netdev - Re: [PATCH net v2] bpf: disallow arithmetic operations on context pointer

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Date:   Mon, 16 Oct 2017 15:02:32 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     netdev@...r.kernel.org, oss-drivers@...ronome.com,
        daniel@...earbox.net, ecree@...arflare.com
Subject: Re: [PATCH net v2] bpf: disallow arithmetic operations on context
 pointer

On Mon, Oct 16, 2017 at 11:16:55AM -0700, Jakub Kicinski wrote:
> Commit f1174f77b50c ("bpf/verifier: rework value tracking")
> removed the crafty selection of which pointer types are
> allowed to be modified.  This is OK for most pointer types
> since adjust_ptr_min_max_vals() will catch operations on
> immutable pointers.  One exception is PTR_TO_CTX which is
> now allowed to be offseted freely.
> 
> The intent of aforementioned commit was to allow context
> access via modified registers.  The offset passed to
> ->is_valid_access() verifier callback has been adjusted
> by the value of the variable offset.
> 
> What is missing, however, is taking the variable offset
> into account when the context register is used.  Or in terms
> of the code adding the offset to the value passed to the
> ->convert_ctx_access() callback.  This leads to the following
> eBPF user code:
> 
>      r1 += 68
>      r0 = *(u32 *)(r1 + 8)
>      exit
> 
> being translated to this in kernel space:
> 
>    0: (07) r1 += 68
>    1: (61) r0 = *(u32 *)(r1 +180)
>    2: (95) exit
> 
> Offset 8 is corresponding to 180 in the kernel, but offset
> 76 is valid too.  Verifier will "accept" access to offset
> 68+8=76 but then "convert" access to offset 8 as 180.
> Effective access to offset 248 is beyond the kernel context.
> (This is a __sk_buff example on a debug-heavy kernel -
> packet mark is 8 -> 180, 76 would be data.)
> 
> Dereferencing the modified context pointer is not as easy
> as dereferencing other types, because we have to translate
> the access to reading a field in kernel structures which is
> usually at a different offset and often of a different size.
> To allow modifying the pointer we would have to make sure
> that given eBPF instruction will always access the same
> field or the fields accessed are "compatible" in terms of
> offset and size...
> 
> Disallow dereferencing modified context pointers and add
> to selftests the test case described here.
> 
> Fixes: f1174f77b50c ("bpf/verifier: rework value tracking")
> Signed-off-by: Jakub Kicinski <jakub.kicinski@...ronome.com>

Acked-by: Alexei Starovoitov <ast@...nel.org>

that's a required fix for net and for net-next I like Ed's idea
to teach convert_ctx_access to recognize such complex pattern
and do fancier rewrite.
iirc we were hitting similar ctx rewrite issue from time to time
and managed to change .c and/or llvm codegen in the past
when compiler was producing code that technically was valid, but
didn't satisfy 'ctx+const' requirement.
Back then verifier wasn't that smart. Now we can do better.