Re: [PATCH net] bpf: disallow arithmetic operations on context pointer

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Mon, 16 Oct 2017 17:47:45 +0100, Edward Cree wrote:
> On 16/10/17 17:30, Jakub Kicinski wrote:
> > On Mon, 16 Oct 2017 17:16:24 +0100, Edward Cree wrote:  
> >> On 16/10/17 16:45, Jakub Kicinski wrote:  
> >>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> >>> index 8b8d6ba39e23..8499759d0c7a 100644
> >>> --- a/kernel/bpf/verifier.c
> >>> +++ b/kernel/bpf/verifier.c
> >>> @@ -1116,7 +1116,12 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> >>>  		/* ctx accesses must be at a fixed offset, so that we can
> >>>  		 * determine what type of data were returned.
> >>>  		 */
> >>> -		if (!tnum_is_const(reg->var_off)) {
> >>> +		if (reg->off) {
> >>> +			verbose("derefence of modified ctx ptr R%d off=%d+%d, ctx+const is allowed, ctx+const+const is not\n",    
> >> This is slightly unclear, it's not that two adds is bad (e.g. r1 += 8;
> >>  r0 = *(u32 *)r1 is bad too), it's that the offset must be in the load,
> >>  not the register; your message might be accurate for some compilers but
> >>  not in full generality (especially for assemblers without compiling).  
> > I'm happy to hear better suggestions :)  I've spent quite a bit of time
> > scratching my head thinking how to phrase this best.  The first
> > part of the message is general enough IMHO, the second is targeted
> > mostly at C developers.  
> Hmm, what really bugs me is that if e.g. the compiler turned
>    *(ctx + 4 + 4)
>  or
>    ctx[4 + 4]
>  or even
>    ctx->arraymemb[4]
>  into this kind of arithmetic on ctx, arguably that would be a bug in the
>  compiler — if it's doing proper constexpr folding on its IR (or something
>  along those lines) it should be able to turn them all into good LDX.  The
>  same even goes for if (ctx + 4) got stored in a local, because there's no
>  reason that has to map to a register.
> So it's not even that "your C source breaks the rules", it's that "your C
>  compiler did something silly that we don't handle".
> Maybe the message should be "compiler maybe mishandled ctx+const+const"?

Hm.  We have no proof of compilers doing such things.  It's probably
more likely that this will be hit if someone does:

struct xxx *X = &skb->cb;
X->field;

Or just tries to add a constant offset to ctx by hand...  "Complier may
have done something silly" is probably implied in all verifier
messages :)

FWIW the pre-tnum error would be:

R%d invalid mem access 'inv'

So we are making this a lot more clear anyway.