From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Laight Subject: RE: get rid of the address_space override in setsockopt v2 Date: Mon, 27 Jul 2020 14:09:09 +0000 Message-ID: <5d958e937db54849b4ef9046e7e12277@AcuMS.aculab.com> References: <20200723060908.50081-1-hch@lst.de> <20200724.154342.1433271593505001306.davem@davemloft.net> <8ae792c27f144d4bb5cbea0c1cce4eed@AcuMS.aculab.com> <20200727134814.GD794331@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Transfer-Encoding: 8BIT Return-path: In-Reply-To: <20200727134814.GD794331@ZenIV.linux.org.uk> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii" To: 'Al Viro' Cc: 'David Miller' , "hch@lst.de" , "kuba@kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "kuznet@ms2.inr.ac.ru" , "yoshfuji@linux-ipv6.org" , "edumazet@google.com" , "linux-crypto@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "netdev@vger.kernel.org" , "bpf@vger.kernel.org" , "netfilter-devel@vger.kernel.org" , "coreteam@netfilter.org" , "linux-sctp@vger.kernel.org" , "linux-hams@vger.kernel.org" From: Al Viro > Sent: 27 July 2020 14:48 > > On Mon, Jul 27, 2020 at 09:51:45AM +0000, David Laight wrote: > > > I'm sure there is code that processes options in chunks. > > This probably means it is possible to put a chunk boundary > > at the end of userspace and continue processing the very start > > of kernel memory. > > > > At best this faults on the kernel copy code and crashes the system. > > Really? Care to provide some details, or is it another of your "I can't > be possibly arsed to check what I'm saying, but it stands for reason > that..." specials? I did more 'homework' than sometimes :-) Slightly difficult without a searchable net-next tree. However, as has been pointed out is a different thread this code is used to update IPv6 flow labels: > > - if (copy_from_user(fl->opt+1, optval+CMSG_ALIGN(sizeof(*freq)), olen)) > > + sockptr_advance(optval, CMSG_ALIGN(sizeof(*freq))); > > + if (copy_from_sockptr(fl->opt + 1, optval, olen)) > > goto done; and doesn't work because the advances are no longer cumulative. Now access_ok() has to take the base address and length to stop 'running into' kernel space, but the code above can advance from a valid user pointer (which won't fault) to a kernel address. If there were always an unmapped 'guard' page in the user address space the access_ok() check prior to copy_to/from_user() wouldn't need the length. So I surmise that no such guard page exists and so the above can advance from user addresses into kernel ones. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)