Re: [PATCH 06/10] ipc: Don't allocate a copy larger than max

[Manfred Spraul <manfred@colorfullife.com>]

Hi Peter,

You wrote:
> When MSG_COPY is set, a duplicate message must be allocated for
> the copy before locking the queue. However, the copy could
> not be larger than was sent which is limited to msg_ctlmax.
>
> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
> ---
>   ipc/msg.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 950572f..31cd1bf 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -820,15 +820,17 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>   	struct msg_msg *copy = NULL;
>   	unsigned long copy_number = 0;
>   
> +	ns = current->nsproxy->ipc_ns;
> +
>   	if (msqid < 0 || (long) bufsz < 0)
>   		return -EINVAL;
>   	if (msgflg & MSG_COPY) {
> -		copy = prepare_copy(buf, bufsz, msgflg, &msgtyp, &copy_number);
> +		copy = prepare_copy(buf, min_t(size_t, bufsz, ns->msg_ctlmax),
> +				    msgflg, &msgtyp, &copy_number);

What about:
- increase msg_ctlmax
- send message
- reduce msg_ctlmax

The side effects of the patch are odd:
- without MSG_COPY, a message can be read regardsless of the size.
   The user could check for E2BIG and increase the buffer size until
   msgrcv succeeds.
- with MSG_COPY, something else would happen.
   As far as I can see, it would oops: msg_ctlmax bytes are allocated,
   then the E2BIG test is against bufsz, and copy_msg() doesn't check
   the size of the target buffer.

I.e.: I would propose to revert the patch.

--
	Manfred