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

[Christian Brauner <christian dot brauner at ubuntu dot com>]

On Wed, Oct 16, 2019 at 11:28:20PM +1100, Michael Ellerman wrote:
> Christian Brauner <christian.brauner@ubuntu.com> writes:
> > On Fri, Oct 11, 2019 at 02:48:10PM +1100, Aleksa Sarai wrote:
> >> 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).
> >
> > 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.

Thank you!
Christian