Messages in this thread | | | From | Kees Cook <> | Date | Tue, 6 Feb 2018 04:31:50 +1100 | Subject | Re: [PATCH v2] socket: Provide put_cmsg_whitelist() for constant size copies |
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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).
> And now we're going to copy it twice, once into the on-stack copy, > and then once again into the CMSG blob. > > Please find a way to make hardened user copy happy without adding > new overhead.
Another idea would be breaking put_cmsg() up into a macro with helper functions, rearrange the arguments to avoid the math, and leaving the copy_to_user() inline to see the const-ness, but that seemed way uglier to me.
I'll think about it some more, but I think having put_cmsg_whitelist() called only in a few places is reasonable here.
-Kees
-- Kees Cook Pixel Security
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