Re: [PATCH v2] usercopy: Avoid soft lockups in test_check_nonzero_user()

Christian Brauner
2019-10-17T06:09:39Z

On Thu, Oct 17, 2019 at 09:00:48AM +1100, Michael Ellerman wrote:
> Christian Brauner <christian.brauner@ubuntu.com> writes:
> > On Wed, Oct 16, 2019 at 11:27:32PM +1100, Michael Ellerman wrote:
> >> On a machine with a 64K PAGE_SIZE, the nested for loops in
> >> test_check_nonzero_user() can lead to soft lockups, eg:
> >>
> >> watchdog: BUG: soft lockup - CPU#4 stuck for 22s! [modprobe:611]
> >> Modules linked in: test_user_copy(+) vmx_crypto gf128mul crc32c_vpmsum virtio_balloon ip_tables x_tables autofs4
> >> CPU: 4 PID: 611 Comm: modprobe Tainted: G L 5.4.0-rc1-gcc-8.2.0-00001-gf5a1a536fa14-dirty #1151
> >> ...
> >> NIP __might_sleep+0x20/0xc0
> >> LR __might_fault+0x40/0x60
> >> Call Trace:
> >> check_zeroed_user+0x12c/0x200
> >> test_user_copy_init+0x67c/0x1210 [test_user_copy]
> >> do_one_initcall+0x60/0x340
> >> do_init_module+0x7c/0x2f0
> >> load_module+0x2d94/0x30e0
> >> __do_sys_finit_module+0xc8/0x150
> >> system_call+0x5c/0x68
> >>
> >> Even with a 4K PAGE_SIZE the test takes multiple seconds. Instead
> >> tweak it to only scan a 1024 byte region, but make it cross the
> >> page boundary.
> >>
> >> Fixes: f5a1a536fa14 ("lib: introduce copy_struct_from_user() helper")
> >> Suggested-by: Aleksa Sarai <cyphar@cyphar.com>
> >> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> >
> > With Aleksa's Reviewed-by I've picked this up:
> > https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=copy_struct_from_user
>
> Thanks. Are you planning to send that to Linus for v5.4 or v5.5 ?

This looks like a pretty straight bugfix to me since it's clearly
causing an issue for you on power so v5.4-rc4 is what I'd aim for. I
just want it to be in linux-next until tomorrow.

Christian