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

[Michael Ellerman]

Christian Brauner <christian.brauner@xxxxxxxxxx> writes:
> On Fri, Oct 11, 2019 at 02:48:10PM +1100, Aleksa Sarai wrote:
>> On 2019-10-11, Michael Ellerman <mpe@xxxxxxxxxxxxxx> 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@xxxxxxxxxx>
>> > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
>> > ---
>> >  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).
>
> Michael, in case you resend, can you make my life a little easier and do
> it on top of
> https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=copy_struct_from_user
> please. I have a fix from Aleksa sitting in there laready that _might_
> cause a conflict otherwise.

No worries, done.

cheers