On Wed, Jan 27, 2021 at 08:06:37PM -0700, Jens Axboe wrote: > 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.
I justed built linux v5.10 with same .config file, and gave it same test. v5.10 (HEAD=2c85ebc57b Linux 5.10) can't reproduce this bug: # ./check generic/013 generic/051 FSTYP -- xfs (non-debug) PLATFORM -- Linux/ppc64le ibm-p9z-xxx-xxxx 5.10.0 #3 SMP Thu Jan 28 04:12:14 EST 2021 MKFS_OPTIONS -- -f -m crc=1,finobt=1,reflink=1,rmapbt=1,bigtime=1,inobtcount=1 /dev/sda3 MOUNT_OPTIONS -- -o context=system_u:object_r:root_t:s0 /dev/sda3 /mnt/xfstests/scratch generic/013 138s ... 77s generic/051 103s ... 143s Ran: generic/013 generic/051 Passed all 2 tests > > -- > Jens Axboe >
