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

[Christian Brauner]

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.

Christian