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Date: Wed, 09 Jun 2021 09:23:39 -0700
From: John Fastabend <john.fastabend@...il.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
John Fastabend <john.fastabend@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, andriin@...com,
netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf 1/2] bpf: Fix null ptr deref with mixed tail calls and
subprogs
Maciej Fijalkowski wrote:
> 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 <maciej.fijalkowski@...el.com>
> > Fixes: a748c6975dea3 ("bpf: propagate poke descriptors to subprograms")
> > Reviewed-by: Daniel Borkmann <daniel@...earbox.net>
> > Signed-off-by: John Fastabend <john.fastabend@...il.com>
> > ---
> > 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'.
I'm not necessarily opposed to a comment there but we don't have a comment
above for the same operation on start offsets. I'll think about it for
a follow up patch assuming no one shouts.
>
> Generally sorry that I missed that, it didn't come to my mind to mix in
> other helpers that include patching.
Thanks for testing.
>
> Anyway:
> Acked-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
> Tested-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
>
> > + 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;
> > }
> >
> >
> >
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