Re: [PATCH bpf-next v3 1/3] bpf: Detect jumping to reserved code during check_cfg()

[Shung-Hsi Yu]

On Wed, Oct 11, 2023 at 06:38:56AM -0700, Alexei Starovoitov wrote:
> On Wed, Oct 11, 2023 at 2:01 AM Hao Sun <sunhao.th@xxxxxxxxx> wrote:
> >
> > Currently, we don't check if the branch-taken of a jump is reserved code of
> > ld_imm64. Instead, such a issue is captured in check_ld_imm(). The verifier
> > gives the following log in such case:
> >
> > func#0 @0
> > 0: R1=ctx(off=0,imm=0) R10=fp0
> > 0: (18) r4 = 0xffff888103436000       ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
> > 2: (18) r1 = 0x1d                     ; R1_w=29
> > 4: (55) if r4 != 0x0 goto pc+4        ; R4_w=map_ptr(off=0,ks=4,vs=128,imm=0)
> > 5: (1c) w1 -= w1                      ; R1_w=0
> > 6: (18) r5 = 0x32                     ; R5_w=50
> > 8: (56) if w5 != 0xfffffff4 goto pc-2
> > mark_precise: frame0: last_idx 8 first_idx 0 subseq_idx -1
> > mark_precise: frame0: regs=r5 stack= before 6: (18) r5 = 0x32
> > 7: R5_w=50
> > 7: BUG_ld_00
> > invalid BPF_LD_IMM insn
> >
> > Here the verifier rejects the program because it thinks insn at 7 is an
> > invalid BPF_LD_IMM, but such a error log is not accurate since the issue
> > is jumping to reserved code not because the program contains invalid insn.
> > Therefore, make the verifier check the jump target during check_cfg(). For
> > the same program, the verifier reports the following log:
> >
> > func#0 @0
> > jump to reserved code from insn 8 to 7
> >
> > Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
> > ---
> >  kernel/bpf/verifier.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index eed7350e15f4..725ac0b464cf 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14980,6 +14980,7 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
> >  {
> >         int *insn_stack = env->cfg.insn_stack;
> >         int *insn_state = env->cfg.insn_state;
> > +       struct bpf_insn *insns = env->prog->insnsi;
> >
> >         if (e == FALLTHROUGH && insn_state[t] >= (DISCOVERED | FALLTHROUGH))
> >                 return DONE_EXPLORING;
> > @@ -14993,6 +14994,12 @@ static int push_insn(int t, int w, int e, struct bpf_verifier_env *env,
> >                 return -EINVAL;
> >         }
> >
> > +       if (e == BRANCH && insns[w].code == 0) {
> > +               verbose_linfo(env, t, "%d", t);
> > +               verbose(env, "jump to reserved code from insn %d to %d\n", t, w);
> > +               return -EINVAL;
> > +       }
> 
> I don't think we should be changing the verifier to make
> fuzzer logs more readable.

Taking fuzzer out of consideration, giving users clearer explanation for
such verifier rejection could save a lot of head scratching.

Compiler shouldn't generate such program, but its plausible to forget to
account that BPF_LD_IMM64 consists of two instructions when writing
assembly (especially with filter.h-like macros) and have it jump to the 2nd
part of BPF_LD_IMM64.

> Same with patch 2. The code is fine as-is.

The only way BPF_SIZE(insn->code) != BPF_DW conditional in check_ld_imm()
can be met right now is when we have a jump to the 2nd part of LD_IMM64; but
what this conditional actually guard against is not straight-forward and
quite confusing[1].


Shung-Hsi

1: https://lore.kernel.org/bpf/0cf50c32-ab67-ef23-7b84-ef1d4e007c33@xxxxxx/