CVE-2024-50063

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

bpf: Prevent tail call between progs attached to different hooks

bpf progs can be attached to kernel functions, and the attached functions can take different parameters or return different return values. If prog attached to one kernel function tail calls prog attached to another kernel function, the ctx access or return value verification could be bypassed.

For example, if prog1 is attached to func1 which takes only 1 parameter and prog2 is attached to func2 which takes two parameters. Since verifier assumes the bpf ctx passed to prog2 is constructed based on func2's prototype, verifier allows prog2 to access the second parameter from the bpf ctx passed to it. The problem is that verifier does not prevent prog1 from passing its bpf ctx to prog2 via tail call. In this case, the bpf ctx passed to prog2 is constructed from func1 instead of func2, that is, the assumption for ctx access verification is bypassed.

Another example, if BPF LSM prog1 is attached to hook fileallocsecurity, and BPF LSM prog2 is attached to hook bpflsmauditruleknown. Verifier knows the return value rules for these two hooks, e.g. it is legal for bpflsmauditruleknown to return positive number 1, and it is illegal for fileallocsecurity to return positive number. So verifier allows prog2 to return positive number 1, but does not allow prog1 to return positive number. The problem is that verifier does not prevent prog1 from calling prog2 via tail call. In this case, prog2's return value 1 will be used as the return value for prog1's hook fileallocsecurity. That is, the return value rule is bypassed.

This patch adds restriction for tail call to prevent such bypasses.