CVE-2021-47607

Summary

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.

Affected Software

VendorProductVersion RangeStatus
LinuxLinux5ffa25502b5ab3d639829a2d1e316cff7f59a41e < f87a6c160ecc8c7b417d25f508d3f076fe346136affected
LinuxLinux5ffa25502b5ab3d639829a2d1e316cff7f59a41e < a82fe085f344ef20b452cd5f481010ff96b5c4cdaffected
LinuxLinux5.12affected
LinuxLinux0 < 5.12unaffected
LinuxLinux5.15.11 <= 5.15.*unaffected
LinuxLinux5.16 <= *unaffected

Weaknesses

ADP Enrichment

CVE Program Container

Additional References

CISA ADP Vulnrichment

  • SSVC:
  • Exploitation: none
    • Automatable: no
    • Technical Impact: partial

References