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    SubjectRe: [bpf-next 3/3] bpf: Fix a use after free in bpf_check()
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    在 2021/7/14 7:17, Alexei Starovoitov 写道:
    > On Sun, Jul 11, 2021 at 7:17 PM He Fengqing <hefengqing@huawei.com> wrote:
    >>
    >>
    >>
    >> 在 2021/7/9 23:12, Alexei Starovoitov 写道:
    >>> On Fri, Jul 9, 2021 at 4:11 AM He Fengqing <hefengqing@huawei.com> wrote:
    >>>>
    >>>>
    >>>>
    >>>> 在 2021/7/8 11:09, Alexei Starovoitov 写道:
    >>>>> On Wed, Jul 7, 2021 at 8:00 PM He Fengqing <hefengqing@huawei.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?

    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.
    > .
    >

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