Re: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies

[David Miller <davem@davemloft.net>]

From: Kees Cook <keescook@chromium.org>
Date: Tue, 6 Feb 2018 04:31:50 +1100

> On Tue, Feb 6, 2018 at 2:03 AM, David Miller <davem@davemloft.net> wrote:
>> From: Kees Cook <keescook@chromium.org>
>> Date: Fri, 2 Feb 2018 02:27:49 -0800
>>
>>> @@ -343,6 +343,14 @@ struct ucred {
>>>
>>>  extern int move_addr_to_kernel(void __user *uaddr, int ulen, struct sockaddr_storage *kaddr);
>>>  extern int put_cmsg(struct msghdr*, int level, int type, int len, void *data);
>>> +/*
>>> + * Provide a bounce buffer for copying cmsg data to userspace when the
>>> + * target memory isn't already whitelisted for hardened usercopy.
>>> + */
>>> +#define put_cmsg_whitelist(_msg, _level, _type, _ptr) ({             \
>>> +             typeof(*(_ptr)) _val = *(_ptr);                         \
>>> +             put_cmsg(_msg, _level, _type, sizeof(_val), &_val);     \
>>> +     })
>>
>> I understand what you are trying to achieve, but it's at a real cost
>> here.  Some of these objects are structures, for example the struct
>> sock_extended_err is 16 bytes.
> 
> It didn't look like put_cmsg() was on a fast path, so it seemed like a
> bounce buffer was the best solution here (and it's not without
> precedent).

For some things like timestamps it can be important.

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