[kernel-hardening] Re: Use-after-free with deferred driver probing and __initconst

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Sun, Dec 03, 2017 at 07:19:53PM +0200, Dan Aloni wrote:
> Hi all,
> 
> [[ CC'ed: folks relating to the original __*_refok family of attributes,
> deferred probing, Open Firmware maintainer, drivers/base/ maintainer,
> kernel harderning, LKML ]]
> 
> It seems that it is possible to cause a use-after-free in the base driver
> platform code using a set of combined circumstances which I describe below.
> The instance of the issue happens on a patched 4.4 kernel at a client of
> mine.
> 
> [    6.173692] Process kworker/u12:3 (pid: 173, stack limit = 0xfffffc3ea92b8000)
> [    6.180902] Call trace:
> [    6.183345] [<ffff12bbcd90746c>] __of_match_node+0x48/0x8c
> [    6.188820] [<ffff12bbcd9074f4>] of_match_node+0x44/0x68
> [    6.194125] [<ffff12bbcd9097e4>] of_match_device+0x34/0x48
> [    6.199603] [<ffff12bbcd4e1ed8>] platform_match+0x34/0xa8
> [    6.204991] [<ffff12bbcd4df3c0>] __device_attach_driver+0x60/0xc8
> [    6.211077] [<ffff12bbcd4dca38>] bus_for_each_drv+0x60/0xac
> [    6.216640] [<ffff12bbcd4ded40>] __device_attach+0x98/0x124
> [    6.222203] [<ffff12bbcd4df494>] device_initial_probe+0x24/0x30
> [    6.228111] [<ffff12bbcd4ddf4c>] bus_probe_device+0x38/0xa0
> [    6.233677] [<ffff12bbcd4de57c>] deferred_probe_work_func+0x108/0x128
> [    6.240111] [<ffff12bbcced1d50>] process_one_work+0x268/0x444
> [    6.245849] [<ffff12bbcced21a0>] worker_thread+0x274/0x404
> [    6.251329] [<ffff12bbcced8c28>] kthread+0xe0/0xe8
> [    6.256114] [<ffff12bbcce92740>] ret_from_fork+0x10/0x50
> [    6.261415] ---[ end trace fccad0f7d2c2142a ]---
> [    6.271293] note: kworker/u12:3[173] exited with preempt_count 1
> [    6.277342] Unable to handle kernel paging request at virtual address ffffffffffffffd8
> 
> It happens while booting, and among other things it requires having platform
> OF drivers marking their `of_driver_id` arrays as `__initconst`, e.g:
> 
>     static const struct of_device_id some_driver_of[] __initconst = {
>         ...
>         {},
>     };
> 
> Given a platform driver that uses deferred probing, these arrays marked as
> `__initconst` could be accessed from a deferred probe path after the init
> section of the kernel has been freed. I have not seen anything in the API
> related deferred probing that can guard from this scenario.
> 
> On kernels prior to KASLR the access is not detected, but it can still happen,
> potentially accessing memory that was returned to the page allocator.
> 
> On 4.15-rc1, the following shows the ratio between instances of `of_device_id`
> arrays and the number of them which are declared`__initconst`:
> 
>     $ git grep 'struct of_device_id.*\[\]' | wc -l
>     3089
> 
>     $ git grep 'struct of_device_id.*\[\]' | grep __initconst  | wc -l
>     117
> 
> Not all of these instances are platform drivers, but perhaps deferred
> probing with other types of drivers may cause similar issues.
> 
> Perhaps it is worthwhile patching stable kernels for the removal of
> `__initconst` on these arrays?

Yes, if those patches are in Linus's tree, that fixes a bug.

> And for the larger question -
> 
> Freeing of init sections poses an exploitable vulnerability if that memory
> is not unmapped, _and_ if there are still accesses taking place due to bugs
> of this kind. Linux's build process is supposed to detect references from
> non-freed sections to the freed sections, but clearly this instance has
> not been detected during build, particularly because we have the `__ref`,
> `__refdata`, and `__refconst` attributes which suppress those checks.
> 
> Perhaps as a harderning measure, older kernels should be patched with a
> config option for not freeing init sections?

Older kernels should just get the fixes that are in newer kernels :)

If you have specific pointers to commits that resolve these issues, I'll
be glad to queue them up in the stable kernels.

Yes, it is a whack-a-mole fixup, a much better option would be what you
suggest.  Perhaps we need to fix the build to properly mark these
section references as errors like other ones are?

thanks,

greg k-h