Messages in this thread | | | Subject | Re: [bpf-next 3/3] bpf: Fix a use after free in bpf_check() | From | He Fengqing <> | Date | Mon, 12 Jul 2021 10:17:39 +0800 |
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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().
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?
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