On 13/11/2018 20:35, Andy Lutomirski wrote: > On Tue, Nov 13, 2018 at 10:26 AM Igor Stoppa <igor.stoppa@xxxxxxxxxx> wrote: [...] >> The high level API could be something like: >> >> wr_memcpy(void *src, void *dst, uint_t size) [...] > If you call a wr_memcpy() function with the signature you suggested, > then you can overwrite any memory of this type. Having a different > mm_struct under the hood makes no difference. As far as I'm > concerned, for *dynamically allocated* rare-writable memory, you might > as well allocate all the paging structures at the same time, so the > mm_struct will always contain the mappings. If there are serious bugs > in wr_memcpy() that cause it to write to the wrong place, we have > bigger problems. Beside bugs, I'm also thinking about possible vulnerability. It might be overthinking, though. I do not think it's possible to really protect against control flow attacks, unless there is some support from the HW and/or the compiler. What is left, are data-based attacks. In this case, it would be an attacker using one existing wr_ invocation with doctored parameters. However, there is always the objection that it would be possible to come up with a "writing kit" for plowing through the page tables and unprotect anything that might be of value. Ideally, that should be the only type of data-based attack left. In practice, it might just be an excess of paranoia from my side. > I can imagine that we'd want a *typed* wr_memcpy()-like API some day, > but that can wait for v2. And it still doesn't obviously need > multiple mm_structs. I left that out, for now, but yes, typing would play some role here. [...] > I think it's entirely reasonable for the API to internally break up > very large memcpys. The criteria for deciding if/how to break it down is not clear to me, though. The single page was easy, but might be (probably is) too much. -- igor
