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

[He Fengqing]

在 2021/7/9 23:12, Alexei Starovoitov 写道:
> 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?
> .
adjust_insn_aux_data() need the new constructed new_prog as an input 
parameter, so we must call bpf_patch_insn_single() before 
adjust_insn_aux_data().

But we can make adjust_insn_aux_data() never return ENOMEM. In 
bpf_patch_insn_data(), first we pre-malloc memory for new aux_data, then 
call bpf_patch_insn_single() to constructed the new_prog, at last call 
adjust_insn_aux_data() functin. In this way, adjust_insn_aux_data() 
never fails.

bpf_patch_insn_data(env) {
	struct bpf_insn_aux_data *new_data = vzalloc();
	struct bpf_prog *new_prog;
	if (new_data == NULL)
		return NULL;

	new_prog = bpf_patch_insn_single(env->prog);
	if (new_prog == NULL) {
		vfree(new_data);
		return NULL;
	}

	adjust_insn_aux_data(new_prog, new_data);
	return new_prog;
}
What do you think about it?