited?

There is no known exploitation of CVE-2021-47607 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as DEBIAN-CVE-2021-47607, GHSA-fp67-w4xm-4hj6, UBUNTU-CVE-2021-47607.