Re: [PATCH] drivers/base: Fix memory leak in error paths

[Jouni HÃgander]

Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:

> On Thu, Nov 14, 2019 at 02:18:40PM +0200, jouni.hogander@xxxxxxxxxx wrote:
>> From: Jouni Hogander <jouni.hogander@xxxxxxxxxx>
>> 
>> Currently error paths are using device_del to clean-up preparations
>> done by device_add. This is causing memory leak as free of dev->p
>> allocated in device_add is freed in device_release. This is fixed by
>> moving freeing dev->p to counterpart of device_add i.e. device_del.
>
> Are you sure that is safe?  The device can still be "alive" after
> device_del() is called.  The only place you know that it should be freed
> is in the release callback.

Now as you pointed this out I'm not.

>
>> This memory leak was reported by Syzkaller:
>> 
>> BUG: memory leak unreferenced object 0xffff8880675ca008 (size 256):
>>   comm "netdev_register", pid 281, jiffies 4294696663 (age 6.808s)
>>   hex dump (first 32 bytes):
>>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
>>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
>>   backtrace:
>>     [<0000000058ca4711>] kmem_cache_alloc_trace+0x167/0x280
>>     [<000000002340019b>] device_add+0x882/0x1750
>>     [<000000001d588c3a>] netdev_register_kobject+0x128/0x380
>>     [<0000000011ef5535>] register_netdevice+0xa1b/0xf00
>>     [<000000007fcf1c99>] __tun_chr_ioctl+0x20d5/0x3dd0
>>     [<000000006a5b7b2b>] tun_chr_ioctl+0x2f/0x40
>>     [<00000000f30f834a>] do_vfs_ioctl+0x1c7/0x1510
>>     [<00000000fba062ea>] ksys_ioctl+0x99/0xb0
>>     [<00000000b1c1b8d2>] __x64_sys_ioctl+0x78/0xb0
>>     [<00000000984cabb9>] do_syscall_64+0x16f/0x580
>>     [<000000000bde033d>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>     [<00000000e6ca2d9f>] 0xffffffffffffffff
>
> How is this a leak?  This is in device_add(), not removing the device.
> When the structure really is freed then it can be removed.

In net/core/net-sysfs.c:netdev_register_kobject device_add allocates
dev->p. Now if register_queue_kobjects fails the error path is calling
device_del and dev->p is never freed. Proper fix here could be to call
put_device after device_del?

>
> Or are you triggering an error in device_add() somehow to trigger this
> callback?

This was found using Syzkaller with fault injection and memory leak
detection enabled. Error is not triggered in device_add but after
device_add.

BR,

Jouni HÃgander