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

[Hao Sun]

On Wed, Oct 11, 2023 at 4:42 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Oct 10, 2023 at 1:33 AM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >
> > On 10/10/23 9:02 AM, John Fastabend wrote:
> > > Hao Sun 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:
> > >
> > > I think we at least would want a test case for this. Also how did you create
> > > this case? Is it just something you did manually and noticed a strange error?
> >
> > Curious as well.
> >
> > We do have test cases which try to jump into the middle of a double insn as can
> > be seen that this patch breaks BPF CI with regards to log mismatch below (which
> > still needs to be adapted, too). Either way, it probably doesn't hurt to also add
> > the above snippet as a test.
> >
> > Hao, as I understand, the patch here is an usability improvement (not a fix per se)
> > where we reject such cases earlier during cfg check rather than at a later point
> > where we validate ld_imm instruction. Or are there cases you found which were not
> > yet captured via current check_ld_imm()?
> >
> > test_verifier failure log :
> >
> >    #458/u test1 ld_imm64 FAIL
> >    Unexpected verifier log!
> >    EXP: R1 pointer comparison
> >    RES:
> >    FAIL
> >    Unexpected error message!
> >         EXP: R1 pointer comparison
> >         RES: jump to reserved code from insn 0 to 2
> >    verification time 22 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >
> >    jump to reserved code from insn 0 to 2
> >    verification time 22 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >    #458/p test1 ld_imm64 FAIL
> >    Unexpected verifier log!
> >    EXP: invalid BPF_LD_IMM insn
> >    RES:
> >    FAIL
> >    Unexpected error message!
> >         EXP: invalid BPF_LD_IMM insn
> >         RES: jump to reserved code from insn 0 to 2
> >    verification time 9 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >
> >    jump to reserved code from insn 0 to 2
> >    verification time 9 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >    #459/u test2 ld_imm64 FAIL
> >    Unexpected verifier log!
> >    EXP: R1 pointer comparison
> >    RES:
> >    FAIL
> >    Unexpected error message!
> >         EXP: R1 pointer comparison
> >         RES: jump to reserved code from insn 0 to 2
> >    verification time 11 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >
> >    jump to reserved code from insn 0 to 2
> >    verification time 11 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >    #459/p test2 ld_imm64 FAIL
> >    Unexpected verifier log!
> >    EXP: invalid BPF_LD_IMM insn
> >    RES:
> >    FAIL
> >    Unexpected error message!
> >         EXP: invalid BPF_LD_IMM insn
> >         RES: jump to reserved code from insn 0 to 2
> >    verification time 8 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >
> >    jump to reserved code from insn 0 to 2
> >    verification time 8 usec
> >    stack depth 0
> >    processed 0 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
> >    #460/u test3 ld_imm64 OK
> >
> > >> func#0 @0
> > >> jump to reserved code from insn 8 to 7
> > >>
> > >> ---
> > >>
> > >>
> > >> Signed-off-by: Hao Sun <sunhao.th@xxxxxxxxx>
> >
> > nit: This needs to be before the "---" line.
> >
> > >> ---
> > >>   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;
> > >> +    }
> >
> > Other than that, lgtm.
>
> We do rely quite a lot on verifier not complaining eagerly about some
> potentially invalid instructions if it's provable that some portion of
> the code won't ever be reached (think using .rodata variables for
> feature gating, poisoning intructions due to failed CO-RE relocation,
> which libbpf does actively, except it's using a call to non-existing
> helper). As such, check_cfg() is a wrong place to do such validity
> checks because some of the branches might never be run and validated
> in practice.
>

Don't really agree. Jump to the middle of ld_imm64 is just like jumping
out of bounds, both break the CFG integrity immediately. For those
apparently incorrect  jumps, rejecting early makes everything simple;
otherwise, we probably need some rewrite in the end.

Also, as you mentioned, libbpf relies on non-existing helpers, not jump
to the middle of ld_imm64. It seems better and easier to not leave this
hole.

> This seems like a pretty obscure case of fuzzer generated test with
> random jumps into the middle of ldimm64 instruction. I think the tool
> should be able to avoid this or handle verifier log just fine in such
> situations. On the other hand, valid code generated by compilers will
> never have such jumps.
>
> So perhaps we can improve existing "invalid BPF_LD_IMM insn" message,
> but let's not teach check_cfg() more checks than necessary?
>

Improving that `invalid BPF_LD_IMM` log does not solve the problem, the
issue here is an invalid jump. Also, there could be various causes that make
the verifier see an invalid BPF_LD_IMM in check_ld_imm().

> >
> > >>      if (e == BRANCH) {
> > >>              /* mark branch target for state pruning */
> > >>              mark_prune_point(env, w);
> > >>
> >