On 1/27/21 8:13 PM, Zorro Lang wrote: > On Thu, Jan 28, 2021 at 10:18:07AM +1000, Nicholas Piggin wrote: >> Excerpts from Jens Axboe's message of January 28, 2021 5:29 am: >>> On 1/27/21 9:38 AM, Christophe Leroy wrote: >>>> >>>> >>>> Le 27/01/2021 à 15:56, Zorro Lang a écrit : >>>>> On powerpc, io_uring test hit below KUAP fault on __do_page_fault. >>>>> The fail source line is: >>>>> >>>>> if (unlikely(!is_user && bad_kernel_fault(regs, error_code, address, >>>>> is_write))) >>>>> return SIGSEGV; >>>>> >>>>> The is_user() is based on user_mod(regs) only. This's not suit for >>>>> io_uring, where the helper thread can assume the user app identity >>>>> and could perform this fault just fine. So turn to use mm to decide >>>>> if this is valid or not. >>>> >>>> I don't understand why testing is_user would be an issue. KUAP purpose >>>> it to block any unallowed access from kernel to user memory >>>> (Equivalent to SMAP on x86). So it really must be based on MSR_PR bit, >>>> that is what is_user provides. >>>> >>>> If the kernel access is legitimate, kernel should have opened >>>> userspace access then you shouldn't get this "Bug: Read fault blocked >>>> by KUAP!". >>>> >>>> As far as I understand, the fault occurs in >>>> iov_iter_fault_in_readable() which calls fault_in_pages_readable() And >>>> fault_in_pages_readable() uses __get_user() so it is a legitimate >>>> access and you really should get a KUAP fault. >>>> >>>> So the problem is somewhere else, I think you proposed patch just >>>> hides the problem, it doesn't fix it. >>> >>> If we do kthread_use_mm(), can we agree that the user access is valid? >> >> Yeah the io uring code is fine, provided it uses the uaccess primitives >> like any other kernel code. It's looking more like a an arch/powerpc bug. >> >>> We should be able to copy to/from user space, and including faults, if >>> that's been done and the new mm assigned. Because it really should be. >>> If SMAP was a problem on x86, we would have seen it long ago. >>> >>> I'm assuming this may be breakage related to the recent uaccess changes >>> related to set_fs and friends? Or maybe recent changes on the powerpc >>> side? >>> >>> Zorro, did 5.10 work? >> >> Would be interesting to know. > > Sure Nick and Jens, which 5.10 rc? version do you want to know ? Or any git > commit(be the HEAD) in 5.10 phase?
I forget which versions had what series of this, but 5.10 final - and if that fails, then 5.9 final. IIRC, 5.9 was pre any of these changes, and 5.10 definitely has them. -- Jens Axboe