bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack() task_pt_regs() can return NULL on powerpc for kernel threads. This is then used in __bpf_get_stack() to check for user mode, resulting in a kernel oops. Guard against this by checking return value of task_pt_regs() before trying to obtain the call chain.

References

• https://git.kernel.org/stable/c/ff6bdc205fd0a83bd365405d4e31fb5905826996
• https://git.kernel.org/stable/c/0bcd484587b3b3092e448d27dc369e347e1810c3
• https://git.kernel.org/stable/c/b82ef4985a6d05e80f604624332430351df7b79a
• https://git.kernel.org/stable/c/b992f01e66150fc5e90be4a96f5eb8e634c8249e