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

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Message-ID: <59E52B1C.5060807@iogearbox.net>
Date:   Mon, 16 Oct 2017 23:56:44 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        netdev@...r.kernel.org
CC:     oss-drivers@...ronome.com, alexei.starovoitov@...il.com,
        ecree@...arflare.com
Subject: Re: [PATCH net v2] bpf: disallow arithmetic operations on context
 pointer

On 10/16/2017 08:16 PM, 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>

Thanks for the fix!

Acked-by: Daniel Borkmann <daniel@...earbox.net>