Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()

[Alexei Starovoitov]

On Fri, Jul 9, 2021 at 4:11 AM He Fengqing <hefengqing@xxxxxxxxxx> wrote:
>
>
>
> 在 2021/7/8 11:09, Alexei Starovoitov 写道:
> > On Wed, Jul 7, 2021 at 8:00 PM He Fengqing <hefengqing@xxxxxxxxxx> wrote:
> >>
> >> Ok, I will change this in next version.
> >
> > before you spam the list with the next version
> > please explain why any of these changes are needed?
> > I don't see an explanation in the patches and I don't see a bug in the code.
> > Did you check what is the prog clone ?
> > When is it constructed? Why verifier has anything to do with it?
> > .
> >
>
>
> I'm sorry, I didn't describe these errors clearly.
>
> bpf_check(bpf_verifier_env)
>      |
>      |->do_misc_fixups(env)
>      |    |
>      |    |->bpf_patch_insn_data(env)
>      |    |    |
>      |    |    |->bpf_patch_insn_single(env->prog)
>      |    |    |    |
>      |    |    |    |->bpf_prog_realloc(env->prog)
>      |    |    |    |    |
>      |    |    |    |    |->construct new_prog
>      |    |    |    |    |    free old_prog(env->prog)
>      |    |    |    |    |
>      |    |    |    |    |->return new_prog;
>      |    |    |    |
>      |    |    |    |->return new_prog;
>      |    |    |
>      |    |    |->adjust_insn_aux_data
>      |    |    |    |
>      |    |    |    |->return ENOMEM;
>      |    |    |
>      |    |    |->return NULL;
>      |    |
>      |    |->return ENOMEM;
>
> bpf_verifier_env->prog had been freed in bpf_prog_realloc function.
>
>
> There are two errors here, the first is memleak in the
> bpf_patch_insn_data function, and the second is use after free in the
> bpf_check function.
>
> memleak in bpf_patch_insn_data:
>
> Look at the call chain above, if adjust_insn_aux_data function return
> ENOMEM, bpf_patch_insn_data will return NULL, but we do not free the
> new_prog.
>
> So in the patch 2, before bpf_patch_insn_data return NULL, we free the
> new_prog.
>
> use after free in bpf_check:
>
> If bpf_patch_insn_data function return NULL, we will not assign new_prog
> to the bpf_verifier_env->prog, but bpf_verifier_env->prog has been freed
> in the bpf_prog_realloc function. Then in bpf_check function, we will
> use bpf_verifier_env->prog after do_misc_fixups function.
>
> In the patch 3, I added a free_old parameter to bpf_prog_realloc, in
> this scenario we don't free old_prog. Instead, we free it in the
> do_misc_fixups function when bpf_patch_insn_data return a valid new_prog.

Thanks for explaining.
Why not to make adjust_insn_aux_data() in bpf_patch_insn_data() first then?
Just changing the order will resolve both issues, no?