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

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Message-ID: <20191016125025.ktxe55l4doirbxbe@wittgenstein>
Date:   Wed, 16 Oct 2019 14:50:26 +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 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>

Thanks!
Acked-by: Christian Brauner <christian.brauner@...ntu.com>