linux-kernel - Re: dm ioctl: Access user-land memory through safe functions.

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Date:	Wed, 6 Jan 2016 18:22:39 -0700
From:	Scotty Bauer <sbauer@....utah.edu>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	dm-devel@...hat.com, linux-kernel@...r.kernel.org, agk@...hat.com,
	jmoyer@...hat.com
Subject: Re: dm ioctl: Access user-land memory through safe functions.

On 01/05/2016 02:13 PM, Mike Snitzer wrote:
> On Tue, Jan 05 2016 at  3:16pm -0500,
> Mike Snitzer <snitzer@...hat.com> wrote:
> 
>> On Tue, Dec 08 2015 at  1:26pm -0500,
>> Scotty Bauer <sbauer@....utah.edu> wrote:
>>
>>> Friendly ping, is anyone interested in this?
>>
>> The passed @user argument is flagged via __user so it can be
>> deferenced directly.  It does look like directly deferencing
>> user->version is wrong.
>>
>> But even if such indirect access is needed (because __user flag is only
>> applicable to @user arg, not the contained version member) we could more
>> easily just do something like this no?:
>>
>>   uint32_t __user *versionp = (uint32_t __user *)user->version;
>>   ...
>>   if (copy_from_user(version, versionp, sizeof(version)))
>>      return -EFAULT;
>>
>> I've staged the following, thanks:
>> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=bffc9e237a0c3176712bcd93fc6a184a61e0df26
> 
> Alasdair helped me understand that we do need your original fix.
> I've staged it for 4.5 (and stable@) here:
> https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.5&id=ead3db62bf10fe143bec99e7b7ff370d7a6d23ef
> 
> Thanks again,
> Mike
> --

This broke linux-next because I'm dumb and didn't test it. I thought it was a trivial enough of a patch that I wouldn't screw it up, but I did.

I incorrectly assumed that user->version was essentially a pointer in userland, not a flat chunk of memory. Ie it was a pointer to some malloc'd region, not an inlined version[3].

I thought it was this:
struct dm_ioctl {

uint32_t *version;
...
}

It is really this:

struct dm_ioctl {

uint32_t version[3];

}

I was trying to get the values out of *version, which would have been a pointer, but instead what the code ended up doing was actually getting 8 bytes of the version (think 4,3,1) out and trying to access that version as a memory address, oops.

It turns out that the original code is correct and doesn't actually touch user memory without a copy_from_user().  Gcc is smart enough to see that version[3] is inlined, and it can emit code which simply takes the userland pointer (struct dm_ioctl __user user), and calculates on offset based on the pointer, thus no actual user dereference occurs. Had the struct looked like the first example I believe the patch would work.

I'm wondering now if we should switch the code a bit to make it less ambiguous, so someone like me doesn't come along again thinking the code dereferences userland memory and waste everyones time.

I've attached a patch based off linux-next-20150616 which reverts my broken code but adds an & to the front of user->version so it looks like the code is doing the right thing.

If I should be basing my patch off something other than linux-next let me know and I'll rewrite it, or we can just revert the old patch and ignore this one.

Thanks and very sorry for the confusion and breakage.

View attachment "0001-dm-ioctl-disambiguate-the-user-pointer-calculation.patch" of type "text/x-patch" (1488 bytes)