[PATCH v3] sysctl: expose sysctl_check_table for unit testing and use it

[John Sperbeck <jsperbeck@google.com>]

In commit b5ffbd139688 ("sysctl: move the extra1/2 boundary check
of u8 to sysctl_check_table_array"), a kunit test was added that
registers a sysctl table.  If the test is run as a module, then a
lingering reference to the module is left behind, and a 'sysctl -a'
leads to a panic.

This can be reproduced with these kernel config settings:

    CONFIG_KUNIT=y
    CONFIG_SYSCTL_KUNIT_TEST=m

Then run these commands:

    modprobe sysctl-test
    rmmod sysctl-test
    sysctl -a

The panic varies but generally looks something like this:

    BUG: unable to handle page fault for address: ffffa4571c0c7db4
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 100000067 P4D 100000067 PUD 100351067 PMD 114f5e067 PTE 0
    Oops: Oops: 0000 [#1] SMP NOPTI
    ... ... ...
    RIP: 0010:proc_sys_readdir+0x166/0x2c0
    ... ... ...
    Call Trace:
     <TASK>
     iterate_dir+0x6e/0x140
     __se_sys_getdents+0x6e/0x100
     do_syscall_64+0x70/0x150
     entry_SYSCALL_64_after_hwframe+0x76/0x7e

Instead of fully registering a sysctl table, expose the underlying
checking function and use it in the unit test.

Fixes: b5ffbd139688 ("sysctl: move the extra1/2 boundary check of u8 to sysctl_check_table_array")
Signed-off-by: John Sperbeck <jsperbeck@google.com>
---
 fs/proc/proc_sysctl.c  | 22 +++++++++++++++++-----
 include/linux/sysctl.h |  8 ++++++++
 kernel/sysctl-test.c   |  9 ++++++---
 3 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 27a283d85a6e..2d3272826cc2 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -1137,11 +1137,12 @@ static int sysctl_check_table_array(const char *path, const struct ctl_table *ta
 	return err;
 }
 
-static int sysctl_check_table(const char *path, struct ctl_table_header *header)
+static int sysctl_check_table(const char *path, const struct ctl_table *table,
+			      size_t table_size)
 {
-	const struct ctl_table *entry;
+	const struct ctl_table *entry = table;
 	int err = 0;
-	list_for_each_table_entry(entry, header) {
+	for (size_t i = 0 ; i < table_size; ++i, entry++) {
 		if (!entry->procname)
 			err |= sysctl_err(path, entry, "procname is null");
 		if ((entry->proc_handler == proc_dostring) ||
@@ -1173,6 +1174,16 @@ static int sysctl_check_table(const char *path, struct ctl_table_header *header)
 	return err;
 }
 
+#if IS_ENABLED(CONFIG_KUNIT)
+int sysctl_check_table_test_helper_sz(const char *path,
+				      const struct ctl_table *table,
+				      size_t table_size)
+{
+	return sysctl_check_table(path, table, table_size);
+}
+EXPORT_SYMBOL(sysctl_check_table_test_helper_sz);
+#endif /* CONFIG_KUNIT */
+
 static struct ctl_table_header *new_links(struct ctl_dir *dir, struct ctl_table_header *head)
 {
 	struct ctl_table *link_table, *link;
@@ -1372,6 +1383,9 @@ struct ctl_table_header *__register_sysctl_table(
 	struct ctl_dir *dir;
 	struct ctl_node *node;
 
+	if (sysctl_check_table(path, table, table_size))
+		return NULL;
+
 	header = kzalloc(sizeof(struct ctl_table_header) +
 			 sizeof(struct ctl_node)*table_size, GFP_KERNEL_ACCOUNT);
 	if (!header)
@@ -1379,8 +1393,6 @@ struct ctl_table_header *__register_sysctl_table(
 
 	node = (struct ctl_node *)(header + 1);
 	init_header(header, root, set, node, table, table_size);
-	if (sysctl_check_table(path, header))
-		goto fail;
 
 	spin_lock(&sysctl_lock);
 	dir = &set->dir;
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 40a6ac6c9713..09caac302333 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -247,6 +247,14 @@ extern int unaligned_enabled;
 extern int unaligned_dump_stack;
 extern int no_unaligned_warning;
 
+#if IS_ENABLED(CONFIG_KUNIT)
+int sysctl_check_table_test_helper_sz(const char *path,
+				      const struct ctl_table *table,
+				      size_t table_size);
+#define sysctl_check_table_test_helper(path, table)	\
+	sysctl_check_table_test_helper_sz(path, table, ARRAY_SIZE(table))
+#endif /* CONFIG_KUNIT */
+
 #else /* CONFIG_SYSCTL */
 
 static inline void register_sysctl_init(const char *path, const struct ctl_table *table)
diff --git a/kernel/sysctl-test.c b/kernel/sysctl-test.c
index 3ac98bb7fb82..247dd8536fc7 100644
--- a/kernel/sysctl-test.c
+++ b/kernel/sysctl-test.c
@@ -410,9 +410,12 @@ static void sysctl_test_register_sysctl_sz_invalid_extra_value(
 		},
 	};
 
-	KUNIT_EXPECT_NULL(test, register_sysctl("foo", table_foo));
-	KUNIT_EXPECT_NULL(test, register_sysctl("foo", table_bar));
-	KUNIT_EXPECT_NOT_NULL(test, register_sysctl("foo", table_qux));
+	KUNIT_EXPECT_EQ(test, -EINVAL,
+			sysctl_check_table_test_helper("foo", table_foo));
+	KUNIT_EXPECT_EQ(test, -EINVAL,
+			sysctl_check_table_test_helper("foo", table_bar));
+	KUNIT_EXPECT_EQ(test, 0,
+			sysctl_check_table_test_helper("foo", table_qux));
 }
 
 static struct kunit_case sysctl_test_cases[] = {
-- 
2.47.1.613.gc27f4b7a9f-goog