Re: [PATCH] [RFC] fs: Possible filp_open race experiment

From: Marek Vasut
Date: Tue Jan 31 2017 - 05:11:47 EST

On 01/31/2017 08:05 AM, Greg Kroah-Hartman wrote:
> On Tue, Jan 31, 2017 at 06:29:36AM +0100, Marek Vasut wrote:
>> +CC Greg, LKML as I don't quite know where this should go.
> You do know about linux-fsdevel, right?

No, wasn't aware of it, sorry.

>> On 01/18/2017 12:16 AM, Marek Vasut wrote:
>>> I believe there is a possible race condition when configfs attributes
>>> trigger filp_open() from the kernel. I initially observed the problem
>>> on Linux 4.4 when loading DT overlay , which in turn loads a driver
>>> which loads firmware. After some further investigation, I came up with
>>> the following minimal-ish example patch, which can trigger the same
>>> behavior on Linux 4.10-rc4 (next 20170117).
> What in-kernel code causes this problem? I didn't think DT overlays
> were a feature in 4.4, are you running with code that isn't in the
> normal releases?

No, it happens in -next as well. I believe if write into configfs binary
attribute triggers filp_open(), the kernel will crash.

>>> The core of the demo is in cfs_over_item_dtbo_write(), which just checks
>>> for valid current->fs . This function is triggered by writing data into
>>> configfs binary attribute, ie.:
> Why are you caring about current->fs?

Because that is what's NULL and is referenced (in set_root_rcu()) when
the configfs binary attribute is written and triggers filp_open() .

>>> $ mkdir /sys/kernel/config/test/overlays/1/dtbo
>>> $ cat file_17201_bytes_long > /sys/kernel/config/test/overlays/1/dtbo
>>> I believe the 'cat' program exits quickly and thus calls fs_exit()
>>> before the cfs_over_item_dtbo_write() is called.
> How can exit be called before write?

I believe the exit happens after write, but this function
cfs_over_item_dtbo_write() is entered only after the fs_exit().

>>> Any attempts to
>>> access FS (like ie. loading firmware from FS) from that function will
>>> therefore fail (by crashing the kernel, NULL pointer dereference in
>>> set_root_rcu() in fs/namei.c).
>>> On the other hand, replacing 'cat' with 'dd' yields different result:
>>> $ dd if=file_17201_bytes_long of=/sys/kernel/config/test/overlays/1/dtbo
>>> The kernel does not crash. I believe this is because dd takes slightly
>>> longer to complete, so the cfs_over_item_dtbo_write() can complete
>>> before the dd process gets to calling fs_exit() and so the filesystem
>>> access is still available, thus current->fs is valid.
> cat and dd act differently, if you strace them, it should show the
> differences, perhaps you can narrow it down there?

I can try.

>>> Note that when using DT overlays (whose configfs interface is not yet
>>> mainline),
> Ah, we can't do anything about code that is not merged, perhaps it is
> just buggy? :)

The configfs stuff is in -next , how is it not merged ? The code below
is an example that triggers the problem.

>>> there can easily be a device which requires a firmware in
>>> the DT overlay. Such device will invoke firmware load, which uses the
>>> filp_open() and will thus trigger the behavior above. Depending on
>>> whether one uses dd or cat, the kernel will either crash or not.
>>> Any ideas ?
> I think you need to fix your device tree overlay code...

This is not related to DTO, I only use that to trigger the problem.

Best regards,
Marek Vasut