sysctl: move the extra1/2 boundary check of u8 to sysctl_check_table_array

Move boundary checking for proc_dou8ved_minmax into module loading, thereby
reporting errors in advance. And add a kunit test case ensuring the
boundary check is done correctly.

The boundary check in proc_dou8vec_minmax done to the extra elements in
the ctl_table struct is currently performed at runtime. This allows buggy
kernel modules to be loaded normally without any errors only to fail
when used.

This is a buggy example module:
	#include <linux/kernel.h>
	#include <linux/module.h>
	#include <linux/sysctl.h>

	static struct ctl_table_header *_table_header = NULL;
	static unsigned char _data = 0;
	struct ctl_table table[] = {
		{
			.procname       = "foo",
			.data           = &_data,
			.maxlen         = sizeof(u8),
			.mode           = 0644,
			.proc_handler   = proc_dou8vec_minmax,
			.extra1         = SYSCTL_ZERO,
			.extra2         = SYSCTL_ONE_THOUSAND,
		},
	};

	static int init_demo(void) {
		_table_header = register_sysctl("kernel", table);
		if (!_table_header)
			return -ENOMEM;

		return 0;
	}

	module_init(init_demo);
	MODULE_LICENSE("GPL");

And this is the result:
        # insmod test.ko
        # cat /proc/sys/kernel/foo
        cat: /proc/sys/kernel/foo: Invalid argument

Suggested-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Wen Yang <wen.yang@linux.dev>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Joel Granados <j.granados@samsung.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: linux-kernel@vger.kernel.org
Reviewed-by: Joel Granados <j.granados@samsung.com>
Signed-off-by: Joel Granados <j.granados@samsung.com>

1 files changed, 2 insertions, 8 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e0b917328cf9..c0a1164eaf59 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -977,16 +977,10 @@ int proc_dou8vec_minmax(struct ctl_table *table, int write,
 	if (table->maxlen != sizeof(u8))
 		return -EINVAL;
 
-	if (table->extra1) {
+	if (table->extra1)
 		min = *(unsigned int *) table->extra1;
-		if (min > 255U)
-			return -EINVAL;
-	}
-	if (table->extra2) {
+	if (table->extra2)
 		max = *(unsigned int *) table->extra2;
-		if (max > 255U)
-			return -EINVAL;
-	}
 
 	tmp = *table;