On Thu, 2018-12-06 at 11:19 -0800, Andy Lutomirski 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? Hi, Sorry, I've been having email troubles today. I found some cases where vmap with PAGE_KERNEL_RO happens, which would not set NP/RO in the directmap, so it would be sort of inconsistent whether the directmap of vmalloc range allocations were readable or not. I couldn't see any places where it would cause problems today though. I was ready to assume that all TLBs don't cache NP, because I don't know how usages where a page fault is used to load something could work without lots of flushes. If that's the case, then all archs with directmap permissions could share a single vmalloc special permission flush implementation that works like Andy described originally. It could be controlled with an ARCH_HAS_DIRECT_MAP_PERMS. We would just need something like set_pages_np and set_pages_rw on any archs with directmap permissions. So seems simpler to me (and what I have been doing) unless I'm missing the problem. If you all think so I can indeed take a shot at it, I just don't see what the problem was with the original solution, that seems less likely to break anything. Thanks, Rick
