> On Dec 6, 2018, at 11:19 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > On Thu, Dec 6, 2018 at 11:01 AM Tycho Andersen <tycho@xxxxxxxx> wrote: >> On Thu, Dec 06, 2018 at 10:53:50AM -0800, Andy Lutomirski wrote: >>>> If we are going to unmap the linear alias, why not do it at vmalloc() >>>> time rather than vfree() time? >>> >>> That’s not totally nuts. Do we ever have code that expects __va() to >>> work on module data? Perhaps crypto code trying to encrypt static >>> data because our APIs don’t understand virtual addresses. I guess if >>> highmem is ever used for modules, then we should be fine. >>> >>> RO instead of not present might be safer. But I do like the idea of >>> renaming Rick's flag to something like VM_XPFO or VM_NO_DIRECT_MAP and >>> making it do all of this. >> >> Yeah, doing it for everything automatically seemed like it was/is >> going to be a lot of work to debug all the corner cases where things >> expect memory to be mapped but don't explicitly say it. And in >> particular, the XPFO series only does it for user memory, whereas an >> additional flag like this would work for extra paranoid allocations >> of kernel memory too. > > I just read the code, and I looks like vmalloc() is already using > highmem (__GFP_HIGH) if available, so, on big x86_32 systems, for > example, we already don't have modules in the direct map. > > So I say we go for it. This should be quite simple to implement -- > the pageattr code already has almost all the needed logic on x86. The > only arch support we should need is a pair of functions to remove a > vmalloc address range from the address map (if it was present in the > first place) and a function to put it back. On x86, this should only > be a few lines of code. > > What do you all think? This should solve most of the problems we have. > > If we really wanted to optimize this, we'd make it so that > module_alloc() allocates memory the normal way, then, later on, we > call some function that, all at once, removes the memory from the > direct map and applies the right permissions to the vmalloc alias (or > just makes the vmalloc alias not-present so we can add permissions > later without flushing), and flushes the TLB. And we arrange for > vunmap to zap the vmalloc range, then put the memory back into the > direct map, then free the pages back to the page allocator, with the > flush in the appropriate place. > > I don't see why the page allocator needs to know about any of this. > It's already okay with the permissions being changed out from under it > on x86, and it seems fine. Rick, do you want to give some variant of > this a try? Setting it as read-only may work (and already happens for the read-only module data). I am not sure about setting it as non-present. At some point, a discussion about a threat-model, as Rick indicated, would be required. I presume ROP attacks can easily call set_all_modules_text_rw() and override all the protections.
