> On 4 Mar 2018, at 23:56, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > Thinking about this more ... > > - When you call munmap, if you pass in the same (addr, length) that were > used for mmap, then it should unmap the guard pages as well (that > wasn't part of the patch, so it would have to be added) > - If 'addr' is higher than the mapped address, and length at least > reaches the end of the mapping, then I would expect the guard pages to > "move down" and be after the end of the newly-shortened mapping. > - If 'addr' is higher than the mapped address, and the length doesn't > reach the end of the old mapping, we split the old mapping into two. > I would expect the guard pages to apply to both mappings, insofar as > they'll fit. For an example, suppose we have a five-page mapping with > two guard pages (MMMMMGG), and then we unmap the fourth page. Now we > have a three-page mapping with one guard page followed immediately > by a one-page mapping with two guard pages (MMMGMGG). I’m analysing that approach and see much more problems: - each time you call mmap like this, you still increase count of vmas as my patch did - now feature vma_merge shouldn’t work at all, until MAP_FIXED is set or PROT_GUARD(0) - the entropy you provide is like 16 bit, that is really not so hard to brute - in your patch you don’t use vm_guard at address searching, I see many roots of bugs here - if you unmap/remap one page inside region, field vma_guard will show head or tail pages for vma, not both; kernel don’t know how to handle it - user mode now choose entropy with PROT_GUARD macro, where did he gets it? User mode shouldn’t be responsible for entropy at all I can’t understand what direction this conversation is going to. I was talking about weak implementation in Linux kernel but got many comments about ASLR should be implemented in user mode what is really weird to me. I think it is possible to add GUARD pages into my implementations, but initially problem was about entropy of address choosing. I would like to resolve it step by step. Thanks, Ilya -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href
