Subject: Re: more on FP operations

From: Daniel Borkmann <[email-masked]>
Date: Thu, 20 Apr 2017 23:52:44 +0200

On 04/20/2017 08:06 PM, David Miller wrote:
>
> I'm running test_verifier for testing, and I notice in my JIT that a
> 32-bit move from the frame pointer (BPF_REG_10) ends up in the JIT.
>
> It is from this test:
>
> 		"unpriv: partial copy of pointer",
> 		.insns = {
> 			BPF_MOV32_REG(BPF_REG_1, BPF_REG_10),
> 			BPF_MOV64_IMM(BPF_REG_0, 0),
> 			BPF_EXIT_INSN(),
> 		},
> 		.errstr_unpriv = "R10 partial copy",
> 		.result_unpriv = REJECT,
> 		.result = ACCEPT,
>
> It seems to suggest that privileged code is allowed to do this, but I
> can't think of a legitimate usage.

One thing I could think of right now would be for use in 32 bit
archs, but that would still need to be taught to the verifier
first. Other patterns f.e. like ...

         {
                 "unpriv: adding of fp",
                 .insns = {
                         BPF_MOV64_IMM(BPF_REG_1, 0),
                         BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_10),
                         BPF_MOV64_IMM(BPF_REG_0, 0),
                         BPF_EXIT_INSN(),
                 },
                 .errstr_unpriv = "pointer arithmetic prohibited",
                 .result_unpriv = REJECT,
                 .result = ACCEPT,
         },

... are currently also possible, but in the above and the partial
copy r1 is always considered as UNKNOWN_VALUE from that point onward
and there's not really much we could do with it anymore, except
perhaps passing to bpf_probe_read() for inspection in tracing for
some reason. Since there are also various other pointers, it is
really only the FP that needs to be special cased for sparc JIT,
right?

> I really want to be able to JIT anything the verifier accepts, but I
> have a hard time justifying adding 32-bit FP register move support,
> adjusting for the stack bias, etc.
>
> Thanks.