On 1/29/20 12:01 AM, Kees Cook wrote: > On Tue, Jan 28, 2020 at 08:58:31AM +0100, Christian Borntraeger wrote: >> >> >> On 28.01.20 00:19, Kees Cook wrote: >>> On Thu, Jan 23, 2020 at 09:14:20AM +0100, Jiri Slaby wrote: >>>> On 14. 11. 19, 22:27, Kees Cook wrote: >>>>> On Tue, Nov 12, 2019 at 01:21:54PM -0800, Kees Cook wrote: >>>>>> How is iucv the only network protocol that has run into this? Do others >>>>>> use a bounce buffer? >>>>> >>>>> Another solution would be to use a dedicated kmem cache (instead of the >>>>> shared kmalloc dma one)? >>>> >>>> Has there been any conclusion to this thread yet? For the time being, we >>>> disabled HARDENED_USERCOPY on s390... >>>> >>>> https://lore.kernel.org/kernel-hardening/9519edb7-456a-a2fa-659e-3e5a1ff89466@xxxxxxx/ >>> >>> I haven't heard anything new. What did people think of a separate kmem >>> cache? >>> >> >> Adding Julian and Ursula. A separate kmem cache for iucv might be indeed >> a solution for the user hardening issue. > > It should be very clean -- any existing kmallocs already have to be > "special" in the sense that they're marked with the DMA flag. So > converting these to a separate cache should be mostly mechanical. > Linux on System z can run within a guest hosted by the IBM mainframe operating system z/VM. z/VM offers a transport called Inter-User Communications Vehicle (short IUCV). It is limited to 4-byte-addresses when sending and receiving data. One base transport for AF_IUCV sockets in the Linux kernel is this Inter-User Communications Vehicle of z/VM. AF_IUCV sockets exist for s390 only. AF_IUCV sockets make use of the base socket layer, and work with sk_buffs for sending and receiving data of variable length. Storage for sk_buffs is allocated with __alloc_skb(), which invokes data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc); For IUCV transport the "data"-address should fit into 4 bytes. That's the reason why we work with GFP_DMA here. kmem_caches manage memory of fixed size. This does not fit well for sk_buff memory of variable length. Do you propose to add a kmem_cache solution for sk_buff memory here? >> On the other hand not marking the DMA caches still seems questionable. > > My understanding is that exposing DMA memory to userspace copies can > lead to unexpected results, especially for misbehaving hardware, so I'm > not convinced this is a generically bad hardening choice. > We have not yet been reported a memory problem here. Do you have more details, if this is really a problem for the s390 architecture? Kind regards, Ursula > -Kees > >> >> For reference >> https://bugzilla.suse.com/show_bug.cgi?id=1156053 >> the kernel hardening now triggers a warning. >> >
