[CVE-2021-29154] Linux kernel incorrect computation of branch displacements in BPF JIT compiler can be abused to execute arbitrary code in Kernel mode

[Piotr Krysiuk <piotras () gmail com>]

An issue has been discovered in the Linux kernel that can be abused by
unprivileged local users to escalate privileges.

The issue is with how BPF JIT compilers for some architectures compute
branch displacements when generating machine code. This can be abused
to craft anomalous machine code and execute it in the Kernel mode,
where the control flow is hijacked to execute unsafe code.

I developed PoCs for x86-64 and x86-32 architectures to demonstrate
shellcode execution in Kernel mode by unprivileged local users.

One of these PoCs has been shared privately with <security () kernel org>
to assist with fix development.

Patches to mitigate the issue for x86-64 and x86-32 architectures are
available. These patches do not attempt to correct the underlying
algorithm and instead assert that all computations were performed
correctly, such that all unsafe inputs are rejected.

The patches were published via BPF subsystem public git repository:
* https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/patch/?id=e4d4d456436bfb2fe412ee2cd489f7658449b098
* https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/patch/?id=26f55a59dc65ff77cd1c4b37991e26497fc68049

# Discoverer

Piotr Krysiuk <piotras () gmail com>

# References

CVE-2021-29154 (reserved via https://cveform.mitre.org/)