[PATCH] kobject: Fix slab-out-of-bounds in fill_kobj_path()

[Wang Hai]

In kobject_get_path(), if kobj->name is changed between calls
get_kobj_path_length() and fill_kobj_path() and the length becomes
longer, then fill_kobj_path() will have an out-of-bounds bug.

The actual current problem occurs when the ixgbe probe.

In ixgbe_mii_bus_init(), if the length of netdev->dev.kobj.name
length becomes longer, out-of-bounds will occur.

cpu0                                         cpu1
ixgbe_probe
 register_netdev(netdev)
  netdev_register_kobject
   device_add
    kobject_uevent // Sending ADD events
                                             systemd-udevd // rename netdev
                                              dev_change_name
                                               device_rename
                                                kobject_rename
                                                |
 ixgbe_mii_bus_init                             |
  mdiobus_register                              |
   __mdiobus_register                           |
    device_register                             |
     device_add                                 |
      kobject_uevent                            |
       kobject_get_path                         |
        len = get_kobj_path_length // old name  |
        path = kzalloc(len, gfp_mask);          |
                                                kobj->name = name;
                                                /* name length becomes
                                                 * longer
                                                 */
        fill_kobj_path /* kobj path length is
                        * longer than path,
                        * resulting in out of
                        * bounds when filling path
                        */

This is the kasan report:

==================================================================
BUG: KASAN: slab-out-of-bounds in fill_kobj_path+0x50/0xc0
Write of size 7 at addr ff1100090573d1fd by task kworker/28:1/673

 Workqueue: events work_for_cpu_fn
 Call Trace:
 <TASK>
 dump_stack_lvl+0x34/0x48
 print_address_description.constprop.0+0x86/0x1e7
 print_report+0x36/0x4f
 kasan_report+0xad/0x130
 kasan_check_range+0x35/0x1c0
 memcpy+0x39/0x60
 fill_kobj_path+0x50/0xc0
 kobject_get_path+0x5a/0xc0
 kobject_uevent_env+0x140/0x460
 device_add+0x5c7/0x910
 __mdiobus_register+0x14e/0x490
 ixgbe_probe.cold+0x441/0x574 [ixgbe]
 local_pci_probe+0x78/0xc0
 work_for_cpu_fn+0x26/0x40
 process_one_work+0x3b6/0x6a0
 worker_thread+0x368/0x520
 kthread+0x165/0x1a0
 ret_from_fork+0x1f/0x30

This reproducer triggers that bug:

while:
do
    rmmod ixgbe
    sleep 0.5
    modprobe ixgbe
    sleep 0.5

When calling fill_kobj_path() to fill path, if the name length of
kobj becomes longer, return failure and retry.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Wang Hai <wanghai38@xxxxxxxxxx>
---
 lib/kobject.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/lib/kobject.c b/lib/kobject.c
index a0b2dbfcfa23..d129f437b200 100644
--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -112,7 +112,7 @@ static int get_kobj_path_length(struct kobject *kobj)
 	return length;
 }
 
-static void fill_kobj_path(struct kobject *kobj, char *path, int length)
+static bool fill_kobj_path(struct kobject *kobj, char *path, int length)
 {
 	struct kobject *parent;
 
@@ -121,12 +121,16 @@ static void fill_kobj_path(struct kobject *kobj, char *path, int length)
 		int cur = strlen(kobject_name(parent));
 		/* back up enough to print this name with '/' */
 		length -= cur;
+		if (length < 0)
+			return false;
 		memcpy(path + length, kobject_name(parent), cur);
 		*(path + --length) = '/';
 	}
 
 	pr_debug("kobject: '%s' (%p): %s: path = '%s'\n", kobject_name(kobj),
 		 kobj, __func__, path);
+
+	return true;
 }
 
 /**
@@ -140,14 +144,20 @@ char *kobject_get_path(struct kobject *kobj, gfp_t gfp_mask)
 {
 	char *path;
 	int len;
+	bool ret;
 
+retry:
 	len = get_kobj_path_length(kobj);
 	if (len == 0)
 		return NULL;
 	path = kzalloc(len, gfp_mask);
 	if (!path)
 		return NULL;
-	fill_kobj_path(kobj, path, len);
+	ret = fill_kobj_path(kobj, path, len);
+	if (!ret) {
+		kfree(path);
+		goto retry;
+	}
 
 	return path;
 }
-- 
2.17.1