netdev - Re: [bpf-next 3/3] bpf: Fix a use after free in bpf

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Date:   Tue, 13 Jul 2021 16:17:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     He Fengqing <hefengqing@...wei.com>
Cc:     Song Liu <song@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()

On Sun, Jul 11, 2021 at 7:17 PM He Fengqing <hefengqing@...wei.com> wrote:
>
>
>
> 在 2021/7/9 23:12, Alexei Starovoitov 写道:
> > On Fri, Jul 9, 2021 at 4:11 AM He Fengqing <hefengqing@...wei.com> wrote:
> >>
> >>
> >>
> >> 在 2021/7/8 11:09, Alexei Starovoitov 写道:
> >>> On Wed, Jul 7, 2021 at 8:00 PM He Fengqing <hefengqing@...wei.com> 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?
4 year old bug then.
I wonder why syzbot with malloc error injection didn't catch it sooner.