[kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

[Al Viro]

On Wed, May 10, 2017 at 04:12:54AM +0100, Al Viro wrote:

> Broken commit: "net: don't play with address limits in kernel_recvmsg".
> It would be OK if it was only about data.  Unfortunately, that's not
> true in one case: svc_udp_recvfrom() wants ->msg_control.
> 
> Another delicate place: you can't assume that write() always advances
> file position by its (positive) return value.  btrfs stuff is sensitive
> to that.
> 
> ashmem probably _is_ OK with demanding ->read_iter(), but I'm not sure
> about blind asma->file->f_pos += ret.  That's begging for races.  Actually,
> scratch that - it *is* racy.

kvec_length(): please, don't.  I would rather have the last remaining
iov_length() gone...   What do you need it for, anyway?  You have only
two users and both have the count passed to them (as *count and *cnt resp.)