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

[He Fengqing]

在 2021/7/14 7:17, Alexei Starovoitov 写道:
> On Sun, Jul 11, 2021 at 7:17 PM He Fengqing <hefengqing@xxxxxxxxxx> wrote:
> > 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().
>
> Right. I forgot about insn_has_def32() logic and
> commit b325fbca4b13 ("bpf: verifier: mark patched-insn with
> sub-register zext flag")
> that added that extra parameter.
>
> > 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?
>
> That's a good idea. Let's do that. The new size for vzalloc is easy to compute.
> What should be the commit in the Fixes tag?
> commit 8041902dae52 ("bpf: adjust insn_aux_data when patching insns")
> right?

Ok, I will add this in the commit message.

> 4 year old bug then.
> I wonder why syzbot with malloc error injection didn't catch it sooner.
> .