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? Thanks, Zorro > > Thanks, > Nick >
