On Thu, Jan 18, 2018 at 5:18 AM, Will Deacon <will.deacon@xxxxxxx> wrote: > Hi Dan, Linus, > > On Thu, Jan 11, 2018 at 05:41:08PM -0800, Dan Williams wrote: >> On Thu, Jan 11, 2018 at 5:19 PM, Linus Torvalds >> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >> > On Thu, Jan 11, 2018 at 4:46 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: >> >> >> >> This series incorporates Mark Rutland's latest ARM changes and adds >> >> the x86 specific implementation of 'ifence_array_ptr'. That ifence >> >> based approach is provided as an opt-in fallback, but the default >> >> mitigation, '__array_ptr', uses a 'mask' approach that removes >> >> conditional branches instructions, and otherwise aims to redirect >> >> speculation to use a NULL pointer rather than a user controlled value. >> > >> > Do you have any performance numbers and perhaps example code >> > generation? Is this noticeable? Are there any microbenchmarks showing >> > the difference between lfence use and the masking model? >> >> I don't have performance numbers, but here's a sample code generation >> from __fcheck_files, where the 'and; lea; and' sequence is portion of >> array_ptr() after the mask generation with 'sbb'. >> >> fdp = array_ptr(fdt->fd, fd, fdt->max_fds); >> 8e7: 8b 02 mov (%rdx),%eax >> 8e9: 48 39 c7 cmp %rax,%rdi >> 8ec: 48 19 c9 sbb %rcx,%rcx >> 8ef: 48 8b 42 08 mov 0x8(%rdx),%rax >> 8f3: 48 89 fe mov %rdi,%rsi >> 8f6: 48 21 ce and %rcx,%rsi >> 8f9: 48 8d 04 f0 lea (%rax,%rsi,8),%rax >> 8fd: 48 21 c8 and %rcx,%rax >> >> >> > Having both seems good for testing, but wouldn't we want to pick one in the end? >> >> I was thinking we'd keep it as a 'just in case' sort of thing, at >> least until the 'probably safe' assumption of the 'mask' approach has >> more time to settle out. > > From the arm64 side, the only concern I have (and this actually applies to > our CSDB sequence as well) is the calculation of the array size by the > caller. As Linus mentioned at the end of [1], if the determination of the > size argument is based on a conditional branch, then masking doesn't help > because you bound within the wrong range under speculation. > > We ran into this when trying to use masking to protect our uaccess routines > where the conditional bound is either KERNEL_DS or USER_DS. It's possible > that a prior conditional set_fs(KERNEL_DS) could defeat the masking and so > we'd need to throw some heavy barriers in set_fs to make it robust. At least in the conditional mask case near set_fs() usage the approach we are taking is to use a barrier. I.e. the following guidance from Linus: "Basically, the rule is trivial: find all 'stac' users, and use address masking if those users already integrate the limit check, and lfence they don't." ...which translates to narrow the pointer for get_user() and use a barrier for __get_user().
