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

Michael Ellerman
2019-10-11T02:24:47Z

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;
+
+ 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 memory
--
2.21.0