Re: [Bug Report] bpf: reg invariant voilation after JSLE

[Hao Sun]

On Wed, Nov 29, 2023 at 6:43 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Nov 21, 2023 at 7:08 AM Hao Sun <sunhao.th@xxxxxxxxx> wrote:
> >
> > Hi,
> >
> > The following program (reduced) breaks reg invariant:
> >
> > C Repro: https://pastebin.com/raw/SRQJYx91
> >
> > -------- Verifier Log --------
> > func#0 @0
> > 0: R1=ctx() R10=fp0
> > 0: (b7) r0 = -2                       ; R0_w=-2
> > 1: (37) r0 /= 1                       ; R0_w=scalar()
> > 2: (bf) r8 = r0                       ; R0_w=scalar(id=1) R8_w=scalar(id=1)
> > 3: (56) if w8 != 0xfffffffe goto pc+4         ;
> > R8_w=scalar(id=1,smin=0x80000000fffffffe,smax=0x7ffffffffffffffe,umin=umin32=0xfffffffe,umax=0xfffffffffffffffe,smin32=-2,smax32=-2,umax32=0xfffffffe,var_off=(0xfffffffe;
> > 0xffffffff00000000))
>
> this part looks suspicious, I'll take a look a bit later
>
> > 4: (65) if r8 s> 0xd goto pc+3        ;
> > R8_w=scalar(id=1,smin=0x80000000fffffffe,smax=13,umin=umin32=0xfffffffe,umax=0xfffffffffffffffe,smin32=-2,smax32=-2,umax32=0xfffffffe,var_off=(0xfffffffe;
> > 0xffffffff00000000))
> > 5: (b7) r4 = 2                        ; R4_w=2
> > 6: (dd) if r8 s<= r4 goto pc+1
> > REG INVARIANTS VIOLATION (false_reg1): range bounds violation
> > u64=[0xfffffffe, 0xd] s64=[0xfffffffe, 0xd] u32=[0xfffffffe, 0xd]
> > s32=[0x3, 0xfffffffe] var_off=(0xfffffffe, 0x0)
> > 6: R4_w=2 R8_w=0xfffffffe
> > 7: (cc) w8 s>>= w0                    ; R0=0xfffffffe R8=scalar()
> > 8: (77) r0 >>= 32                     ; R0_w=0
> > 9: (57) r0 &= 1                       ; R0_w=0
> > 10: (95) exit
> >
> > from 6 to 8: safe
> >
> > from 4 to 8: safe
> >
> > from 3 to 8: safe
> > processed 14 insns (limit 1000000) max_states_per_insn 0 total_states
> > 1 peak_states 1 mark_read 1
> >
> >
> > Besides, the verifier enforces the return value of some prog types to
> > be zero, the bug may lead to programs with arbitrary values loaded.
>
> Generally speaking, if the verifier reports "REG INVARIANTS VIOLATION"
> warning above, it doesn't necessarily mean that verifier has some bug.
> We do know that in some conditions verifier doesn't detect conditions
> that *will not* be taken, and in such cases we might get reg
> invariants violation. But in such case verifier will revert to
> conservative unknown scalar state, which is correct, even if
> potentially unnecessarily pessimistic.
>

Yes, I'm aware of that, which is why I only selected two suspicious cases
to report. Also, this is true after the check (5f99f312bd3be: bpf: add
register bounds sanity checks and sanitization), but these cases may
cause some issues in the previous releases. Your recent improvement in
return value check also helps.

I will see what I can do, maybe add more checks by using both tnum and
ranges information in is_scalar_branch_taken().

Thanks!