Messages in this thread | | | Subject | Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check() | From | He Fengqing <> | Date | Wed, 14 Jul 2021 09:53:46 +0800 |
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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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