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

[David Miller]

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.