Re: [PATCH bpf] bpf: Use pointer type whitelist for XADD

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Apr 15, 2020 at 10:47:43PM +0200, Jann Horn wrote:
> At the moment, check_xadd() uses a blacklist to decide whether a given
> pointer type should be usable with the XADD instruction. Out of all the
> pointer types that check_mem_access() accepts, only four are currently let
> through by check_xadd():
> 
> PTR_TO_MAP_VALUE
> PTR_TO_CTX           rejected
> PTR_TO_STACK
> PTR_TO_PACKET        rejected
> PTR_TO_PACKET_META   rejected
> PTR_TO_FLOW_KEYS     rejected
> PTR_TO_SOCKET        rejected
> PTR_TO_SOCK_COMMON   rejected
> PTR_TO_TCP_SOCK      rejected
> PTR_TO_XDP_SOCK      rejected
> PTR_TO_TP_BUFFER
> PTR_TO_BTF_ID
> 
> Looking at the currently permitted ones:
> 
>  - PTR_TO_MAP_VALUE: This makes sense and is the primary usecase for XADD.
>  - PTR_TO_STACK: This doesn't make much sense, there is no concurrency on
>    the BPF stack. It also causes confusion further down, because the first
>    check_mem_access() won't check whether the stack slot being read from is
>    STACK_SPILL and the second check_mem_access() assumes in
>    check_stack_write() that the value being written is a normal scalar.
>    This means that unprivileged users can leak kernel pointers.
>  - PTR_TO_TP_BUFFER: This is a local output buffer without concurrency.
>  - PTR_TO_BTF_ID: This is read-only, XADD can't work. When the verifier
>    tries to verify XADD on such memory, the first check_ptr_to_btf_access()
>    invocation gets confused by value_regno not being a valid array index
>    and writes to out-of-bounds memory.

> Limit XADD to PTR_TO_MAP_VALUE, since everything else at least doesn't make
> sense, and is sometimes broken on top of that.
> 
> Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
> I'm just sending this on the public list, since the worst-case impact for
> non-root users is leaking kernel pointers to userspace. In a context where
> you can reach BPF (no sandboxing), I don't think that kernel ASLR is very
> effective at the moment anyway.
> 
> This breaks ten unit tests that assume that XADD is possible on the stack,
> and I'm not sure how all of them should be fixed up; I'd appreciate it if
> someone else could figure out how to fix them. I think some of them might
> be using XADD to cast pointers to numbers, or something like that? But I'm
> not sure.
> 
> Or is XADD on the stack actually something you want to support for some
> reason, meaning that that part would have to be fixed differently?

yeah. 'doesnt make sense' is relative.
I prefer to fix the issues instead of disabling them.
xadd to PTR_TO_STACK, PTR_TO_TP_BUFFER, PTR_TO_BTF_ID should all work
because they are direct pointers to objects.
Unlike pointer to ctx and flow_key that will be rewritten and are not
direct pointers.

Short term I think it's fine to disable PTR_TO_TP_BUFFER because
prog breakage is unlikely (if it's actually broken which I'm not sure yet).
But PTR_TO_BTF_ID and PTR_TO_STACK should be fixed.
The former could be used in bpf-tcp-cc progs. I don't think it is now,
but it's certainly conceivable.
PTR_TO_STACK should continue to work because tests are using it.
'but stack has no concurrency' is not an excuse to break tests.

Also I don't understand why you're saying that PTR_TO_STACK xadd is leaking.
The first check_mem_access() will check for STACK_SPILL afaics.