Re: [PATCH] net: Whitelist the skbuff_head_cache "cb" field

[Kees Cook]

On Fri, Feb 9, 2018 at 7:16 AM, David Miller <davem@xxxxxxxxxxxxx> wrote:
> From: Kees Cook <keescook@xxxxxxxxxxxx>
> Date: Wed, 7 Feb 2018 17:44:38 -0800
>
>> Most callers of put_cmsg() use a "sizeof(foo)" for the length argument.
>> Within put_cmsg(), a copy_to_user() call is made with a dynamic size, as a
>> result of the cmsg header calculations. This means that hardened usercopy
>> will examine the copy, even though it was technically a fixed size and
>> should be implicitly whitelisted. All the put_cmsg() calls being built
>> from values in skbuff_head_cache are coming out of the protocol-defined
>> "cb" field, so whitelist this field entirely instead of creating per-use
>> bounce buffers, for which there are concerns about performance.
>>
>> Original report was:
>  ...
>> Reported-by: syzbot+e2d6cfb305e9f3911dea@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> ---
>> I tried the inlining, it was awful. Splitting put_cmsg() was awful. So,
>> instead, whitelist the "cb" field as the least bad option if bounce
>> buffers are unacceptable. Dave, do you want to take this through net, or
>> should I take it through the usercopy tree?
>
> Thanks Kees, I'll take this through my 'net' tree.

Cool, thanks. And just to be clear, if it's not already obvious, this
patch needs kmem_cache_create_usercopy() which just landed in Linus's
tree last week, in case you've not merged yet.

-Kees

-- 
Kees Cook
Pixel Security