From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B2D12C48BCD for ; Wed, 9 Jun 2021 16:09:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9CBFD60FDC for ; Wed, 9 Jun 2021 16:09:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235239AbhFIQLY (ORCPT ); Wed, 9 Jun 2021 12:11:24 -0400 Received: from mga17.intel.com ([192.55.52.151]:55126 "EHLO mga17.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229685AbhFIQLX (ORCPT ); Wed, 9 Jun 2021 12:11:23 -0400 IronPort-SDR: sAt8pXMOnbMBMDmdv5SJVqzoTpizSWIh53dNLjHcaT/rI67nw0IvchHdr5543vVZHpGrdokxPD krtnHJ5X2EDQ== X-IronPort-AV: E=McAfee;i="6200,9189,10010"; a="185477303" X-IronPort-AV: E=Sophos;i="5.83,261,1616482800"; d="scan'208";a="185477303" Received: from fmsmga007.fm.intel.com ([10.253.24.52]) by fmsmga107.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Jun 2021 09:04:02 -0700 IronPort-SDR: CgE1yeU5oBzGahGwILLMEy1eTwerjPOQIObEBLmB4MPFrZ8jaNIyrB3wTT8hL8p1D7TmoSYf1M 1vZQUsendSCg== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.83,261,1616482800"; d="scan'208";a="413789382" Received: from ranger.igk.intel.com ([10.102.21.164]) by fmsmga007.fm.intel.com with ESMTP; 09 Jun 2021 09:04:00 -0700 Date: Wed, 9 Jun 2021 17:51:31 +0200 From: Maciej Fijalkowski To: John Fastabend Cc: ast@kernel.org, daniel@iogearbox.net, andriin@fb.com, netdev@vger.kernel.org, bpf@vger.kernel.org Subject: Re: [PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and subprogs Message-ID: <20210609155131.GA12061@ranger.igk.intel.com> References: <162318053542.323820.3719766457956848570.stgit@john-XPS-13-9370> <162318061518.323820.4329181800429686297.stgit@john-XPS-13-9370> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <162318061518.323820.4329181800429686297.stgit@john-XPS-13-9370> User-Agent: Mutt/1.12.1 (2019-06-15) Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, Jun 08, 2021 at 12:30:15PM -0700, John Fastabend wrote: > The sub-programs prog->aux->poke_tab[] is populated in jit_subprogs() and > then used when emitting 'BPF_JMP|BPF_TAIL_CALL' insn->code from the > individual JITs. The poke_tab[] to use is stored in the insn->imm by > the code adding it to that array slot. The JIT then uses imm to find the > right entry for an individual instruction. In the x86 bpf_jit_comp.c > this is done by calling emit_bpf_tail_call_direct with the poke_tab[] > of the imm value. > > However, we observed the below null-ptr-deref when mixing tail call > programs with subprog programs. For this to happen we just need to > mix bpf-2-bpf calls and tailcalls with some extra calls or instructions > that would be patched later by one of the fixup routines. So whats > happening? > > Before the fixup_call_args() -- where the jit op is done -- various > code patching is done by do_misc_fixups(). This may increase the > insn count, for example when we patch map_lookup_up using map_gen_lookup > hook. This does two things. First, it means the instruction index, > insn_idx field, of a tail call instruction will move by a 'delta'. > > In verifier code, > > struct bpf_jit_poke_descriptor desc = { > .reason = BPF_POKE_REASON_TAIL_CALL, > .tail_call.map = BPF_MAP_PTR(aux->map_ptr_state), > .tail_call.key = bpf_map_key_immediate(aux), > .insn_idx = i + delta, > }; > > Then subprog start values subprog_info[i].start will be updated > with the delta and any poke descriptor index will also be updated > with the delta in adjust_poke_desc(). If we look at the adjust > subprog starts though we see its only adjusted when the delta > occurs before the new instructions, > > /* NOTE: fake 'exit' subprog should be updated as well. */ > for (i = 0; i <= env->subprog_cnt; i++) { > if (env->subprog_info[i].start <= off) > continue; > > Earlier subprograms are not changed because their start values > are not moved. But, adjust_poke_desc() does the offset + delta > indiscriminately. The result is poke descriptors are potentially > corrupted. > > Then in jit_subprogs() we only populate the poke_tab[] > when the above insn_idx is less than the next subprogram start. From > above we corrupted our insn_idx so we might incorrectly assume a > poke descriptor is not used in a subprogram omitting it from the > subprogram. And finally when the jit runs it does the deref of poke_tab > when emitting the instruction and crashes with below. Because earlier > step omitted the poke descriptor. > > The fix is straight forward with above context. Simply move same logic > from adjust_subprog_starts() into adjust_poke_descs() and only adjust > insn_idx when needed. > > [ 88.487438] BUG: KASAN: null-ptr-deref in do_jit+0x184a/0x3290 > [ 88.487455] Write of size 8 at addr 0000000000000008 by task test_progs/5295 > [ 88.487490] Call Trace: > [ 88.487498] dump_stack+0x93/0xc2 > [ 88.487515] kasan_report.cold+0x5f/0xd8 > [ 88.487530] ? do_jit+0x184a/0x3290 > [ 88.487542] do_jit+0x184a/0x3290 > ... > [ 88.487709] bpf_int_jit_compile+0x248/0x810 > ... > [ 88.487765] bpf_check+0x3718/0x5140 > ... > [ 88.487920] bpf_prog_load+0xa22/0xf10 > > CC: Maciej Fijalkowski > Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms") > Reviewed-by: Daniel Borkmann > Signed-off-by: John Fastabend > --- > kernel/bpf/verifier.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 94ba5163d4c5..ac8373da849c 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -11408,7 +11408,7 @@ static void adjust_subprog_starts(struct bpf_verifier_env *env, u32 off, u32 len > } > } > > -static void adjust_poke_descs(struct bpf_prog *prog, u32 len) > +static void adjust_poke_descs(struct bpf_prog *prog, u32 off, u32 len) > { > struct bpf_jit_poke_descriptor *tab = prog->aux->poke_tab; > int i, sz = prog->aux->size_poke_tab; > @@ -11416,6 +11416,8 @@ static void adjust_poke_descs(struct bpf_prog *prog, u32 len) > > for (i = 0; i < sz; i++) { > desc = &tab[i]; Can we have a comment below that would say something like: "don't update taicall's insn idx if the patching is being done on higher insns" ? What I'm saying is that after a long break from that code I find 'off' as a confusing name. It's the offset within the flat-structured bpf prog (so the prog that is not yet sliced onto subprogs). Maybe we could find a better name for that, like "curr_insn_idx". I'm not sure what's your view on that. OTOH I'm aware that whole content of bpf_patch_insn_data operates on 'off'. Generally sorry that I missed that, it didn't come to my mind to mix in other helpers that include patching. Anyway: Acked-by: Maciej Fijalkowski Tested-by: Maciej Fijalkowski > + if (desc->insn_idx <= off) > + continue; > desc->insn_idx += len - 1; > } > } > @@ -11436,7 +11438,7 @@ static struct bpf_prog *bpf_patch_insn_data(struct bpf_verifier_env *env, u32 of > if (adjust_insn_aux_data(env, new_prog, off, len)) > return NULL; > adjust_subprog_starts(env, off, len); > - adjust_poke_descs(new_prog, len); > + adjust_poke_descs(new_prog, off, len); > return new_prog; > } > > >