linux-kernel - Re: [PATCH v2] usercopy: Avoid soft lockups in test_check_nonzero

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Message-ID: <20191017060938.p4tmr5ruv6frgse4@wittgenstein>
Date:   Thu, 17 Oct 2019 08:09:39 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     cyphar@...har.com, mingo@...hat.com, peterz@...radead.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, christian@...uner.io, keescook@...omium.org,
        linux@...musvillemoes.dk, viro@...iv.linux.org.uk,
        torvalds@...ux-foundation.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usercopy: Avoid soft lockups in
 test_check_nonzero_user()

On Thu, Oct 17, 2019 at 09:00:48AM +1100, Michael Ellerman wrote:
> Christian Brauner <christian.brauner@...ntu.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@...har.com>
> >> Signed-off-by: Michael Ellerman <mpe@...erman.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