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

From: Kees Cook <redacted>
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@syzkaller.appspotmail.com
Fixes: 6d07d1cd300f ("usercopy: Restrict non-usercopy caches to size 0")
Signed-off-by: Kees Cook <redacted>
---
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.