Unverified Commit f418dddf authored Oct 16, 2019 by Michael Ellerman Committed by Christian Brauner Oct 16, 2019

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: f5a1a536 ("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>

parent c90012ac

lib/test_user_copy.c

+19 −3

Original line number	Diff line number	Diff line
		@@ -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

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