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

[Greg Kroah-Hartman]

On Fri, Nov 15, 2019 at 12:05:17PM +0200, Jouni Högander wrote:
> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> 
> >> >> 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?
> >
> > Hm, this sounds like you have a reference count leak here, as
> > put_device() should be properly called already in this case.  You might
> > want to look further to see where exactly the register_queue_kobjects()
> > call fails in order to see if we grabbed a reference we forgot to put
> > back on an error path.
> 
> Ok, did some more debugging on
> this. net/core/net-sysfs.c:netdev_register_kobject is doing
> device_initialize(dev). This is in
> drivers/base/core.c:device_initialize:
> 
>  * NOTE: Use put_device() to give up your reference instead of freeing
>  * @dev directly once you have called this function.
> 
> My understanding is that remaining reference on error path is taken by
> device_initialize and as instructed in the note above it should be given
> up using put_device?

Yes, that is correct.

> Tested this and it's fixing the memory leak I found in my Syzkaller
> exercise. Addition to that it seems to be fixing also this one:
> 
> https://syzkaller.appspot.com/bug?id=f5f4af9fb9ffb3112ad6e30f717f769decdccdfc

Great!  Care to submit a patch for this?

thanks,

greg k-h