# CVE-2021-47607: linux: In the Linux kernel, the following vulnerability has been resolved:  bpf: Fix kernel address leakage in atomic cmpxch...

CVE-2021-47607 is fixed in linux on 1 platform, first by DEBIAN-CVE-2021-47607, released June 19, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2021-47607
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0025
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jun 19, 2024

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.15.15-1 | [DEBIAN-CVE-2021-47607](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47607.md) | Jun 19, 2024 |
| Linux | linux Debian 13.5 (trixie) | 5.15.15-1 | [DEBIAN-CVE-2021-47607](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47607.md) | Jun 19, 2024 |
| Linux | linux Debian 14 | 5.15.15-1 | [DEBIAN-CVE-2021-47607](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47607.md) | Jun 19, 2024 |

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