usercopy: Avoid soft lockups in test_check_nonzero_user()

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>
Reviewed-by: Aleksa Sarai <cyphar@cyphar.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191016122732.13467-1-mpe@ellerman.id.au
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
index ad23727..5ff04d8 100644 (file)
--- a/lib/test_user_copy.c
+++ b/lib/test_user_copy.c
@@ -47,9 +47,25 @@ 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 < 2 * PAGE_SIZE, "buffer too small"))
+               return -EINVAL;
+
+       /*
+        * We want to cross a page boundary to exercise the code more
+        * effectively. 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.
+        */
+       size = 1024;
+       start = PAGE_SIZE - (size / 2);
+
+       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