Re: [PATCH bpf RESEND 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access for 32-bit architecture

[Yang Jihong]

Hello,

On 2022/11/5 7:37, Alexei Starovoitov wrote:
> On Fri, Nov 4, 2022 at 3:43 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> > On Thu, Nov 3, 2022 at 11:15 AM Alexei Starovoitov
> > <alexei.starovoitov@xxxxxxxxx> wrote:
> > > On Thu, Nov 3, 2022 at 4:23 AM Russell King (Oracle)
> > > <linux@xxxxxxxxxxxxxxx> wrote:
> > > > On Thu, Nov 03, 2022 at 05:21:16PM +0800, Yang Jihong wrote:
> > > > > The error code -EACCES is returned when bpf prog is tested in 32-bit environment,
> > > > > This is because bpf_object__relocate modifies the instruction to change memory
> > > > > size to 4 bytes, as shown in the following messages:
> > > > >
> > > > > libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
> > > > > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
> > > > > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4
> > > > >
> > > > > As a result, the bpf_skb_is_valid_access check fails. For 32-bit architecture,
> > > > > unnecessary checks need to be deleted.
> > > >
> > > > Isn't the purpose of this check to ensure that the entire pointer is
> > > > written, and BPF can't write half of it?
> > > >
> > > >
> > > > >        case offsetof(struct __sk_buff, sk):
> > > > > -             if (type == BPF_WRITE || size != sizeof(__u64))
> > > > > -                     return false;
> > > >
> > > > Wouldn't "(size != sizeof(struct bpf_sock *) && size != sizeof(__u64))"
> > > > be more appropriate here, so 32-bit can only write the 32-bit pointer
> > > > or the full 64-bit value, and 64-bit can only write the 64-bit pointer?
> > > > Or is there a reason not to? bpf folk?
> > >
> > > You're correct. The patch is completely wrong.
> > > The bug is elsewhere.
> >
> > So I looked at this a bit (and replied to the old version of this
> > patch). What happens in the kernel is that we expect 64-bit load but
> > rewrite it to 32-bit load on 32-bit architectures (because we just use
> > sizeof(struct sk_buff, sk) which is 4 bytes on 32-bit arch.
> >
> > The problem here is that libbpf adjusts such pointer accesses from
> > 8-byte read to 4-byte reads for preserve_access_index (because libbpf
> > sees that pointer is really 4 byte long), which is what we actually
> > want in the general case. Here the assumption was made before CO-RE
> > that __sk_buff is a stable (and fake) UAPI and the correct BPF program
> > will access sk as a 64-bit pointer because BPF-side pointers always
> > appear as 64-bit.
> >
> > But from a correctness standpoint I think it should be fine to enable
> > both 32- and 64-bit loads for such pointers in __sk_buff for 32-bit
> > host arch. This will work well with CO-RE and will be correctly
> > rewritten to 32-bit or 64-bit accesses, depending on host
> > architecture.
> >
> > We should still reject 32-bit load on 64-bit host arch, though.
>
> Replied in the other thread as well :)
> The CO_RE screws up access here.
> Since it's a load of a pointer the verifier has to see it as a 8-byte load.
> When CO-RE converts it to 4 byte the verifier won't recognize it
> as a pointer load anymore.
> We cannot easily convert 64-bit BPF ISA into 32-bit.
> It's a massive amount of work.
> .

From the above discussion, there are two different solutions:
1. Modify bpf_skb_is_valid_access to ensure that 32-bit can only load 
the 32-bit pointer or the full 64-bit value, and 64-bit can only load 
the 64-bit pointer
2. Modify libbpf, CO_RE skips adjust load's mem size and retains the 
8-byte load.
The two fixes will be added in the next version.
Please review the solution to be selected.

Thanks,
Yang