Null pointer dereference

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

bpf: Fix re-attachment branch in bpf_tracing_prog_attach

The following case can cause a crash due to missing attach_btf:

1. load rawtp program
2. load fentry program with rawtp as target_fd
3. create tracing link for fentry program with target_fd = 0
4. repeat 3

In the end we have:

• prog->aux->dst_trampoline == NULL

• tgt_prog == NULL (because we did not provide target_fd to link_create)

• prog->aux->attach_btf == NULL (the program was loaded with attach_prog_fd=X)

• the program was loaded for tgt_prog but we have no way to find out which one

  BUG: kernel NULL pointer dereference, address: 0000000000000058
  Call Trace:
  <TASK>
  ? __die+0x20/0x70
  ? page_fault_oops+0x15b/0x430
  ? fixup_exception+0x22/0x330
  ? exc_page_fault+0x6f/0x170
  ? asm_exc_page_fault+0x22/0x30
  ? bpf_tracing_prog_attach+0x279/0x560
  ? btf_obj_id+0x5/0x10
  bpf_tracing_prog_attach+0x439/0x560
  __sys_bpf+0x1cf4/0x2de0
  __x64_sys_bpf+0x1c/0x30
  do_syscall_64+0x41/0xf0
  entry_SYSCALL_64_after_hwframe+0x6e/0x76

Return -EINVAL in this situation.