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

[Aleksa Sarai <cyphar at cyphar dot com>]

On 2019-10-11, Michael Ellerman <mpe@ellerman.id.au> 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>
> ---
>  lib/test_user_copy.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> How does this look? It runs in < 1s on my machine here.
> 
> cheers
> 
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 950ee88cd6ac..9fb6bc609d4c 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -47,9 +47,26 @@ static bool is_zeroed(void *from, size_t size)
>  static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
>  {
>  	int ret = 0;
> -	size_t start, end, i;
> -	size_t zero_start = size / 4;
> -	size_t zero_end = size - zero_start;
> +	size_t start, end, i, zero_start, zero_end;
> +
> +	if (test(size < 1024, "buffer too small"))
> +		return -EINVAL;
> +
> +	/*
> +	 * We want to cross a page boundary to exercise the code more
> +	 * effectively. We assume the buffer we're passed has a page boundary at
> +	 * size / 2. We also don't want to make the size we scan too large,
> +	 * otherwise the test can take a long time and cause soft lockups. So
> +	 * scan a 1024 byte region across the page boundary.
> +	 */
> +	start = size / 2 - 512;
> +	size = 1024;

I don't think it's necessary to do "size / 2" here -- you can just use
PAGE_SIZE directly and check above that "size == 2*PAGE_SIZE" (not that
this check is exceptionally necessary -- since there's only one caller
of this function and it's in the same file).

> +
> +	kmem += start;
> +	umem += start;
> +
> +	zero_start = size / 4;
> +	zero_end = size - zero_start;
>  
>  	/*
>  	 * We conduct a series of check_nonzero_user() tests on a block of memory

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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