linux-kernel - [PATCH v2] usercopy: Avoid soft lockups in test_check_nonzero

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20191016122732.13467-1-mpe@ellerman.id.au>
Date:   Wed, 16 Oct 2019 23:27:32 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     cyphar@...har.com
Cc:     mingo@...hat.com, peterz@...radead.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, christian@...uner.io, keescook@...omium.org,
        linux@...musvillemoes.dk, viro@...iv.linux.org.uk,
        torvalds@...ux-foundation.org, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2] 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@...har.com>
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
---
 lib/test_user_copy.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

v2: Rework calculation to just use PAGE_SIZE directly.
    Rebase onto Christian's tree.

diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
index ad2372727b1b..5ff04d8fe971 100644
--- 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
-- 
2.21.0