linux-kernel - Re: [PATCH 0/2] Lustre debugfs fixes

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Message-Id: <8D55EF19-8930-47C4-B7A5-27A158A1B6DB@linuxhacker.ru>
Date:	Sun, 7 Feb 2016 18:51:09 -0500
From:	Oleg Drokin <green@...uxhacker.ru>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	devel@...verdev.osuosl.org,
	Andreas Dilger <andreas.dilger@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lustre Development List <lustre-devel@...ts.lustre.org>
Subject: Re: [PATCH 0/2] Lustre debugfs fixes


On Feb 7, 2016, at 4:39 PM, Greg Kroah-Hartman wrote:

> On Sat, Feb 06, 2016 at 02:01:49AM -0500, green@...uxhacker.ru wrote:
>> From: Oleg Drokin <green@...uxhacker.ru>
>> 
>> These two patches tie some loose ends from the Lustre debugfs conversion,
>> but while investigating them I also accumulated some questions
>> that would be good to get answers for.
>> 
>> 1. Unlike procfs, debugfs does not really guard your back and if root
>> comes in and tries to write to a readonly file (or read a write-only one),
>> it's allowed (as are permission changes too) as long as the appropriate write
>> (or read) method is provided.
>> So apparently there's whole class of bugs related to this, sample
>> exhibits are in e.g. acpi_ec_add_debugfs creating a totally noop module
>> parameter to control writes that does not really prevent any writes
>> (patch submitted separately).
>> But also things like wil_debugfs_create_iomem_x32 where when called from
>> e.g. wil6210_debugfs_init_offset, some read-only attributed get a generic
>> write method that would write straight to hardware registers (who knows
>> what would happen when you write there, possibly they are readonly, but
>> you are not getting an error).
>> At first it looked like an easy way to catch this would be to just check
>> for RO/WO mode with write/read handler set, but this is thwarted by
>> the simple attribute defines that always assign read and write methods,
>> but do the check internally for the get/set method instead.
>> But also some fault injection code that sets readonly access on some files,
>> but provides a fully functional write method that works as desired.
>> 
>> Would it make sense to redo the simple-attribute framework to easy such
>> cases detection (and also update writeable attributes to have permissions
>> reflecting this) and have a correspinding kernel debug compile option
>> to check for these?
> 
> If a developer provides the write hooks for a debugfs file, and
> userspace changes the permissions to write to it, why would you prevent
> this?  Perhaps this is what is intended.

Well, it works differently for procfs where you cannot really change the permissions.
I understand the developer might envision permission changes (or even ignoring of
permissions by root) and there is such a code out there even.
But DEFINE_SIMPLE_ATTRIBUTE code for one always provides both read and write methods
and then internally checks if get and set are available and returns -EACCESS if not.
That's totally ok too (other than -EACCESS that I think it not permitted as an errno
from read/write with EINVAL being used for that), I am just trying to figure out if
there is a way to more automatically detect cases where the write or read access is
not desired and is left there by mistake.

> Remember, debugfs is only for debugging stuff, never rely on it for
> actual device/system use.

Yes, I understand that. But debugfs is on by default pretty much anywhere and if
there's a bug that lets you write to some hardware register you were supposed to only
read and get some elevated privileges, or you read some write-only file and crash
the kernel - that's bad too.
We have kernel debugging options to check for all sorts of stuff, like use after free,
double unlocks and so on.
This sounded like another class of bugs that would have benefitted from that.

>> Now, I also see that drm_debugfs_create_files allows anybody to
>> insert any debugfs file anywhere and it is a non-gpl EXPORT_SYMBOL as well,
>> should it be converted too, or is it sysfs access only that is restricted?
> No, these should be as well, a patch for that would be great, thanks.

Done.

Bye,
    Oleg